Meterology UHS

Air masses

Air mass refers to a large body of air. These masses are usually 1600 kilometers or more across and are characterized by having the same properties at a given altitude. There are four main types of air masses. They are distinguished by properties of temperature and moisture content. Air masses that originate over land tend to be dry and are called continental. If they originate over the ocean, they are more humid and are called maritime. Warm air masses usually come from tropical regions, so the word tropical is used to describe warm air masses. Similarly, colder air masses are described as polar. To describe a particular air mass, there must be one descriptor of its humidity (continental or maritime) and one descriptor of its temperature (tropical or polar). Thus, there are four classifications for air masses: continental polar, continental tropical, maritime polar, and maritime tropical. See below for a brief description of each type of air mass

Cyclonic storms

Cyclones form along fronts and proceed through a somewhat predictable life cycle. Wave cyclones begin along a front where air masses of different densities (temperatures) are moving parallel to the front in opposite directions. This would usually be continental polar air from the polar easterlies and maritime tropical air from the westerlies south of the front. The result of this opposing flow of air is the development of cyclonic shear (see figure 5.3) and a general counterclockwise rotation. This creates a wave-like appearance. As a small wave appears, warm air invades the weak spot along the front and moves poleward, forcing cold air to move toward the equator Cyclonic shear and counterclockwise rotation: Place a pencil between your palms. Now move your right hand ahead of your left. Notice that the pencil follows a counterclockwise rotation. This change causes a readjustment in the pressure field that results in almost circular isobars with the low pressure centered on the crest of the "wave." Warm air from the southwest advances on cooler air, resulting in a warm front. At the tail end of the warm air, cold air from the northwest advances under the warmer air, resulting in a cold front. So a cyclonic storm could be described as a cold front advancing toward a warm front with the fronts meeting at a low pressure center. The cold front eventually overtakes the warm front, causing an occluded front.

Weather fronts

Fronts are located at the boundary of two air masses that have different densities. Fronts may form between any two types of air masses. As one air mass advances upon another, a little mixing does occur, but for the most part they both retain their original air mass characteristics. The four types of fronts are explained below.

Humidity

Humidity is a general term used to denote the amount of water in the air. Any device that is used to measure the humidity in the air is called a hygrometer. There are many different methods used to measure humidity. These include absolute humidity, mixing ratio, and relative humidity. In order to understand the different types of humidity, it is necessary to understand the concept of saturation. Saturation occurs when air holds exactly the largest amount of water that it possibly can. For example, as water evaporates, more water molecules are present in the air. This continues until a point is reached in which the air can't hold any more water molecules and some of them are forced out of the air as liquid water again. Eventually a balance is reached, and the air will hold as much water vapor as it can. Changing the temperature of air will change the saturation. Warmer air can hold more water vapor than cooler air. Another important idea related to humidity is the dewpoint. The dewpoint is the temperature at which the air would be saturated (100 percent relative humidity). Think of what happens when you buy a can of soda on a warm day. The cold can is cooler than the dewpoint, so water begins to condense on the outside of the can. When air is saturated, the air temperature is equal to the dewpoint and the air cannot hold any more water vapor. Water will then condense very easily. Absolute humidity is a direct measurement of the mass of water vapor in a given volume of air. Since volume can change as air moves or changes temperature, the absolute humidity can change without losing or gaining any water vapor. The mixing ratio is expressed as the mass of water vapor in a given mass of air. Because it is measured in units of mass, the mixing ratio is not affected by changes in pressure or temperature. Unfortunately, neither the absolute humidity nor the mixing ratio can be easily measured, but an easier measurement of humidity is the relative humidity. Relative humidity is the ratio of the amount of water in the air divided by the amount of water the air can hold at that temperature. Thus, if air is saturated, the relative humidity is 100 percent. Relative humidity can be changed in one of two ways: by changing the amount of water in the air or by changing the temperature. (Recall that air at different temperatures will hold different amounts of water.) Changes in relative humidity usually occur in one of three ways: daily temperature variation, movement of air from one location to another, or vertical movement of air in the atmosphere. Instruments used to measure humidity are called hygrometers. The most common hygrometer is a psychrometer. The psychrometer consists of two thermometers side by side. One of these has a piece of cloth over the bulb and is called the wet bulb. The other is left exposed to air and is called the dry bulb. To use the psychrometer, the cloth is dipped in water. As air is blown over the wet bulb, water evaporates. Evaporation requires the absorption of heat which comes from the thermometer and therefore shows a lower temperature. The amount of cooling that takes place is directly proportional to the dryness of the air. The drier the air, the more water will evaporate and the cooler the wet bulb temperature will be. A chart is necessary to read the humidity from a psychrometer. (See figure 2.3.) Look down the left side of the chart for the current air temperature (dry bulb temperature). Then follow it across until it lines up with the wet bulb depression (dry bulb minus wet bulb temperatures) at the top of the chart. The relative humidity is shown. For example, if the dry bulb reads that it is 75 degrees and the wet bulb reads 66 degrees, then we find the wet bulb depression by taking 75 minus 66 to get 9. Then we look at the psychrometer chart to find 75 under the dry bulb temperature and go across to where the wet bulb depression reads 9. We can then see that the relative humidity is 62 percent. The combination of high temperature with high humidity can cause people a great deal of discomfort. Meteorologists have developed a scale to measure the level of discomfort and have called it the heat index. This takes into account both the air's actual temperature and relative humidity and tries to convert it to the human-perceived equivalent temperature. As mammals, we maintain a constant body temperature by burning food calories to warm up and sweating to cool down. The evaporation of sweat on our bodies absorbs heat from our bodies, which cools us. When the air is very humid, the rate of evaporation decreases, so it is more difficult for this process to effectively cool us. It is for this reason that we feel like we sweat profusely when in a humid area. Our bodies are trying to cool us by sweating, but it is more difficult.

Hurricanes

Hurricanes have been called the "Greatest Storms on Earth" because they cause the most widespread severe damage. They are the only natural disasters with their own names. While winds in tornadoes can be above 300 miles per hour, hurricane winds above 150 miles per hour are extremely rare. While a tornado that is a mile across is huge, a hurricane that is one hundred miles across is very small. Few tornadoes last even an hour, and a damage path of a hundred miles goes into the record books. Hurricanes, on the other hand, last more than a week and can devastate islands around the Caribbean days before slamming into the United States. A large hurricane stirs up more than a million cubic miles of the atmosphere every second. They can kick up fifty-foot or higher waves in the open ocean. When a hurricane hits land, it brings a storm surge (a wall of water as high as twenty feet that can flood one hundred miles of coast with ten feet of water). A typical hurricane dumps six to twelve inches of rain when it comes ashore, which contributes to severe flooding. Hurricanes form in the tropics. The tropical region supplies the key ingredients needed for tropical cyclones: wide, warm oceans; warm, humid air; and normally weak upper air winds. Think of a hurricane as a huge machine that takes heat and moisture and turns it into wind and waves. Water is the source of energy for a hurricane. Hurricanes begin as a low pressure center in the tropics. The low pressure pulls in warm, moist air, forcing it to rise. When water vapor condenses in rising air, it releases latent heat, which warms the surrounding air. Warmer air rises, so a cycle of rising air begins. As warm air rises, more air flows in to replace it, causing winds. Hurricanes die out over land because there is no water to continue to fuel the storm. The stages of development of a hurricane are tropical depression, tropical storm, and hurricane. A tropical depression is kind of like a "baby hurricane" and has wind speeds below 39 miles per hour. It is called a tropical depression because it is a low pressure center, or a depression of air pressure, formed in the tropical region. If the pressure drops further, a tropical storm is the next step, with winds reaching as high as 74 miles per hour. (Recall that wind speeds increase as the pressure gradient increases.) Further drops in air pressure strengthen the pressure gradient enough to produce wind speeds in excess of 74 miles per hour (119 kilometers per hour). At this point the tropical storm has become a hurricane. Hurricanes in the Atlantic Ocean usually form in the trade winds and move from west to east at about 25 kilometers per hour (15 miles per hour). They are then deflected northward into the westerlies which curves their path and increases their speed to around 100 kilometers per hour (60 miles per hour). A location only a few hundred kilometers away (within one day's striking distance) may have clear skies and calm winds. Before the use of satellites, this condition made it extremely difficult to evacuate coastal cities before the hurricane destruction occurred. In fact, the worst natural disaster to hit the U.S. was a hurricane that struck an unprepared Galveston, Texas in September 1900. Over eight thousand Americans lost their lives because there was no adequate warning. The year 2004 was a bad hurricane season. Four hurricanes made landfall in the state of Florida (Hurricanes Charley, Frances, Ivan, and Jeanne). This was the first time that four hurricanes hit a single U.S. state since 1886 when four hurricanes hit Texas. The year 2005 saw the costliest single-hurricane damage on record when Hurricane Katrina passed near New Orleans in August. Several news outlets reported that New Orleans had dodged a bullet because the eye of Katrina passed to the east. However, the storm surge, combined with high winds and waves, managed to break the levees of Lake Pontchartrain, resulting in massive flooding. The damage and destruction were widespread. Much of the city found itself under water. Katrina was followed by Hurricane Rita in September 2005. Rita was the third strongest hurricane on record in the Atlantic Basin and the seventeenth named tropical storm in 2005. Today we have a good warning system. We are able to see and track the entire development of tropical storms and hurricanes through the use of satellites. As a result, there are much fewer deaths resulting from hurricanes. However, property damage has increased dramatically because of the rapid population growth in coastal cities. The National Weather Service is concerned about such population growth. Evacuation of very large numbers of people could require more warning time than is presently available. A hurricane watch is an announcement to specific coastal cities that could experience damage within thirty-six hours. A hurricane warning is issued when sustained winds of 119 kilometers per hour (74 miles per hour) or higher are expected within twenty-four hours. A hurricane warning may remain in effect if dangerously high water or a combination of high water and exceptionally high waves continue even after winds have died down. An attempt was made to modify hurricanes to make them less intense. Project Stormfury was a government project from 1962 to 1983. They studied the effects of seeding the clouds in a hurricane. The theory was that since hurricanes get their energy from latent heat of condensation, if you could artificially speed up the condensation process by seeding, then the hurricane would run out of energy and die out faster. Unfortunately, as experiments were performed, it was found that many hurricanes did not exhibit the necessary characteristics for cloud seeding to have any effect

Air pressure

Imagine what would happen to a bathroom scale if a person were to throw a baseball at it. The scale would show a measurement of some weight and then return to normal. If enough people were to throw baseballs at it, it would not have enough time to return to its normal position and would show a fairly steady measurement of weight. The baseballs represent the individual air molecules in the atmosphere, which push on everything. Air pressure is the force exerted by continuous collisions of air molecules. Another way to think of air pressure is the weight of a vertical column of air. However, it is important to remember that air pressure is the same in every direction (air is pushing on you from the right just as hard as it is pushing down on you). Two factors affect air pressure: temperature and density. As the temperature of a gas rises, the individual molecules move faster. This means that they will exert a stronger force, and hence, air pressure rises. Equally, a drop in temperature means a drop in pressure. Also, increasing the density of a gas will serve to increase the number of gas molecules in a given volume. This has the effect of increasing the number of collisions that take place, thereby increasing the air pressure. It might appear to make sense that on warm days the atmospheric pressure would be highest and on cold days it would be lowest. However, this is not generally true for reasons that we will discuss. Recall that the atmosphere has no "lid." As air temperature rises, the air molecules move faster and try to spread out. This results in a decrease in density. (Hot air tends to rise because it is less dense and literally floats on cooler air.) This decrease in density is accompanied by an overall decrease in pressure. Also of interest is the fact that, because of gravity, air molecules are more concentrated near the earth's surface. Air pressure drops by a factor of approximately one-half per five kilometers as you gain altitude. Thus, at an altitude of five kilometers, pressure would be one-half of the sea-level pressure. At ten kilometers, it would be one-fourth, and so on. To measure atmospheric pressure, meteorologists use a unit called a millibar (mb). This unit comes from the standard metric unit of force, the newton (N). One millibar equals a pressure of one hundred newtons per square meter. This being the case, the normal atmospheric pressure at sea-level is 1013 mb. Even though the millibar is the standard unit for pressure in weather applications, you may be more familiar with the term inches of mercury. Television stations still use this term in their weather broadcasts. The expression dates back to 1643 when an Italian scientist named Torricelli invented the mercurial barometer. To measure the force that air exerts on all of us, he filled a glass tube that was closed on one end with mercury. The tube was then inverted and put into a dish of mercury. Torricelli discovered that the mercury flowed out of the tube until the weight of the column was balanced by the pressure exerted on the surface of the mercury by the air above. He noticed that when air pressure increased, the height of the column of mercury in the tube rose. When pressure dropped, so did the mercury. The length of the column of mercury became the measure of air pressure. The mercurial barometer is still one of the standard devices used for measuring air pressure. Standard pressure at sea-level is 29.92 inches of mercury, or 760 mm of mercury. The need for smaller and more portable equipment led to the development of the aneroid barometer. Aneroid means "without liquid." This instrument consists of partially evacuated chambers that have a spring attached inside to keep them from collapsing. The metal chambers change shape because they are very sensitive to changes in pressure. When pressure increases, they become compressed; when pressure decreases, they expand. One of the major advantages of the aneroid barometer is that it can be adapted to recording equipment. These instruments are called barographs and provide a continuous record of fluctuations in air pressure. A pen is attached to an arm of the barometer. A column slowly rotates, allowing the pen to mark the position of air pressure continuously. Another important application of the aneroid barometer is its use in planes as altimeters. Since air pressure decreases with an increase in altitude, the barometer can be calibrated to show the plane's altitude.

Lake effect

Knowing about the different air masses can help us understand lake effect snow. As we stated earlier, water takes a long time to release heat. When a continental polar air mass passes over a large body of water, it picks up moisture from the relatively warm lake. The lake is relatively warm, which causes warm, moist air to rise and mix with the cold air passing over it. This moisture is then dumped on the other side of the lake. Thus, areas on the east side of lakes usually receive some lake effect snow. It does not add enough moisture to sustain large snowfall for very long. Only those areas that are quite close to shore (within several miles) will experience the lake effect. For example, Toledo, Ohio may receive two inches of snow from the same snowstorm that dumps two feet of snow on Buffalo, New York. (Toledo is on the west side of Lake Ontario; Buffalo is on the east side.)

Optical effects of water

Light that comes from the sun is composed of all colors. Mixing the colors of light produces white light. Try holding a prism up to a light source. You will see an array of colors. This splitting of white light into colors is called dispersion, and it occurs because different colors of light are bent at different angles through the prism. The interaction of white light from the sun with our atmosphere produces many interesting optical effects. To better understand these optical phenomena, it is necessary to understand something of the nature of light. Four possible interactions of light are reflection, refraction, diffraction, and interference. In this course, you only need to know about reflection and refraction. Reflection is simply the bouncing back of a wave that strikes a barrier. The law of reflection states that the angle of incidence equals the angle of reflection. A special type of reflection, called internal reflection, is of particular interest. This occurs when light that travels through a transparent substance, such as glass or water, reaches the opposite side and some of the light is reflected back into the substance. Internal reflection combined with dispersion is the primary mechanism for creating rainbows. Refraction is the bending of a wave due to a change of speed as the wave changes medium. An example of this can be seen if you place a pencil in a container of water. The pencil appears to be bent because the eye perceives the light coming from the pencil as if it followed a straight line instead of a refracted path. Following are descriptions of the most common optical phenomena in the atmosphere. Rainbow: Rainbows are the most well known of the optical phenomena. Light of different colors are refracted at different angles. The combined effects of refraction and internal reflection allow us to see the rainbow. Mirage: Light travels more rapidly in hot air near the surface. As downward-directed rays enter this warm zone, they are bent upward so that they reach the observer from below eye level. Halo: A halo is a narrow, whitish ring having a large diameter centered on the sun or moon. They form in a fashion similar to rainbows, except they form from ice crystals rather than raindrops. Glory: A glory forms in a similar way as rainbows, but cloud droplets are involved instead of raindrops. Light rays are back-scattered toward the sun. A person observing this would need to be between a cloud and the sun, looking back at the cloud. He would see bright circles enshrouding his head. This is rarely seen. (Sometimes it can be seen from an airplane window or on top of a mountain looking down on the clouds.) Solar pillar: These are vertical shafts of light usually seen near sunset extending above the setting sun. They are caused by sunlight that reflects off of flat ice crystals in some clouds. Corona: Commonly seen with the moon as a bright, whitish disk centered on the moon, a corona is produced by a thin layer of clouds. Corona is formed by the diffraction of light around cloud droplets. Interference of the light sometimes produces colored patterns. Sun dog: Two bright regions, or "mock suns," form in conjunction with halo if many ice crystals have a vertical orientation. Sun dogs occur slightly below the level of the real sun and about 22º to each side.

Tornado injuries

Most injuries result from flying debris. On average, tornadoes are responsible for more deaths in the U.S. than any other weather event except for lightning. The average is about ninety deaths per year. Because tornadoes are short-lived, localized phenomena, they are among the most difficult to predict, but scientists do know the atmospheric conditions necessary for tornadoes to form. Whenever these conditions are present, the National Weather Service will issue a tornado watch. This means that conditions are such that a tornado could form. If a tornado is spotted, then a tornado warning is issued. If the direction and probable speed of a storm is known, then the probable path of the tornado can be identified. Since the 1960s when tornado warnings were first issued, scientists believe that the warning system has prevented deaths.

Precipitation

One of the most interesting topics of weather is precipitation. We always want to know if it will rain, when it will snow, and so on. There are many different types of precipitation. We will discuss two minor and seven major types of precipitation. The first two types are classified as minor precipitation. Dew is simply water that has condensed on objects whose temperature has dropped below the dewpoint. This is, in fact, how the term dewpoint received its name. White frost is another example of minor precipitation. This occurs when the dewpoint is below freezing. Thus, frost is deposited directly from a gas to a solid form. Frost is not frozen dew. Deposition creates delicate patterns of ice crystals that often appear on windows. There is much more to the formation of precipitation than just condensing water from water vapor. Two very different processes are responsible for the various forms of precipitation. The first precipitation process is called the Bergeron Process. It received its name from the Swedish meteorologist who discovered it. This process relies on two interesting properties of water. First, pure water suspended in air does not freeze at 0ºC as we might expect. Water in the liquid state below 0ºC is called supercooled water. This supercooled water will freeze very quickly if it is agitated. Supercooled water droplets will also freeze quickly around solid particles that have a crystal structure similar to ice. These materials have been called freezing nuclei. Thus, freezing nuclei play a role in freezing very similar to the role that condensation nuclei play in condensation. The second property of water which is important to the Bergeron Process is that the saturation level of ice crystals is much lower than that of liquid water. This means that when air is saturated with respect to liquid water, it is supersaturated with respect to ice. With these two important facts, we can explain the Bergeron Process. Try to think of a cloud around -10ºC. At this temperature, some ice crystals have formed. They are surrounded by supercooled water droplets. Now, because the air was saturated with respect to liquid water, it is supersaturated with respect to the newly formed ice crystals. This allows the ice crystals to collect much more water out of the air. Because of the supersaturated condition of the air, the growth of snow crystals is often sufficiently rapid that the ice crystals become large enough to fall. As they fall, many of them pick up more water on the way down, increasing their size. Others may have their delicate crystal structure shattered. These fragments then serve as additional freezing nuclei and speed up the precipitation process. Large snowflakes usually consist of ten to thirty individual ice crystals. If the temperature near the surface is near or above 4ºC, then snowflakes usually will melt before reaching the surface. They fall as rain. Even some summer rain showers begin as snow that has melted on the way down. The other precipitation process is called the collision-coalescence process. Some clouds are made entirely of liquid water droplets. Some of the droplets may become relatively large if large condensation nuclei are present or when hygroscopic nuclei are present. These giant droplets fall much faster than smaller ones, bumping and knocking into some smaller droplets on their way down. They become even larger, and they begin to fall even faster. Because of the huge size required (rain drops have a diameter one thousand times greater than cloud droplets, meaning they contain one billion times as much water as a cloud droplet), this process takes place most effectively in thick, vertical clouds.

Converting between temperature scales

See picture

Localized wind

See the picture

Analogy forecasting

Suppose you notice that about 24 to 30 hours before a rainstorm the wind usually blows from the south. You notice that today the wind is blowing from the south all day. Based on this you would predict that tomorrow a storm should arrive. This type of prediction is an analogy prediction. This type of forecast depends on comparing patterns to predict how nature works. Here are some old sayings that are examples of Analogy forecasting. "RED SKY AT NIGHT, SAILORS DELIGHT. RED SKY IN MORNING, SAILORS TAKE WARNING." "THE HIGHER THE CLOUDS, THE FINER THE WEATHER." "CLEAR MOON, FROST SOON." "WHEN CLOUDS APPEAR LIKE TOWERS, THE EARTH IS REFRESHED BY FREQUENT SHOWERS." "RAINBOW IN THE MORNING GIVES YOU FAIR WARNING."

Mature stage

The mature stage begins as precipitation begins to fall. This falling water creates a drag on the air, pulling some of it down with it. This creates strong downdrafts. At the surface, cool downdrafts spread laterally and can be felt even before precipitation reaches there. During this stage, the most severe weather is present with downdrafts and updrafts found side by side. The heaviest precipitation occurs here, with possibilities for hail, lightning, gusty winds, or tornadoes. Once downdrafts begin, air from outside the cloud is pulled in. This air is generally colder and much drier than the air within the cloud. This literally cools and evaporates the cloud. In the dissipating stage, the updrafts stop and only downdrafts are found in the cloud. Without a continuous supply of warm, moist air, the cloud simply "dies."

States of matter

There are three main states of matter: solid, liquid, and gas. The largest difference between the states of matter is the motion of the molecules. In a solid, molecules are not really free to move about. They stay in relatively fixed positions, although they do vibrate. A solid maintains both its shape and volume. Molecules in a liquid are arranged a little more loosely. They can move about and are freer than in a solid. Liquids will maintain their volume, but their shape will change to fit whatever container they are found in. Molecules in a gas are very free to move about and are bound much less than even the molecules of a liquid. Gases can be compressed or expanded. A gas will try to completely fill whatever container it is found, and its volume is free to change as it does so.

Synoptic forecasting

This is the most sophisticated type of forecasting. It depends on the use of weather maps that show current conditions over a wide area. Here is a synoptic weather chart from NOAA. With this type of data, meteorologists can use computer models and their own understanding of weather dynamics to make predictions for several days and even weeks in advance. This is the most common and accurate type of prediction.

Forces and wind

We have already discussed the importance of the vertical movement of air in the formation of clouds. What about the horizontal movement of air? We normally refer to this as wind. What causes air to move horizontally? To put it simply, wind is the result of horizontal differences in air pressure. Warm air is less dense than cool air and will rise. Air flows from areas of high pressure toward areas of low pressure. Wind is nature's attempt to equalize air pressure. Because unequal heating of the earth's surface generates these pressure differences, the sun is the ultimate driving force of wind. If the earth did not rotate, and if there were no friction, air would move directly from high pressure to low pressure. Because these factors are present, there are three forces that affect wind: Pressure gradient force Coriolis force Friction According to Newton's second law of motion, an unbalanced force is required to cause anything to accelerate. The force that drives winds results from horizontal differences in pressure. The greater the difference in pressure, the stronger the winds that result. Variations in air pressure over the earth's surface are determined by taking readings at numerous locations. This information is then displayed on a weather map in the form of isobars. These are lines of equal pressure on a map. Thus, any location along a single isobar has the same air pressure. The spacing of these isobars indicates the pressure change over a given distance and is expressed as the pressure gradient. Think of the slope of a hill. A steep hill has a rapid change of altitude for a small change in horizontal position. The same is true for isobars. Closely spaced isobars indicate a steep pressure gradient, resulting in strong winds. Notice in the figure below that where isobars are closely spaced, the distance from high to low pressure is very small. This results in strong winds. In contrast, look where the isobars are widely spaced. The pressure gradient force pushes air from high pressure towards low pressure. Thus, the air must cross the isobars. The earth's rotation produces noticeable effects in the direction that winds blow. The result is called the Coriolis effect. Winds moving from the equator toward the poles are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Winds do not increase in speed indefinitely. Upper-level winds are deflected by the Coriolis force until it balances the pressure gradient force. This causes airflow to be deflected in such a way that upper-level winds will flow in straight lines parallel to isobars. These are called geostrophic winds. Similar to this are the gradient winds, which occur when winds blow at a constant speed parallel to curved isobars. Gradient winds demonstrate a counterclockwise rotation around low pressure areas and clockwise rotation around high pressure areas. It is common to call low pressure centers cyclones and the flow around them cyclonic. Cyclonic flow has the same direction as the rotation of the earth—counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. High pressure centers are called anticyclones. Troughs and ridges occur when isobars curve to form elongated regions of low pressure and high pressure, respectively. Flow around a trough is cyclonic; flow around a ridge is anticyclonic. Wind is measured in terms of its speed and direction. A device known as an anemometer measures the wind speed. A wind vane will tell you the wind direction. Wind directions are always stated as the direction the wind is coming from. For example, a southerly wind is a wind which is blowing from the south. This is because knowing where the wind is coming from might give you some indication of the type of weather to expect. Wind combined with low temperatures can create the feeling of much colder temperatures. Meteorologists have developed a system to describe how cold it feels with a combination of wind and low temperatures. This is called the wind chill. It is based on how quickly the body loses heat from exposed skin. To read the wind chill chart below, find the current air temperature across the top. Next, find the wind speed along the side. Where these two line up, we have the wind chill. This is a measurement of how cold it feels (in degrees Fahrenheit). For example, if the air temperature is ten degrees Fahrenheit and the wind is blowing at five miles per hour, we look on the chart and see that it feels like it is one degree Fahrenheit outside. (Look for ten across the top to find the temperature and five miles per hour along the side to find the wind speed. Where these two line up, you find one degree Fahrenheit.) Wind Chill

Atmospheric conditions

We know that if air rises it expands and cools, and if it sinks it compresses and warms. Why would air rise on some occasions and sink on others? The answer is related to atmospheric stability. Stability in the air is defined as the tendency to resist vertical movement or the tendency to remain in or return to its original altitude. What might cause air to be unstable? If a parcel of air were warmer than the surrounding air, then it would be less dense (more spread out) and would therefore rise. Conversely, if it were cooler than the environment, then it would be denser and would sink. There are three different conditions of stability: absolute stability, absolute instability, and conditional instability. Absolute stability means that at any altitude, air will remain in its original position, or if forced to rise, it will return to its original position. This happens when the environmental lapse rate is less than the wet rate (ER < WR). The rising air parcel will always be cooler than the surrounding air and will thus be forced to sink back to its original position. A temperature inversion is the most stable condition of air. This occurs when temperatures actually rise with increasing altitude. Thus, any air that is forced to rise will become much colder (by following the dry rate or the wet rate) than the environment, which increases with altitude in this case. Many cities experience inversions during winter months. This is why pollution seems to be trapped when there is an inversion. The warmer air above acts like a lid to trap the air below. Any air that does rise simply gets pushed back down by the warmer air above. Absolute instability occurs when air that is forced to rise slightly suddenly becomes warmer than the environment and continues to rise. This happens when the environmental lapse rate is greater than the dry rate (ER > DR). The rising parcel of air will always be warmer than the environment and will continue to rise. The most common type of stability is conditional instability. This occurs when air is stable up to a certain altitude and then unstable for the rest of the atmosphere. When the environmental lapse rate is between the wet rate and the dry rate, we have conditional instability (WR < ER < DR). Air must be forced to rise some distance before it will become unstable. We know that stable air resists vertical movement and unstable air rises freely, but how does this apply to daily weather? Convective lifting is an example of how stability can impact our daily weather. Instability often occurs on warm summer afternoons as solar heating is intense. Surface irregularities cause pockets of air to be heated more than the surrounding air, thus becoming unstable. These unstable parcels of air will rise. This is the process of convective lifting and is responsible for many afternoon showers or thunderstorms. Instability is enhanced by the following: intense solar heating that warms the air from below (convective lifting) heating of an air mass from below as it passes over a warm surface forceful lifting caused by orographic lifting, frontal wedging, or convergence radiation cooling from the tops of clouds Stability is enhanced by the following: radiation cooling of the earth's surface after sunset cooling of an air mass from below as it passes over a cold surface subsidence of an air column (subsidence is a general downward flow of air) Forceful lifting usually occurs by one of three mechanisms: orographic lifting, frontal wedging, or convergence. Orographic lifting occurs when airflow is blocked by surface features, such as mountains. Elevated terrain acts as a barrier to the flow of air. As air ascends a mountain slope, adiabatic cooling takes place. This often results in cloud formation and significant precipitation. The windward sides of mountain ranges (the side where wind blows towards) are some of the rainiest places on earth. (Wind blowing towards mountains blows on the windward side.) By the time air reaches the leeward side of mountains (the opposite side), much of the moisture has been lost. This explains why many of the world's deserts lie on the leeward side of mountains. These deserts are called rain shadow deserts. The Great Basin Desert of the United States (throughout Arizona, New Mexico, and so on), the Gobi Desert of Mongolia, the Takla Makan of China, and the Patagonia Desert of Argentina are examples of rain shadow deserts found on the leeward side of mountains. Look back at figure 2.4 and you can see why this is the case, especially if you realize that some of the mountain ranges involved are several thousands of meters high! If orographic lifting were the only mechanism that forced air to rise, then most of the United States would be a desert. The flat central plains would have no mountains to cause orographic lifting. Here, the mechanism for forceful lifting is frontal wedging. This occurs when warm air masses interact with cold air masses. The cold air is denser and acts somewhat like a "mountain of air." The warm air will rise above the cold air. Convergence is the third mechanism of forceful lifting. Whenever air flows together, convergence occurs. As moving air comes together, it is forced to rise. In other words, as air converges horizontally, it must move vertically. Try bringing your hands together with palms down. As you push the fingertips of both of your hands together they are forced upward. This is similar to what happens when air masses flow towards each other. The air is forced upward. This leads to instability in the atmosphere

Temperature and altitude

We know that temperatures drop with an increase in altitude in the troposphere. This may not seem to make sense because we also know that warm air rises. The key to understanding this mystery lies in the fact that an adiabatictemperature change takes place. This means that temperature changes without adding or removing heat. Think of what happens to the air you breathe as you move to higher altitudes. The air is "thinner." Now imagine a parcel of air that is forced to rise (without any heat flowing into or out of the air parcel). As it rises, it is now in thinner air so it expands. Expansion cooling occurs. As a gas expands, its temperature drops. When you allow air to escape out of a tire, the air feels cold. The air itself was not cold before, but by causing it to expand rapidly, a cooling effect is noticeable. Temperature is the average kinetic energy of molecules in a given amount of space, so if you suddenly spread them out, there would be fewer molecules in that space, and hence, a lower temperature. Compression heating describes just the opposite. As you compress a gas, its temperature rises. Therefore, adiabatic expansion is responsible for the lower temperatures at higher altitudes.

Solar radiation

All objects absorb light energy from the sun and then re-emit that energy as lower energy infrared light. Most heat is transmitted in the form of infrared rays. This is how infrared scopes for night vision work. They are sensitive to infrared instead of the visible light our eyes can see. Although the atmosphere is transparent to visible light, only about 25 percent of visible light penetrates directly to the surface without interference from the atmosphere; the remainder is affected by reflection, absorption, or scattering. About 51 percent of incoming solar radiation is absorbed at the surface. Reflection is responsible for about 30 percent of incoming solar energy being bounced back into space. This energy is lost to the earth and has no impact on our weather. The fraction of total radiation encountered that is reflected at the surface is called its albedo. The albedo of the earth as a whole would be about 30 percent. Locally, the albedo, or reflectivity of a place, can vary. There are four major factors which affect the albedo on a local scale: The nature of the surface: Fresh snow, for example, will reflect most of the sunlight that reaches it, while the ground underneath would certainly absorb more. Imagine what could happen if the polar ice caps were covered with black soot. They would then absorb instead of reflect incoming solar energy. This would cause them to melt. With no polar ice caps, the albedo of the earth would go down, causing a general rise in global temperatures. Interesting, wouldn't you say? Cloud cover and weather: Thick clouds have an albedo of 70 to 80 percent. Thin clouds have an albedo of only 25 to 50 percent. Particulates in the air: The presence of dust, smoke, and pollution certainly affects how much sunlight is reflected and how much reaches the surface. The angle of the sun: Imagine a sunset over the ocean. At such a low angle, most of the sunlight is reflected off of the surface of the water. At noon, not as much is reflected. Absorption is the second cause for atmospheric interference of incoming solar radiation. Gases are selective absorbers, meaning that they will absorb certain wavelengths of radiation much better than others. When a gas molecule absorbs light waves, this energy is transformed into internal molecular motion, which manifests itself as an increase in temperature. Nitrogen is a very poor absorber of light. Oxygen and ozone are very good absorbers of ultra-violet radiation. The absorption of ultra-violet radiation by ozone accounts for the higher temperatures in the stratosphere. Water vapor and carbon dioxide are good absorbers of infrared radiation. Only about 19 percent of total incoming solar energy is absorbed in the atmosphere. This is not the most important mechanism of heating the atmosphere. Even though light travels in straight lines, the gases and dust particles in the atmosphere can redirect this energy. This process is called scattering. This explains why shaded areas or rooms without direct sunlight may be illuminated. Scattering is also responsible for the diffused light of our daytime sky. This is why the sky is blue. Atmospheric gases are more efficient at scattering blue light than any other color, so we see blue light all over the sky. At sunrise or sunset we may see the sky a reddish-orange color. Scattering also accounts for this. As the sun is rising or setting, it is at a very low angle and sunlight must pass through much more of the atmosphere before reaching an observer. Most of the blue light will be scattered away before it can reach the observer, so only the reds and oranges remain. This reddish-orange light will then illuminate any clouds in the vicinity, creating a reddish-orange sky. The balance of incoming radiation and outgoing radiation is called the heat budget. In order for the temperature of any object to remain the same, the total heat absorbed must equal the total heat radiated. If the earth received one hundred units of solar energy, fifty-one of them would be absorbed at the surface. It loses seven units because of convection and turbulence of the atmosphere near the surface. Twenty-three units would be transported upward to the atmosphere through evaporation. The remaining twenty-one units are radiated as long wave (infrared) radiation. Thus, the earth as a whole maintains a constant temperature. Daily variations of this heat budget explain the rise and fall of temperatures. During the day, incoming radiation exceeds outgoing radiation, so the temperature at the surface rises. At night, the opposite is true and the temperature drops. The same argument applies to latitudinal changes. In the tropics, where sunlight is more intense, incoming sunlight exceeds outgoing radiation. The opposite is true for higher latitudes. Where more radiation is received than given off, temperature must increase.

Lightning

According to the National Oceanic and Atmospheric Administration (NOAA), over the continental forty-eight states, an average of twenty million cloud-to-ground flashes are detected each year. Cloud-to-ground lightning can injure or kill people by direct or indirect means. The lightning current can branch off to a person from a tree, fence, pole, or other tall object. It is not known if all people who are killed are directly struck by the flash itself. In addition, flashes may conduct their current through the ground to a person after the flash strikes a nearby tree, antenna, or other tall object. The current also may travel through power or telephone lines or plumbing pipes to a person who is in contact with an electric appliance, telephone, or plumbing fixture. Similarly, objects can be directly struck, and this impact may result in an explosion, burn, or total destruction. The damage may be indirect when the current passes through or near it. Sometimes, a current may enter a building and transfer through wires or plumbing and damage everything in its path. The best shelter is a substantial building that has plumbing and wiring. A very unsafe building for lightning has only a roof and some supports but no wiring or pipes extending into the ground. A vehicle with a metal roof provides good shelter and is much better than being in the open or in an ungrounded building, but it is not as good as being in a building that is grounded by wires and pipes

Atmospheric gases

Ancient Greeks believed that all matter in the universe was made up of four distinct elements—water, earth (soil), fire, and air. Air was one of these fundamental substances. We often speak of air as if it were a pure substance. In truth, it is not a pure substance. Air is made up of a variety of gases. Nitrogen is the most common gas in the atmosphere, comprising about 78 percent of the air. Oxygen comes in second with about 20.9 percent. Argon is a distant third with 0.9 percent. Other gases account for less than 0.2 percent of the atmosphere. You may have heard about carbon dioxide (CO2) as a gas found in the atmosphere. Only about 0.035 percent of the air is made up of CO2. Carbon dioxide is an excellent absorber of heat radiated by the earth, so it has a profound effect on energy transfers in the atmosphere. For this reason, it plays a role in the earth's weather. Concentrations of CO2 have been slowly increasing over the past century. This is largely caused by the burning of fossil fuels (coal, oil, natural gas). As these fuels burn, CO2 is given off as a by-product. Some scientists believe that our society's increasing dependence on the use of fossil fuels will cause even greater concentrations of CO2 in the atmosphere and cause a warming of the lower atmosphere called the greenhouse effect. This effect is sometimes referred to on a larger scale as global warming. There are scientists who also contend that mankind affects the global climate in other ways that tend to offset any warming effects that CO2 release might have (such as planting trees, or recycling). Besides CO2 there are a variety of other gases that can be found in the atmosphere in small percentages. Ozone, dust particles, and water vapor are just a few of these. Water vapor concentrations may be as high as 4 percent in warm, tropical regions, as opposed to a very small fraction of a percent in desert areas. Water in the atmosphere obviously plays a crucial role in the continued existence of life on Earth. Besides the fact that water vapor is the source of all clouds and precipitation, it has the ability to absorb some solar energy in addition to radiant energy given off by the earth. Water is also the only substance on earth which can exist in all three phases (solid, liquid, and gas) at the temperatures and pressures normally found on the earth.

Global circulation

As early as 1735, George Hadley recognized that the driving force for the winds was solar energy. He proposed that the extreme temperature difference between the equator and the poles would create a large convection cell in each hemisphere. The more intensely heated air at the equator would rise and move poleward. When this air reached the poles, it would sink and spread out back toward the equator. Hadley had the right idea, but because of the Coriolis effect, there are actually three wind belts in each hemisphere. These are the trade winds, prevailing westerlies, and polar easterlies. The flow toward the equator is deflected by the Coriolis effect and forms the reliable trade winds. These winds may be found between the equator and 30º N or S. In the Northern Hemisphere, the trade winds flow from the northeast, where they provided the sail power for explorers from Europe to the New World. In the Southern Hemisphere, the trade winds come from the southeast. The trade winds from both hemispheres meet near the equator where the doldrums are found. The doldrums are a region where winds are light and weather is monotonous. Between 30º and 60º latitude are the prevailing westerlies. The surface flow is toward the poles, but the Coriolis effect causes it to be deflected. This creates a westerly flow of air. From 60º to 90º latitude, we find the polar easterlies. Subsidence at the poles creates a surface flow toward the equator. The Coriolis effect causes this to be deflected into an easterly wind. These cold, easterly winds eventually meet the warmer flow of the westerlies at the middle latitudes. The region where these conflicting winds clash is called the polar front. Another effect of the three-cell global circulation is the creation of four distinct pressure zones. In regions of rising air, there is generally low pressure. In regions of sinking air, high pressure is dominant. Thus, the ideal pressure zones are the following: Equatorial low—rising air near the equator (0º latitude) Subtropical high—sinking air around 30º latitude Subpolar low—rising air around 60º latitude Polar high—sinking air around the poles (90º latitude) El Niño (the Spanish word for "the child (masculine)") refers to a warm current appearing in the Pacific Ocean. Every three to seven years, an El Niño event may last for many months, having significant economic and atmospheric consequences worldwide. The warm ocean affects global weather as well as the fish population. Some areas that rely on the fishing industry see drastic reductions in their catch during El Niño. For example, off the coast of Peru, the ocean is usually cool and full of nutrients for local fish populations. As the warmer, nutrient-poor waters from El Niño come in, fish have less to feed on and many die.

Forms of precipitation

As mentioned previously, many forms of precipitation begin as snow crystals, which fall, possibly melting along the way down. There are seven different forms of major precipitation. They are sleet, freezing rain (glaze), hail, rime, graupel, rain, and snow. Sleet: This is snow crystals that pass through a layer of warm air that melts them, after which they fall through a layer of freezing air. Think of them as "frozen raindrops." Freezing rain: Also known as glaze, this is rain that falls as supercooled liquid onto a surface that is below the freezing point. The surface is coated with a thin layer of ice as the rain hits and freezes. Hail: These are hard, rounded pellets or irregular lumps of ice. They form in cumulonimbus clouds where updrafts are strong enough to overcome the pull of gravity on raindrops. This pushes them into sub-freezing layers of the cloud where they freeze and collect more ice. Rime: These delicate "ice feathers" form into the wind on surfaces as fog passes through a sub-freezing region. Graupel: Commonly called "soft hail," graupel forms as rime collects on snow crystals. Rain: Rain is generally produced by nimbostratus or cumulonimbus clouds that are often the result of snow at higher altitudes, which passes through a warm layer of air and melts. Snow: The crystalline structure of ice allows snowflakes to have six sides, though there is an indefinite amount of variability possible (no two snowflakes are identical).

Fog

Fog could be described as being a cloud on the ground. Different types of fog are distinguished based upon the way in which they form. There are two major processes that form fog: cooling and evaporation. The major difference between evaporation fogs and cooling fogs is the mechanism for reaching saturation. Cooling fogs simply reduce the capacity of air to hold water by lowering temperature, while evaporation fogs actually increase the water content of the air. Fogs formed by cooling include radiation fog, advection fog, and upslope fog. Radiation fog results from the radiation cooling of the earth's surface. During the night, there is no incoming radiation to warm the surface, so outgoing radiation causes the temperature to drop. This in turn causes the temperature of air near the surface to drop. When the air temperature drops below the dewpoint, fog forms. Radiation fog is thickest in valleys because the air containing the fog is generally cool and dense. Advection fog occurs when warm, moist air passes over a cold surface. It becomes colder by contacting and mixing with cooler air. If cooling is sufficient, a blanket of fog forms. This type of fog forms best when there are winds of ten to twenty miles per hour to "mix things up." Advection fog is usually thick and persistent. Upslope fog is created when relatively warm, moist air moves up a gentle slope. Recall that as air rises, it cools adiabatically. This cooling may be sufficient to generate fog. Evaporation is the other category of fog formation. Steam fog occurs when cool air moves over warm water. As the rising water vapor meets the cold air, it immediately condenses and rises with air that is being warmed from below. This is the type of fog that occurs when you take a bath or shower on a cool day. Frontal fog, or precipitation fog, forms when warm air is lifted over colder air, clouds with rain form, and the cold air below is near the dewpoint. Enough rain evaporates to reach saturation and fog forms.

Man's attempt to control damaging effects of weather

For centuries, man has been at the mercy of the earth's dynamic weather systems. With increasing knowledge and technology, we have tried to control some of the damaging effects of weather. Cloud seeding is one example of how we have made this attempt. Fog creates problems of visibility for travelers both in the air and on the ground. One of the applications of cloud seeding is the fact that by over-seeding a cloud, you can virtually wipe it out. Unfortunately, this will only work if the cloud contains supercooled droplets. Not much fog contains supercooled droplets, though. One of the ways that we have attempted fog dispersal is through the mixing of drier air with the moist, foggy air. Another method has been to try to add heat to the fog, thus evaporating it. The Orly Airport in Paris has a system that consists of eight jet engines located at various places along the runway. This system has been shown to improve visibility for nearly a kilometer. Another weather-related problem is the damage done to property and crops by hail. Hail suppression efforts can be traced all the way back to the early Greek civilization. Scientists believe that hail forms by the collection and freezing of supercooled droplets around a nucleus. These are forced upward in a cloud by strong updrafts and collect more and more layers of ice until they are too heavy and fall. There are usually only a relatively small number of "hail embryos" that form because freezing nuclei are scarce. Modern attempts at hail suppression have been based on the idea that seeding storm clouds would interrupt the formation of hailstones. This has unfortunately not been met with much success. Frost prevention is a major agricultural concern. Fruit growers understand the damage that frost can do to their crops. Methods of frost prevention include high volume sprinklers and wind machines. The sprinklers continuously cover the outside of plants with water until the outside temperature is above 0°C. As the cold air moves through, this water freezes and releases latent heat. This latent heat keeps the crops above the freezing level. Wind machines work when air aloft is warmer. They simply mix the warmer air with the cooler air below. Air is often cooler right at the surface during the night because the surface radiates heat away and cools quickly.

Heat vs. temperature

Heat and temperature are often treated as basically the same thing. This is surely not the case. Heat is a form of energy. As you add heat to a substance, its temperature will rise. So temperature could be described as a degree of hotness. A teaspoon of boiling water will have the same temperature as five gallons of boiling water, but the five gallons of water contains much more energy than the teaspoon. A cup of boiling water certainly has a higher temperature than a tub full of warm water, yet the cup does not have more heat energy. The quantity of heat depends on the mass of an object. The temperature does not. Temperature measures the average kinetic energy of the molecules of a substance. It does not matter if there are five or five billion molecules. Heat, on the other hand, does take into account the number of molecules. The corona of the sun (the visible, outer layer), for example, is about two million degrees, but there is more heat in an ice cube of the same volume because there are so few molecules in the corona. All forms of matter are composed of atoms or molecules, which are in constant motion. Because of this motion, all matter is said to have thermal energy. Whenever a substance is heated, its atoms move faster and faster. This results in an increase in thermal energy. It is the average motion of the atoms or molecules that we sense when we determine how hot or cold it is. We often describe this as the temperature. Temperature is really a measure of the average motion of atoms or molecules in a substance.

Persistence forecasting

Humans have never been very good at predicting the weather. Usually those predictions were just our "best guess" of what the weather would bring. But there are patterns which we can use to forecast the weather. Here is an example: What kind of weather would you predict for January of next year? Most people would predict that it will be very much like January of this year. People make this prediction because they know that January weather is about the same year after year. Meteorologists would say there is persistence in the weather for any given month of the year. Of course January in Argentina is very different from January in Canada so persistence is really only valid for locations in the Northern Hemisphere or for locations in the Southern Hemisphere. We cannot claim persistence across the whole globe. This type of weather prediction is what scientists call persistence forecasting. Humans do persistence forecasting naturally. Here is another example for springtime weather in Washington D.C. Based on today's weather data in the left column, what would you predict tomorrow will be like? Since the weather data today doesn't show any indication of change, one would likely predict that the pattern should persist until tomorrow. We should expect to have calm winds, clear skies, and temps about 70 degrees - a nice spring day. Based on the data we predict the weather will persist without much change. Humans have some great weather data sensors built right into our bodies. We can feel the wind. Detect temperature and see the movement of clouds. We also have powerful brains to process the data and see patterns. One thing we cannot sense is the air pressure. The first barometers were simple tubes filled with mercury. They were scientific devices used to measure blood pressure and pressure in chemistry experiments. But soon a pattern was noticed. When the mercury in the barometer went down, storms would often follow. When the mercury rose, the skies would clear. They realized that changes in air pressure could help predict the weather. If a barometer is steady it means the conditions will likely stay the same. If the barometer is falling it means storms are on their way. If the barometer is rising it means clear and sunny skies ahead. So our senses and weather instruments help us with persistence forecasting.

Cloud seeding

In 1946, Vincent J. Schaefer discovered that if you dropped dry ice into a cloud of supercooled droplets, ice crystals formed quickly, thus speeding up the precipitation process. This is called cloud seeding. Shortly after this discovery, it was learned that silver iodide could also be used for cloud seeding. This is because of the similarity between the crystal structure of silver iodide and ice. Silver iodide has a big advantage in that it may be supplied from airplanes and from burners on the ground. In either case, two conditions must be present to be successful: one, clouds must be present; and two, at least the top portion of the cloud must be supercooled. Two basic strategies of seeding exist. Static seeding is based on the assumption that cumulus clouds do not have enough freezing nuclei to form the ice crystals necessary for precipitation. The object is to produce just the right amount of ice crystals. Dynamic seeding, on the other hand, uses huge quantities of seeding in order to rapidly convert supercooled liquid droplets to ice during the growth phase of a cloud

Weather analysis and forecasting

In order to attempt to forecast weather, it is necessary to know and understand the atmospheric conditions. For this, a synoptic weather chart is used. Synoptic means coincident in time. Thus, a synoptic weather chart shows the weather conditions for the same time across a large area. This information usually includes temperature, dewpoint, air pressure, cloud cover, wind speed and direction, and both current and past weather. An abbreviated weather station model is shown for various weather reporting stations. Once data are plotted, isobars and fronts are drawn on the maps. Fronts can be identified fairly easily because they represent boundaries between contrasting air masses. Four good indicators of fronts are: sharp temperature change over a short distance change of wind direction over a short distance (as much as 90 degrees) humidity variations by observing changes in dewpoint over short distance clouds and precipitation patterns Sharp temperature change over a short distance, change of wind direction over a short distance as much as 90 degrees, humidity variations by observing changes in dew point over short distance, clouds and precipitation patterns Persistence assumes that the weather patterns occurring upstream that are headed your way will produce the same type of weather that it is producing right now.

The seasons

Our planet rotates around an imaginary axis, which is tilted 23.5 degrees from the plane of its orbit around the sun. This means that the sun will be directly overhead at different locations throughout the year. Direct, head-on sunlight allows the most energy per unit area to reach the surface. The greater the angle of incoming light, the more spread out the energy will be. For this reason, the earth has four seasons. The sun is directly above the equator on the vernal equinox (approximately March 21). As the earth revolves around the sun, its axis does not change. Thus, on the summer solstice (approximately June 21), the sun is directly above the Tropic of Cancer (23.5 degrees N latitude). This is the farthest north that the sun can be found straight above the ground. On the autumnal equinox (approximately September 23), the sun is once again above the equator. Finally, on the winter solstice (approximately December 21), the sun is above the Tropic of Capricorn (23.5 degrees S latitude). In figure 1.6, notice how a beam of light striking from directly above concentrates the energy into a small area, while the same beam striking from an angle will spread the energy out over a larger area. Sunlight hits most directly above the equator on the two equinoxes, most directly above the Tropic of Cancer on the summer solstice, and most directly above the Tropic of Capricorn on the winter solstice. The Northern Hemisphere enjoys summer while the sun's rays are hitting it most directly. Solar energy is more concentrated in the Northern Hemisphere between the vernal equinox and the autumnal equinox. Conversely, the Southern Hemisphere receives more concentrated solar energy between the autumnal equinox and the vernal equinox. For this reason, winter in the Northern Hemisphere is at the same time as summer in the Southern Hemisphere. Aside from the fact that solar energy is less concentrated when spread out, another reason that winters are so much colder than summers is the fact that solar energy coming to the earth at more of an angle must pass through more of the atmosphere. This allows more of the solar radiation to be reflected and absorbed before reaching the surface. Thus, solar energy is even less concentrated than it would otherwise have been. Calculating the noon sun angle for any location is relatively simple. Recall that on any given day, only one latitude receives vertical rays of the sun (90 degrees). A place located one degree north or south of this latitude will receive sunlight at 89 degrees. Two degrees away will receive sunlight at 88 degrees and so on. To calculate the noon sun angle, find the number of degrees of latitude separating the desired location from the location of the vertical sun, then subtract that value from 90 degrees. It is interesting to note that the sun will never be directly overhead many locations throughout the world. This is because the furthest north that the sun shines from directly overhead is the Tropic of Cancer (23.5 degrees N latitude) on the summer solstice and the furthest south is the Tropic of Capricorn (23.5 degrees S latitude) on the winter solstice.

Measurement of precipitation

Precipitation is vital to life on Earth. As such, we have developed various ways to measure precipitation. There are three major devices or gauges used to measure precipitation. The standard rain gauge is simply a container left open at the top to collect precipitation as it falls. It is usually collected and funneled into a narrower container with an appropriate magnifying factor taken into account to obtain a more accurate measurement. A second measuring device is called a tipping bucket gauge. This device has two compartments at the base of a funnel. When one of the buckets fills with water, it tips over and dumps the water out. Each time the bucket tips, an electric circuit is tripped and the amount of precipitation is automatically recorded on a graph or computer. The third device is called a weighing gauge and is similar to the standard rain gauge, except that its weight is measured instead of the water depth. This weight is recorded on a graph automatically. When snow measurements are kept, two measurements are taken: depth and water equivalent. Depth measurement is relatively simple but is usually done in various locations so as to obtain a good average and nullify any drifting effects. To obtain the water equivalent, samples are taken and melted. They can then be measured as rain. Water content in snow can vary greatly. A good rule of thumb is ten centimeters of snow equals one centimeter of water. (In reality it can be as much as thirty centimeters of dry, powdery snow to one centimeter of water or as little as four centimeters of wet snow to one centimeter of water.)

Heat transfer

Temperature is not the same thing as heat. Heat is the energy that flows because of differences of temperature. For example, when you touch a hot pan, heat is transferred from the pan to your fingers. Also, when you touch an ice cube, heat flows from your hand to the ice cube. Thus, if two objects of different temperatures come into contact, the warmer object will become cooler and the cooler object will become warmer until they reach the same temperature. Thus, heat is transferred. There are three mechanisms of heat transfer: conduction, convection, and radiation. Energy radiates from the fire, making the metal hot to the touch (conduction). Water in the bottom of the pan heats by conduction. Then it rises as cooler water sinks, creating convection currents. Conduction is very familiar to anyone who has tried to grasp a metal spoon that has been left in a hot pan. Heat is directly transferred through the atoms or molecules of the metal. The vibration of molecules forces the adjacent molecules to vibrate more, and the next ones and so on. The ability of substances to conduct heat varies. Metals are good conductors. Air, however, is a very poor conductor. Consequently, conduction is only important between the earth's surface and the air directly in contact with the surface. As a means of heat transfer for the atmosphere as a whole, conduction is the least significant. Convection is the transfer of heat because of the movement of mass from one location to another. It takes place only in liquids and gases. Heat gained by either conduction or radiation by the lowest layer of the atmosphere is most often transferred by convection. The statement "warm air rises and cool air sinks" is an example of convection. Radiation is the third and probably most important mechanism of heat transfer. The sun is the source of nearly all energy on the earth. This solar energy is transmitted through space by means of radiation. Radiation, or electromagnetic radiation, is how energy from the sun is transmitted to the earth. This is the only type of wave that requires no medium and can travel through the vacuum of space. In fact, they travel at nearly 300,000 kilometers per second (186,000 miles per second)! Energy from the sun is more than just visible light. There is an entire spectrum of electromagnetic radiation. Electromagnetic radiation waves, from lowest to highest energy, are radio waves, microwaves, infrared, visible light, ultra-violet, x-rays, and gamma rays.

Layers of atmosphere

The atmosphere has four distinct layers: the troposphere, stratosphere, mesosphere, and thermosphere. Each layer is defined by how the temperature changes as the altitude increases. The boundaries between layers are given the names tropopause, stratopause, and mesopause. The lowest layer is the troposphere. As altitude increases, air temperature drops. You may have experienced this simply by going up into some mountains. As you arrived at higher elevations, the air was colder. The temperature decrease in the troposphere is called the environmental lapse rate. This averages to about 6.5ºC every thousand meters, though it varies at any given location. Occasionally, pockets of air may be found in the troposphere where temperature actually increases with an increase in altitude. This is known as a temperature inversion. The troposphere has an average thickness of about twelve kilometers. It is somewhat thicker at equatorial regions and thinner at the poles. The troposphere is the layer of the atmosphere that interests meteorologists the most. This is the layer in which almost all weather phenomena occur. For this reason, it has sometimes been called the "weather sphere." Some areas of the world experience temperature inversions during the winter months. During these inversions, air near the ground is very cold when compared to air at higher altitudes. This has the effect of creating a kind of "lid" to trap air in valleys. Air that warms near the ground rises and encounters air above that is warmer. Since the air above is warmer, the rising air will sink again because it is cooler than the air above. This causes problems like pollution that builds up until storms move through the area to clean up the air. Beyond the troposphere lies the stratosphere. Temperatures remain rather constant for several kilometers before sharply rising from altitudes of about twenty to fifty kilometers. Higher temperatures occur in the stratosphere because it is in this layer that the bulk of the atmosphere's ozone may be found. Ozone is comprised of three oxygen atoms joined into a single molecule. Ozone absorbs potentially harmful ultra-violet radiation from the sun. This splits the ozone molecule. Temperatures rise because solar energy is absorbed then released in the form of heat. The maximum concentration of ozone is found from fifteen to thirty km above the earth's surface. In the mesosphere, temperatures drop again with altitude until an altitude of eighty kilometers. Temperatures here reach as low as -90ºC. This is the coldest temperature reached in the atmosphere. Extending upward from the mesopause and having no clearly defined upper boundary is the thermosphere. Temperatures again rise with altitude. It is here that the highest temperatures are reached. It reaches temperatures exceeding 1000ºC!

Lapse rate

The drop in temperature as air rises can be measured by observing how much it drops for some rise in altitude. This is called a lapse rate. There are four main lapse rates that describe the rate of cooling as one increases in elevation. The first is the dry adiabatic lapse rate (dry rate or DR). This describes the rate of cooling of a rising parcel of air as long as the dewpoint (the temperature at which the air is saturated) is lower than the air temperature. The dry rate is 10ºC per 1000 meters or 1ºC per 100 meters. It does not change. The dewpoint lapse rate describes the rate of cooling of the dewpoint and is 2ºC per 1000 meters or 0.2ºC per 100 meters. This is constant as long as the dewpoint is lower than the temperature of the air parcel as well. Eventually, however, since the dewpoint is dropping more slowly than the air parcel temperature, the two will be equal at a certain altitude. This altitude is called the lifting condensation level (LCL). At this point, water begins to condense from the air because the air is at the same temperature as the dewpoint. Recall that condensation releases heat into the environment. This latent heat of condensation causes the air parcel to cool more slowly (from 5-9ºC per 1000 meters). This is the wet adiabatic lapse rate (wet rate or WR). Above the LCL, both the rising air parcel and the dewpoint will follow the wet rate. In figure 2.4 you can see an example of how these different lapse rates affect rising parcels of air. So far we have discussed lapse rates for rising air only. The environmental lapse rate (ER) is the rate of cooling of the environment and is usually about 6.5ºC per 1000 meters. However, the environmental lapse rate can range anywhere from 5-12ºC per 1000 meters depending on conditions. In figure 2.3, notice that the parcel of air begins at the lower left with air temperature of 26ºC and a dewpoint of 21.2ºC. As it rises, its temperature drops at the dry lapse rate of 1ºC per 100m. (This is equal to 2ºC per 200m!) The dewpoint drops at 0.2ºC per 100m. Note that at an altitude of 800m, the air temperature is equal to the dewpoint. Clouds form at this elevation, called the lifting condensation level. As the air continues to rise, both the air temperature and the dewpoint change at the wet lapse rate (0.5ºC per 100m in this case). As the air descends down the other side of the mountain, the air follows the dry lapse rate, and the dewpoint follows the dewpoint lapse rate. It is interesting to notice that the air on this side of the mountain is warmer, and the dewpoint is lower than it was at the same elevation on the left side of the mountain. This phenomenon is often responsible for what are called rain shadow deserts. The air on the leeward side of a mountain range is often warmer and drier because of this.

Defining features

The features that define a thunderstorm are lightning and thunder. Lightning occurs when electricity travels between areas of opposite electrical charge within a cloud, between clouds, or from a cloud to the ground. It generally follows a process of formation. First, charges separate within a cloud. Air is not a very good conductor of electricity; but with a huge negative charge built up in a cloud, some of these negative charges ionize air molecules (put an electric charge on them), forming a narrow conductive path. This is called the step leader. The electrons near the surface are pulled downward as the path is completed. As they do, electrons positioned higher begin to flow downward as well. This is called the return stroke because the path of electron flow is continually being built upward. The return stroke is the bright flash of lightning that we are able to see. The first stroke is usually followed by several other strokes in order to drain the electric charge from the clouds. Each subsequent stroke begins with a dart leader, which re-ionizes the path. If the electric current has stopped for more than one-tenth of a second between strokes, a new step leader has to form a new conductive path. The rapid electrical discharge of lightning heats the air and causes it to expand explosively. This expansion produces the sound waves that we hear as thunder. We often hear a rumbling sound with thunder. This is caused when sound waves from various parts of the lightning bolt reach our ears at different times. It happens because the speed of sound is relatively slow. There is usually a delay between the time we see a flash of lightning and when we hear the thunder. This is because light travels much faster than sound. For our purposes, we can assume that we see lightning at the exact instant it really occurs. Sound travels about one mile in five seconds. Thus, if we see lightning and hear the thunder five seconds later, then the lightning struck one mile away. Each second of delay time represents 0.2 miles.

Thunderstorms

The most common type of severe weather is the thunderstorm. We will be discussing various aspects of thunderstorms as well as other types of severe weather. Thunderstorms are small compared to hurricanes or winter storms. A typical thunderstorm is only fifteen miles across and lasts only thirty minutes. Even the biggest thunderstorms are usually less than twenty-five miles across and will not last longer than a few hours (USA Today 1995 Weather Almanac, p.103). The National Weather Service estimates that one hundred thousand thunderstorms hit the U.S. each year. About ten thousand of these are classified as severe (three-fourths-inch or larger hail, tornado, or winds above 58 mph). Thunderstorms are most common in the spring and summer months but can occur during the fall or winter.

Tornado warning sysfem

The tornado warning system relies heavily on people actually seeing a tornado and then calling authorities with this information. This is prone to many mistakes and errors. People have called in when harmless clouds appear in the shape of funnel clouds. Many tornadoes go unnoticed at night until it is too late. So on the one hand there are unnecessary warnings, and on the other, insufficient warning. Using weather radar, tornadoes often show up as a hook-shaped echo, but this unfortunately does not help in all cases. With new Doppler Radar, scientists have been able to predict the touchdown of a tornado about twenty-one minutes early as opposed to about two minutes before touchdown by visual means. When conditions are present that can cause tornados the National Weather Service will put in a tornado watch, tornado warnings are where the national weather service has seen a tornado in the area.

Westerlies and the jet stream

The upper troposphere at nearly all latitudes is dominated by a westerly flow of air. This westerly flow aloft is largely responsible for the flow of weather systems. Why do we have these westerly winds? Think about the density of a column of air. Cold air is less dense than warm air. This means that air pressure decreases more rapidly at the poles than at the equator because of the warm air over the equator and the cold air over the poles. Thus, at altitudes above the surface, air pressure is higher in tropical regions than in polar regions. This creates a pressure gradient toward the poles. The Coriolis effect then balances this gradient force to create geostrophic winds flowing from a westerly direction. Within the westerlies are narrow ribbons of high speed winds that are called jet streams. The best known jet streams are found in the middle latitudes between elevations of 7,500 and 12,000 meters. Wind speeds here are often in excess of 200 kilometers per hour. The bigger the difference in temperature at the surface, the greater the pressure gradient will be, and hence, the stronger the jet stream winds will be. For example, in winter, it is not unusual for Florida to be quite warm while just a few hundred kilometers to the north it could be below freezing. Large contrasts of temperature result in very fast jet streams. The jet streams were first discovered during World War II when bombers on their way to Japan made little headway before exhausting a lot of fuel. On the return trip, they traveled extremely fast using very little fuel. Even today, aircrafts use the jet stream to their advantage when traveling eastward. All upper-air westerly flows have been observed to follow wavy paths with long wavelengths. The "waviness" of the flow experiences seasonal variations. It is generally flatter in the summertime and wavier in the wintertime. The waviness of the polar jet stream affects surface temperatures. The colder air to the north of the polar jet stream can bring temperatures below freezing as far south as Georgia or Florida, and the warmer air south of the polar jet stream brings clear, warm weather to Utah or Oregon. It all depends on how "wavy" the polar jet stream is.

Clouds

There are three main types of clouds: cirrus, stratus, and cumulus. Cirrus clouds are found only at very high altitudes and appear wispy or thin and white. Stratus clouds are layered clouds (in Latin, strata means "layer"). These are usually middle level clouds, but certain conditions allow stratus clouds to reach low altitudes. Cumulus clouds are "cotton ball" clouds. They usually develop at lower altitudes but may then develop vertically to form cumulonimbus clouds. They are sometimes referred to as clouds of vertical development. Recall that condensation occurs when water vapor changes into liquid water. This can result in rain, dew, fog, or clouds, among other possibilities. Even though the various types of condensation may be different, they all share two common properties: the air must be saturated, and there must be a surface on which the water will condense. If condensation occurs above the ground, tiny particles known as condensation nuclei serve as the surfaces. Examples of condensation nuclei include particles of dust, smoke, pollution, or salt, just to name a few. Without condensation nuclei, air would have to reach relative humidities much higher than one hundred percent in order for clouds to form. The most effective condensation nuclei are hygroscopic, which means they absorb water. Once condensation begins, it progresses rapidly, forming very small cloud droplets. The available water vapor in the air is quickly consumed by the billions of tiny cloud droplets. They are so small that they remain suspended in the air. The following images show common formations of the three main types of clouds in the sky. Note the distinct differences in each, and how each affects water vapor content.

Three stages of thunderstorms

There are three stages to the development of a thunderstorm. They are the cumulus stage, mature stage, and dissipating stage. The cumulus stage is marked by rapid growth. Unstable air is forced to rise quickly, resulting in strong updrafts or upward winds. Cumulus clouds develop into cumulonimbus clouds. Moisture in the air condenses as the lifting condensation level is reached. This causes latent heat of condensation to be released, causing further instability and upward movement of the air.

Temperature scales

There are three temperature scales in common use today. The first was developed by Gabriel Fahrenheit in 1714. He assigned zero to the lowest temperature he could get by mixing salt with ice. Human body temperature was originally assigned a value of twelve, but he later divided these twelve into 98.6 degrees. He found that pure water froze at 32 degrees and boiled at 212 degrees. The Fahrenheit scale is used today only in the United States. The second temperature scale was developed in 1742 by a Swedish astronomer named Andres Celsius. He originally called this scale the centigrade scale because he based it upon 100 degrees between the freezing and boiling points of water. Water would freeze at zero and boil at 100º centigrade. Today we call this the Celsius scale. It is used in nearly every other country in the world as part of the metric system. Lord Kelvin of England developed the third temperature scale based upon the expansion and contraction of gases. He observed that for every 1ºC change, volumes would change by 1/273. He theorized that at -273ºC, gas would have zero volume! This temperature is called absolute zero, and it is the temperature at which all molecular motion would stop. Lord Kelvin based his temperature scale on this absolute zero. In fact, absolute zero is the coldest temperature possible. There cannot be any temperatures below this; therefore, there are no negative temperatures in the Kelvin scale. On this scale, water freezes at 273 K and boils at 373 K. Thus, the increments, or degrees, are equivalent to those of the Celsius scale but merely start with zero at a different location.

Tornado safety

Tornado Safety: What You Can Do Before the Storm: Develop a plan for you and your family for home, work, school and when outdoors. Have frequent drills. Know the county/parish in which you live, and keep a highway map nearby to follow storm movement from weather bulletins. Have an NOAA Weather Radio with a warning alarm tone and battery back-up to receive warnings. Listen to radio and television for information. If planning a trip outdoors, listen to the latest forecasts and take necessary action if threatening weather is possible. If a Warning is issued or if threatening weather approaches: In a home or building, move to a pre-designated shelter, such as a basement. If an underground shelter is not available, move to an interior room or hallway on the lowest floor and get under a sturdy piece of furniture. Stay away from windows. Get out of automobiles. Do not try to outrun a tornado in your car; instead, leave it immediately. Mobile homes, even if tied down, offer little protection from tornadoes and should be abandoned. Occasionally, tornadoes develop so rapidly that advance warning is not possible. Remain alert for signs of an approaching tornado. Flying debris from tornadoes causes most deaths and injuries.

Tornados

Tornadoes are often called "twisters" or "cyclones." They are violent windstorms that take the form of a rotating column of air (called a vortex) that extends downward from a cumulonimbus cloud. A tornado that extends below a cloud but does not touch the ground is called a funnel cloud. Pressure within a tornado can be ten percent lower than outside. Air is pulled in from all directions. This air spirals upward until it combines with the general airflow of the parent thunderstorm. Because of the large pressure drop, air that is sucked in expands and cools adiabatically, which often results in condensation. This condensation, or dust and debris, makes the funnel visible.

Severe thunderstorms

Tornadoes form in association with severe thunderstorms. Fortunately, less than one percent of thunderstorms produce tornadoes. Tornadoes are most commonly found with the cold fronts or squall lines of mid-latitude cyclones. During the spring, air masses found in mid-latitude cyclones are likely to have the greatest contrast. Continental polar air will still be quite frigid, while maritime tropical air will be warm, humid, and unstable. The greater the contrast, the more intense the storm. The central United States records the most tornadoes of any location in the world because of the likelihood that the two contrasting air masses will meet here. An average of 770 tornadoes are reported each year in the U.S. The average tornado travels across the land at about 45 kilometers per hour from the southwest toward the northeast and cuts a path about 26 kilometers long. About half of the tornadoes reported in the U.S. are short-lived and have lifetimes of three minutes or less and paths that rarely exceed one kilometer in length and one hundred meters wide. A guide to the strength of a tornado was developed by T. Theodore Fujita from the University of Chicago. It is called the Fujita Intensity Scale or the F-scale. Because tornado winds cannot be measured directly, a rating on the F-scale is determined by looking at the worst damage done by a tornado.

Pressure gradient

Tornadoes represent the steepest pressure gradient found on earth and result in the strongest winds. Wind speeds can reach 480 km/h (300 mph). This number is an estimate based on observations of tornadoes captured on movies or video. Tornadoes are extremely localized phenomena, usually no larger than six hundred meters in diameter. Because of this, it is difficult to obtain accurate weather data from a tornado. Even in the region most frequently visited by tornadoes, it would be unlikely to capture a tornado passing by any one specific point. The probability would be only once in two hundred and fifty years that a tornado would pass right over an observing point.

Unique properties of water

Water is a substance with the rare quality of being able to exist in all three phases of matter on the surface of the earth. Solid water is called ice. Ice can be found across many parts of the earth in cooler regions, atop mountains, and during winter months. Liquid water can be found in rivers, lakes, streams, and oceans. Gaseous water is usually called water vapor. Water vapor in the form of steam occurs whenever you boil water. However, liquid water left out will become water vapor through a process called evaporation. There are special names given to the processes of changing from one state of matter to another. Whenever a substance changes from one state to another, heat must be transferred to or from the environment. This hidden heat from the change of states of matter is called latent heat. When a substance, such as water, changes from a solid to a liquid or a liquid to a gas, it must absorb heat energy to do so. Think of the process involved in ice becoming liquid water. We call this process melting. In order to melt ice, you must allow the ice to absorb heat. As it does, it will change from a solid to a liquid. The same is true for evaporation, the changing from a liquid to a gas. Heat must be absorbed. A third type of change that requires heat to be absorbed is the change from a solid to a gas. This is called sublimation. This process is not very common to observe directly, but if you have ever seen dry ice (solid CO2), you have seen vapors forming directly from the solid. This is an example of sublimation. Deposition is just the opposite, a phase transfer directly from a gas to a solid. So we see that as substances change from a solid to a liquid to a gas, heat must be absorbed. The reverse is true when changing from a gas to a liquid to a solid. Heat must be given off when substances change this way. See the following table and figure.

The water cycle

Water is such a simple molecule—two hydrogen atoms bonded to a single oxygen atom—yet this simple product of nature is so vital to life on earth. Naturally, scientists are very concerned about the earth's water resources. Water continuously circulates through air, ocean, and ground in what has been called the hydrologic cycle, or water cycle. Let's begin at the largest source of water—the ocean. Water evaporates from the ocean and then enters the atmosphere as water vapor. This vapor is condensed when clouds form and falls over land as rain. This water then flows in rivers back to the ocean, joins groundwater, or evaporates back into the air. Nature has a way of balancing itself. For example, studies have shown that continents receive much more precipitation than is lost to evaporation. Conversely, evaporation is greater over the oceans than precipitation. Since ocean levels do not show a significant change, runoff must equal the deficit of precipitation over the oceans. Thus, the hydrologic cycle, or water cycle, illustrates the movement of water from ocean to atmosphere, from atmosphere to land, and from land back to the ocean. The earth will never run out of water. It is continuously being cycled through this process. The water molecules you drink today may be some of the same ones that Julius Caesar drank two thousand years ago! Some people have expressed concern about the earth's water resources. We will never run out of water, but the vast majority of the earth's water is found in the oceans as saltwater. If we pollute our fresh water sources, then there may not be enough fresh water for us to use. Drought is also a concern that many people have. If a lack of fresh water occurs locally because of drought, it would be very expensive to transport enough fresh water to meet everyone's needs. Even though there may be a lack of water because of drought, what that really means is that there is a lack of inexpensive water.