Meteorology Chapter 4

Why are human hairs often used in a hair hygrometer?

Because a strand of human hair is known to increase by 2.5% as the relative humidity increases from 0% to 100%.

Humidity

- Refers to any one of a number of ways of specifying the amount of water vapor in the air.

Brief Review Points

- Relative humidity tells us how close the air is to being saturated - Relative humidity can change when the air's water-vapor content changes, or when the air temperature changes. - With a constant amount of water vapor, cooling the air raises the relative humidity and warming the air lowers it. - The dew-point temperature is a good indicator of the air's water vapor content; High dew points indicate high water vapor content and low dew points indicate low water vapor content. - Where the air temperature and dew-point are close together, the relative humidity is high; when they are far apart, the relative humidity is low. - Dry air can have a high relative humidity when the air is very cold and the air temperature and dew point are close together.

dew-point

- Represents the temperature to which air would have to be cooled (with no change in air pressure or moisture content) for saturation to occur. - Determined with respect to a flat surface of water. - Good indicator of the air's actual water vapor content. High dew points indicate high water vapor content; low dew points, low water vapor content - Addition of water vapor to the air increases the dew point; removing water vapor lowers it. - When the air temperature and dew point are equal, the air is saturated and the relative humidity is 100%

Absolute Humidity

- The mass of water vapor in a given volume of air - Mass of water vapor/volume of air - represents the water vapor density (mass/volume) in the measured parcel and normally is expressed as grams of water vapor in a cubic meter of air (g/m^3) - A rising or descending parcel of air will experience a change in its volume because of the changes in surrounding air pressure. Consequently, when a volume of air fluctuates, the absolute humidity changes--even when the air's vapor content has remained constant. For this reason the absolute humidity is not commonly used in atmospheric studies.

Describe how a sling psychrometer works. What does it measure? Does it give you dew point and relative humidity? Explain.

A psychrometer consists of two liquid-in-glass thermometers mounted side by side. One of the thermometers has a moist cloth placed over its bulb. When air is passed over the thermometer bulbs (by twirling or "slinging" it around), water evaporates from the moistened cloth. The latent heat needed for this evaporation comes partially from the thermometer bulb, thus reducing the wet-bulb thermometer's temperature. The wet bulb temperature measured by the psychrometer is similar to, but not exactly the same as the dew point temperature. Wet bulb temperature is the wiliest temperature that can be attained by evaporating water into the air, whereas relative humidity is a measure of the amount of water vapor in the air with respect to the water saturation of air, and dew point is the air temperature required for saturation. Relative humidity is determined with the help of a chart called a psychometric table, which uses the dry bulb temperature and the difference in temperature between the two bulbs.

Why are specific humidity and mixing ratio more commonly used in representing atmospheric moisture than absolute humidity?What is the only way to change the specific humidity or mixing ratio of an air parcel?

A rising or descending parcel of air will experience a change in its volume because of the changes in the surrounding air pressure. Consequently, when a volume of air fluctuates, the absolute humidity changes--even though the air's vapor content has remained constant. For this reason, the absolute humidity is not commonly used in atmospheric studies. Specific humidity and mixing ratios, on the other hand, do not depend on the volume of air. Specific humidity and mixing ratio can only change if water vapor is added or taken away from an air parcel.

In a volume of air, how does the actual vapor pressure differ from the saturation vapor pressure? When are they the same?

Actual vapor pressure indicates the number of water vapor molecules in an air parcel, whereas saturation vapor pressure describes how much water vapor is necessary to make the air saturated at any given temperature. Actual vapor pressure would be the same as saturation vapor pressure if the air were saturated.

Supersaturated

Air with a relative humidity greater than 100%

Mixing ratio

Another convenient way to express humidity is to compare the mass of the water vapor in the parcel to the mass of the remaining dry air. - Mass of water vapor/mass of dry air - Expressed as grams of water vapor per kilogram of air (g/kg) - Remain constant as long as water vapor vis not added to or removed from the parcel

electrical hygrometer

Another instrument that measures humidity - consists of a flat plate coated with a film of carbon. An electric current is sent across the plate. As water vapor is absorbed, the electrical resistance of the carbon coating changes.

Explain why condensation occurs primarily when the air is cooled.

As temperatures drop, the speed of water molecules decreases.

Explain why, during a summer day, the relative humidity will change as shown in figure 4.12 (pg. 101).

As temperatures increase, the amount of water vapor required for saturation increases. Since this represents the denominator of the fraction in the RH equation, RH must be inversely proportional to temperature.

Sublimation

As we observe ice crystals in freezing air we can see an occasional molecule gain enough energy to break away from its neighbors and enter into the air above. The molecule changes from an ice molecule directly into a vapor molecule without passing through the liquid state - ice-to-vapor phase change

Explain why it takes longer to cook vegetables in the mountains than at sea level.

Atmospheric pressure is lower at higher altitudes, so the saturation vapor pressure of the puddles escaping from boiling water is also lower. This means that at higher altitudes, boiling occurs at lower temperatures, so food prepared in boiling water needs more time to cook.

When outside air is brought indoors on a cold winter day, the relative humidity of the heated air inside often drops below 25%. Explain why this situation occurs.

Because building furnaces usually heat air without adding water vapor to it. This causes the air's saturation vapor pressure to become much larger than its actual vapor pressure, thus reducing the relative humidity.

heat index

Combines air temperature with relative humidity to determine an apparent temperature (What the temperature feels like to the average person for various combinations of air temperature and relative humidity.)

Actual vapor pressure

Expresses the amount of water vapor in terms of the amount of pressure that the water vapor molecules exert

Specific Humidity

Humidity can be expressed in ways that are not influenced by changes in air volume. When the mass of the water vapor in the air parcel is compared with the mass of all the air in the parcel (including vapor) - Mass of water vapor/total mass of air - Expressed as grams of water vapor per kilogram of air (g/kg) - Remain constant as long as water vapor vis not added to or removed from the parcel

Deposition

If a water vapor molecule should attach itself to an ice crystal, the vapor-to-ice phase change is called this

psychrometer

Instrument used to obtain dew points and relative humidity. - Consists of two liquid-in-glass thermometers mounted side by side and attached to a piece of metal that either has a handle or chain at one end.

hygrometer

Instruments that measure humidity

How can a region have a high specific humidity and a low relative humidity? Give an example.

Large amounts of water vapor in the air will cause the specific humidity to be high. Very high hair temperatures cause the RH to be low. So the combination of large amounts of water vapor and very high air temperatures gives high specific humidity and low RH. An example would be an offshore wind blowing onto a hot desert, such as is sometimes found in middle-eastern countries.

infrared hygrometer

Measures atmospheric humidity by measuring the amount of infrared energy absorbed by water vapor in a sample of air, and the dew cell determined the amount of water vapor in the air by measuring the air's actual water vapor pressure.

dew-point hygrometer

Measures the dew-point temperature by cooling the surface of a mirror until condensation (dew) forms.

Precipitation

Process where liquid or solid cloud particles may grow in size and fall to the surface through this process - Rain, snow, or hail

Condensation

Process where water vapor changes back into liquid water - Winds transport the moist air to other regions where the water vapor changes back into liquid, forming clouds

Evaporation

The sun's energy transforms liquid water into water vapor in this process - Transforms enormous quantities over the ocean

Basically, how do the three states of water differ?

Solid phase (ice): crystal, slow vibrational speed of the molecules Liquid phase (liquid water): faster vibrational speeds Gaseous phase (water vapor): fastest vibrational speeds.

What does saturation vapor pressure primarily depend upon?

Temperature

What are the primary factors that influence evaporation?

Temperature, relative humidity, and wind

Saturated

The condition when the total number of molecules escaping the liquid (evaporating) is balanced by the number returning (condensing) - Air is _________ with water vapor - Under these conditions for every molecule that evaporates, one must condense, and no net loss of liquid or vapor molecules result. - When air is cool water vapor molecules move slowly and stick and condense to condensation nuclei (microscopic bits of dust, smoke, salt, etc mixed in with the air molecules. They are called this because water vapor condenses on them). When air is warm, water vapor molecules move faster and bounce off the condensation nuclei. When billions of water vapor molecules condense onto the nuclei, tiny liquid cloud droplets form. - Condensation occurs primarily when the air is cooled

Hydrologic cycle

The cycle of moving and transforming water molecules from liquid to vapor and back to liquid again.

Briefly explain the movement of water in the hydrologic cycle.

The hydrologic cycle is an unending circulation of water between the ocean, land, and atmosphere. The sun's energy evaporates ocean water, atmospheric winds carry the water vapor to distant regions, and the water vapor condenses into liquid, forming clouds, and liquid (or solid) precipitation falls out of the clouds. When the precipitations falls on land, the water eventually returns to the oceans in rivers or evaporates back into the atmosphere. When precipitation falls onto the ocean, the cycle is complete.

Wet-bulb temperature

The lowest temperature that can be reached by evaporating water into the air. - Good measure of how cool the skin can become through evaporation of water vapor or perspiration from the skin.

Relative Humidity

The most common way of describing atmospheric moisture, it is also most misunderstood. - Does not indicate the actual amount of water vapor in the air, instead tells us how close the air is to being saturated. - The ratio of the amount of water vapor actually in the air to the maximum amount of water vapor required for saturation at that particular temperature (and pressure) - Ratio of the air's water vapor content to its capacity thus... - Water vapor content/water vapor capacity - We can think of the actual vapor pressure as a measure of the air's actual water vapor content, and the saturation vapor pressure as a measure of air's total capacity for water vapor hence RH can be expressed as... - Actually vapor pressure/saturation vapor pressure X 100% - Always given as a percent. - Changes can be brought about by changing the air's water vapor content or by changing the air temperature - With no change in temperature, adding water vapor to the air increases the relative humidity, and removing water vapor from the air lowers the relative humidity - As the air temperature increases (with no change in water vapor content), the relative humidity decreases because in warmer air the water vapor molecules are zipping about at such high speeds that they are unlikely to join together and condense. The higher the temperature, the faster the molecular speed, the less likely saturation will occur, and the lower the relative humidity. - With no change in water vapor content, an increase in air temperature lowers the relative humidity, while a decrease in air temperature raises the relative humidity.

Saturation vapor pressure

The pressure that the water vapor molecules would exert if the air were saturated with vapor at a given temperature. - When both water and ice exist at the same temperature below freezing, the saturation vapor pressure just above the water is greater than the saturation vapor pressure over the ice.

How are evaporation and condensation related to saturated air above a flat water surface?

The rate of evaporation equals the rate of condensation above a saturated water surface.

Why is cold polar air described as "dry" when the relative humidity of that air is very high?

The very cold temperatures of polar air mean that the amount of water vapor required for saturation is very small. Since this amount is the denominator of the fraction in the RH equation, a small value gives a large RH.

Why is the wet-bulb temperature a good measure of how cool human skin can become?

The wet-bulb temperature is the lowest temperature that can be reached by evaporating water into the air. On a hot day, when the wet-bulb temperature is low, rapid evaporation (and hence cooling) takes place at the skin's surface. As the wet-bulb temperature approaches the air temperature, less cooling occurs and the skin temperature may begin to rise. When the wet-bulb temperature exceeds the skin's temperature, no net evaporation occurs, and body temperature can rise quite rapidly. Fortunately most of the time, the wet-bulb temperature is considerably below the temperature of the skin.

How are the dew-point temperature and wet-bulb temperature different? Can they ever read the same? Explain.

The wet-bulb temperature is the lowest temperature that can be reached by evaporating water into the air. The dew point temperature is the temperature to which air would have to be cooled (with no change in air pressure or moisture content) for saturation to occur. The wet-bulb temperature is generally slightly larger than the dew point temperature. The two can be the same when the air is saturated.

Why is the air from the Gulf of Mexico so much more humid than air from the Pacific Ocean at the same latitude?

Warmer temperatures in the Gulf of Mexico region increase the saturation vapor pressure of the air, allowing more water vapor to be present in the air.

Explain why the air on a hot, humid day is less dense than on a hot, dry day.

Water vapor molecules weigh less than the other molecules that make up most of the air (N2 and O2). On a hot humid day the percentage of lighter water vapor molecule sis greater than on a hot, dry day, thus the air on a hot humid day is lighter and less dense than on a hot dry day.

apparent temperature

What the temperature feels like to the average person for various combinations of air temperature and relative humidity.

Why do hot, humid summer days usually feel hotter than hot, dry summer days?

When the air temperature is high and the relative humidity is low, perspiration on the skin evaporates quickly, often making us feel that the air temperature is lower than it really is. However when both the air temperature and relative humidity are high and the air is nearly saturated with water vapor, body moisture does not readily evaporate; instead, it collects on the skin as beads of perspiration. Less evaporation means less cooling, and so we usually feel warmer than we did with a similar air temperature but a lower relative humidity.

frost point

When the dew point is determined with respect to a flat surface of ice it is called this

How does condensation differ from precipitation?

With condensation, water vapor is removed from the atmosphere as it is changed from vapor to liquid water on surfaces. With precipitation, solid or liquid water is removed from the atmosphere as it falls to the Earth as rain or snow.

a) What does the relative humidity represent? b) When the relative humidity is given, why is it also important to know the air temperature? c) Explain two ways the relative humidity may be changes.

a) The ratio of the amount of water vapor actually in the air to the maximum amount of vapor required for saturation at that particular temperature (and pressure) b) Because the amount of water vapor required for saturation depends strongly on the temperature c) By changing the amount of water vapor in an air parcel, and by changing the temperature of the air.

a) What is the dew-point temperature? b) How is the difference between dew point and air temperature related to the relative humidity?

a) the temperature to which air would have to be cooled (with no change in air pressure or moisture content) for saturation to occur. b) When the difference between the dew point and the air temperature is large, the relative humidity is low. When the difference between the dew point and the air temperature is small, the relative humidity is high.

hair hygrometer

constructed on the principle that the length of human hair increases by 2.5% as the relative humidity increases from 0 to 100%