L.6: Moisture in the Atmosphere

Relative humidity can change when:

(1) for a given temperature (the denominator remains unchanged), adding moisture (increasing the numerator) results in an increase in water vapor content and an increase in relative humidity, and vice versa. (2) for a given amount of water vapor in the air (the numerator remains unchanged), increasing the temperature results in an increase in water vapor capacity and a decrease in relative humidity, and vice versa.

If it is 95 degrees outside with an 80% humidity, it will 'feel' like _______ degrees.

133

Which of these locations have a comparatively low relative humidity?

30 degrees North and South

According to the heat index, which is the hottest?

90 with 90%

Which city has the HIGHEST actual vapor pressure?

City D 50 42

Relative humidity is usually lowest around ______________________.

Mid-afternoon

Saturation vapor pressure can tell us what?

Pressure being exerted by water molecules at saturation

Relative Humidity

Relative humidity (symbolized as "RH") Is the ratio of the actual amount of water vapor in the air to the maximum amount of water vapor the air can hold. In other words, it is a ratio between water vapor content and water vapor capacity. Relative Humidity = water vapor content / water vapor capacity Because water vapor content can also be measured with vapor pressure, and capacity can be measured with saturation vapor pressure, relative humidity can also be expressed as: Relative Humidity = actual vapor pressure / saturation vapor pressure Relative humidity is particularly important concept, and it is the most common way of describing atmospheric moisture. We will spend the next few sections exploring it in more detail. Let us look at that equation again: Relative Humidity = water vapor content / water vapor capacity ** The denominator (water vapor capacity) depends on temperature. Recall from the discussion about saturation vapor pressure that the higher the air temperature, the higher capacity for water vapor in that air. So, unlike specific humidity, relative humidity depends on not only the water vapor amount in the air but also the temperature. So, higher relative humidity does NOT necessarily mean more water vapor in the air. Relative humidity can change when: (1) for a given temperature (the denominator remains unchanged), adding moisture (increasing the numerator) results in an increase in water vapor content and an increase in relative humidity, and vice versa. (2) for a given amount of water vapor in the air (the numerator remains unchanged), increasing the temperature results in an increase in water vapor capacity and a decrease in relative humidity, and vice versa. Relative humidity is proportional to the amount water vapor in the air, and inversely proportional to the temperature. DAILY VARIATIONS At what time of the day is relative humidity usually highest? Lowest? Usually the amount of water vapor in the air depends on what kind of air mass is in the area, which usually does not change in a 24-hour daily cycle (unless a front has passed through). The temperature dependence of relative humidity thus results in a diurnal change of relative humidity: low relative humidity occurs during the late afternoon (when temperatures are the highest), and high relative humidity occurs at night and early morning when temperatures are the lowest. REGIONAL VARIATIONS Relative humidity is high in two regions: (1) over tropical latitudes, where the amount of water vapor is high due to evaporation from warm tropical oceans (2) over the poles, due to low temperatures ??On the other hand, specific humidity, which depends only on moisture amount in the air, only has a single peak over tropical oceans and decreases toward both poles (see the figure on page 7 if you do not remember this trend). EXAMPLE As we mentioned earlier, there is a common misperception that air with high relative humidity must have more water vapor content than air with lower relative humidity. Let us take a look at another example to illustrate why this is not the case. On the morning of a mid-winter day, International Falls, Minnesota, has a temperature of -9° C and a measure of relative humidity at 100%. On that same morning, Phoenix, Arizona, has a temperature of 10° C and 30% relative humidity. Which city has more water vapor in the air? At T = -9° C (at International Falls), the saturation vapor pressure (es) = 3 mb, At T = 10° C (Phoenix), the saturation vapor pressure = 12.3 mb Calculating ACTUAL vapor pressure in International Falls (with 100% RH) RH = e / es 1.00 = e / es e = 1.00 x 3 mb e = 3 mb (therefore, our actual vapor pressure is 3 mb) Calculating ACTUAL vapor pressure in Phoenix (with 30% RH) RH = e / es 0.30 = e / es e = 0.30 x 12.3 mb e = 3.69 mb (therefore, our actual vapor pressure is 3.69 mb) After doing the calculations, you can now see that although International Falls was at saturation with 100% relative humidity, it actually had LESS water vapor content in the air. We know this because the vapor pressure (which refers to the actual amount of water vapor in the air) was 3mb in International Falls while the vapor pressure was 3.69 mb in Phoenix. This may seem confusing, BUT REMEMBER THAT THE AMOUNT OF MOISTURE THE AIR CAN HOLD DEPENDS ON TEMPERATURE---and because the temperature was much higher in Phoenix, it will take a lot more moisture to reach saturation. While 30% RH may not seem like the air would contain a lot of moisture, it does contain more moisture than a location that is 100% saturated, but at a temperature of -9° C. Here is another way of looking at it. Imagine the air in Phoenix, Arizona, is like the 30-gallon gas tank of a Ford F-350 truck, because warmer air can hold more water vapor. And because cooler air can not hold as much water vapor, imagine the air in International Falls, Minnesota, is like the much smaller 10-gallon gas tank of a Mini Cooper. If you are at 50% relative humidity in BOTH locations, you would have 15 gallons of gas (or water vapor) in the truck and 5 gallons in the car. Obviously we can see that 50% RH does NOT mean we have the same AMOUNT of actual water vapor in the air since we have three times more gas in the truck, and more than the Mini Cooper can even hold to begin with! In summary, this example shows that at 30% relative humidity, Phoenix has more water vapor (higher saturation vapor pressure) in the air compared to International Falls, even though relative humidity there is 100%. THE TEMP IN PHOENIX IS MUCH HIGHER, AND THEREFORE, THE AMOUNT OF WATER VAPOR REQUIRED FOR SATURATION IS MUCH HIGHER THAN THAT IN INTERNATIONAL FALLS. Using what you learned in the previous example about Minnesota and Arizona, you may now be able to answer the following question: Q: why is the climate of the southwest coast of the United States warm and dry, while the Gulf coast is hot and moist? Think about this for a moment: both coasts are adjacent to large bodies of water and both coasts experience onshore wind flow on a regular basis. So, why does one have a desert-like climate and the other ample moisture and rainfall? A: Part of the answer lies in the temperature of the adjacent water bodies. The southwest United States is influenced by air from the Pacific Ocean, where the water temperature is around 60° F, while the Gulf Coast is influenced by air from the Gulf of Mexico, where the water temperature is around 80° F. Cooler Pacific waters create lower humidity in the southwest, while warmer Gulf waters generate high humidity along Gulf Coast.

The ratio of the mass of water vapor in a given volume (parcel) of air to the mass of the remaining dry air describes the:

mixing ratio

Relative humidity is:

proportional to the amount water vapor in the air, and inversely proportional to the temperature.

As the air temperature increases, with no addition of water vapor to the air, the dew point will:

remain the same

Temperature determines ______ while dew point temperature determines ______.

saturation vapor pressure; vapor pressure

Relative humidity is usually highest around ______________________.

sunrise

Absolute humidity is:

the mass of water vapor molecules in a unit volume of air.

Saturation vapor pressures are higher at with warmer temperatures.

true

Water Vapor

<4% of the atmosphere Liquid Solid Gas The change of water vapor from one form to another, referred to as phase change, is an important source of energy for many weather phenomena. Covers 75% of the earth, 96.5% is salt water. Water is the only substance on earth that is present naturally in three states: 1. As a vapor 2. Liquid 3. Solid It has the highest heat capacity of all common solids and liquids (1 cal / g / °C). In other words, it requires more heat to warm a body of water than it does to warm another common solid or liquid. It has the highest surface tension of all common liquids (72.8 Dynes/cm at 20 °C). In other words, it requires more energy to break the surface and allow water to escape through the process of evaporation. It has a different density in different states. - As a vapor, water's density is ~10-5 g / cm3 - As a liquid, water's density is ~ 1.0 g / cm3 - As a solid, water's density is : ~ 0.9 g / cm3 All water states have different albedos: Liquid: 10% Ice: 30-40% Snow: 20-95% Cloud: 30-90% Water can change its phase through the absorption or release of heat energy. As liquid water cools, the motion of the water molecules slows and additional hydrogen bonds are formed, linking more of the water molecules. When water cools to 0 degrees Celsius (32 degrees Fahrenheit), the water freezes or changes its phase to ice. During this process, energy is released as the latent heat of freezing. When heat energy is applied and absorbed by ice, the activity of the water molecules increases and the hydrogen bonds begin to break. The ice melts or changes its phase to water. During this process, energy is absorbed as the latent heat of melting. Additional heat completely liberates the water molecules and they move into the air. This added heat is called the latent heat of evaporation. During the process of evaporation water changes its phase to a gas known as water vapor, the least dense phase of water. When the water reverses this phase change, and cools (say, then it ascends in the atmosphere), water vapor condenses to a liquid and heat is released as the latent heat of condensation. L = ± 2.5 x 106 J / kg condensation/evaporation L = ± 0.334 x 106 J / kg freezing or melting

CHILLED MIRROR HYGROMETER

A device called a chilled mirror hygrometer measures dew point temperature by cooling the surface of a mirror until condensation forms. A beam of light is reflected off of a chilled mirror. When the mirror temperature is cooled enough for dew to form on it, the light beam scatters off the dew drops instead of reflecting from the mirror. A photo detector records this change.

The ratio of the mass of water vapor in a given volume (parcel) of air to the total volume of that (parcel) air describes the:

Absolute humidity

Vapor Pressure (e)

All gas molecules in the air contribute to air pressure, in other words, each subset of molecules (for example, N2, O2, H2O) exerts a partial pressure. Vapor pressure, then, is the pressure exerted by water molecules in the air, and it is a fraction of the total air pressure. It is also an absolute measure of the amount of water vapor in the air, *****that is, the higher the vapor pressure, the more water vapor molecules are in the atmosphere. !!!!!!! How much water vapor the air can hold depends on temperature. As temperature increases, the capacity for air to hold water increases. This is because at higher temperatures, faster water molecules in liquid escape more frequently, causing the saturation water vapor amount to rise. Therefore, saturation vapor pressure is higher at higher temperature. For example, at 30 degrees F, the saturation vapor pressure is 5.6 mb, while at 70 degrees F, it is at 25 mb. Air can also be saturated over ice surfaces. Because it is easier for molecules to break away from a water surface than from an ice surface, the saturation vapor pressure above solid ice is less than above liquid water, es(water) > es(ice) at all temperatures. In other words, at the time of saturation, there is more water vapor over a liquid water drop than over an ice cube.

Which of these locations would have the highest average specific humidity?

An ocean in the tropics

Psychrometer

Another instrument for measuring dew point and relative humidity is the psychrometer. A psychrometer is a simple device consisting of a dry bulb thermometer (which measures air temperature - T) and a wet bulb thermometer (which measures wet bulb temperature). Wet bulb temperature (Twb) is the lowest temperature that can be obtained by evaporating water into the air (which cools air). The difference between T and Twb is called wet bulb depression. Given a wet bulb depression, one can determine relative humidity and dew point temperature for a given air temperature by using a table called the Psychrometric Table.

Question: Given what you have just learned about absolute humidity and specific humidity, will the specific humidity of a volume of air change as it rises or descends in the atmosphere?

Answer: No. Because specific humidity is a ratio of the mass of water vapor as compared to the mass of air, volume (which can change as the air rises or descends) is not a factor.

Heat Index

As you may have noticed on a hot and humid summer day, humidity plays an important role in how comfortable (or uncomfortable) warm temperatures can be for people. An 85° F day with 20% humidity can feel very different than an 85° F day with 80% humidity! The index that combines air temperature AND humidity to give an apparent temperature (that is, how hot it feels for average person) is called the heat index. The chart below shows the relationship between temperature and relative humidity, and how the heat index increases as humidity increases. Here is a quick explanation to help you better understand the chart: 90° F with a RH of 90% feels like 121° F. However, 105° F with a RH of 40% also feels like 121° F. This means that a 15-degree difference can occur but still feel the same to us because of the differences in relative humidity! You may have heard people refer to Arizona as having dry heat--this is what they are referring to! With low humidity levels, high summertime temperatures feel much more comfortable in Arizona (in the southwestern United States), than they do in Louisiana (in the southeast). Over 200 Americans die from heatstroke every year, and many more fall ill from the taxing demands of heat. Our bodies dissipate heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands, and as a last resort, by panting, when blood is heated above 98.6° F. Sweating cools the body through evaporation. However, high relative humidity retards evaporation, robbing the body of its ability to cool itself. When heat gain exceeds the level the body can remove, body temperature begins to rise and heat related illnesses and disorders may develop. The heat index is the temperature the body feels when heat and humidity are combined. The chart above shows the heat index that corresponds to the actual air temperature and relative humidity. (This chart is based upon shady, light wind conditions. Exposure to direct sunlight can increase the heat index by up to 15° F.) In a hot environment, for example, the body absorbs heat and its core temperature increases--and sweating is the way the body cools itself.

Which city has the HIGHEST saturation vapor pressure?

City A 65 36

Instruments for measuring humidity

DON'T ****ING KNOW

Absolute Humidity (Rv)

Defined as the density of water vapor which is a measure of the mass (kg) of water vapor molecules in a unit volume of air (1 m3). Mathematically, absolute humidity can be expressed as mass of water vapor divided by the volume of air. Absolute Humidity = mass of water vapor (kg) / volume of air (m3)

Dew Point Depression

Dew point temperature is an absolute measure of the amount of water vapor in the air. Dew point depression = T- Td Key Point: Here is one quick rule of thumb: for every 10° C (18° F) increase in dew point, the amount of water vapor in the air is nearly doubled. Question: What does a large value for dew point depression signify? Answer: A large dew point depression would signify that the temperature and dew point are far apart, therefore indicating that the air is dry.

For every 10 degrees Celsius increase in dew point, the amount of water vapor in the air is almost:

Doubled

Where are the locations on the Earth with the highest levels of specific humidity?

Equatorial regions

Air with high relative humidity will have more water vapor content than air with low relative humidity.

F

Heat stroke is a possibility with prolonged exposure to temperatures between 80 and 90 degrees F.

FALSE

Higher relative humidity means that there is more water vapor in the air.

FALSE

Like most solids, water is at its most dense state when it is solid.

FALSE

Vapor pressure is the pressure exerted by water molecules in the air. It is also an absolute measure of the amount of liquid water in the air.

False

When there is more water vapor in the air, more of cooling is needed for the air to become saturated.

False

Use the table above to answer the following question: In a chilly summer morning in East Lansing, the temperature is 10° C and air is saturated (R.H. = 100%). In the afternoon, the air warms to 29° C. If the same air mass has been in the area, that is, the moisture content in the air has not changed from the morning to the afternoon, would relative humidity increase or decrease in the afternoon from the morning? What would be the relative humidity in the afternoon?

From the table: In the morning T = 10° C, saturation vapor pressure (es) = 12.3 mb In the afternoon, T = 29° C, es = 41 mb In the morning RH = e / es, therefore e = RH x es e = 1.0 x 12.3 mb (RH = 1.0 because the relative humidity equaled 100%) e = 12.3 mb (notice that because relative humidity was 100%, actual water vapor pressure and saturation water vapor pressure are equal). In the afternoon RH = e / es RH = 12.3 mb / 41 mb RH = 38% The relative humidity decreased from 100% in the morning to 38% in the afternoon.

If you decrease the water vapor content, but keep the temperature the same the relative humidity:

Goes down

If you increase the water vapor content, but keep the temperature content the same the relative humidity:

Goes up

Saturation Vapor Pressure

One specific type of vapor pressure is saturation vapor pressure. Water molecules move between the liquid and gas phases. When an equilibrium status is reached, which means, for every water molecule evaporated into the air, a vapor molecule would return to the liquid phase, the air is said to be saturated with water vapor at this point. Water molecules evaporated = vapor molecules liquified The pressure exerted by the water vapor molecules at the point of saturation is called saturation vapor pressure. When air is saturated, the level of water vapor in the air is at its maximum amount for a given temperature and pressure. Saturation vapor pressure is a measure of how much water vapor is necessary for the air to be saturated at any given temperature. The concept of saturation can be applied for other humidity measures--for example, the saturation specific humidity is the specific humidity at the point of saturation. Therefore, saturation vapor pressure is higher at higher temperature.

Sublimation

SOLID ICE-->WATER VAPOR Input of heat is required for this process. The heat released during deposition equals the summation of heat from condensation and freezing. L = ± 2.83 x 106 J / kg deposition or sublimation

Which form of water has the ability to have the highest albedo?

Snow

Saturation vapor pressures are higher at with warmer temperatures.

T

Higher temperatures can have higher water vapor content.

TRUE

The relative humidity is often very high in the polar regions.

TRUE

Concepts of Humidity

The concept of absolute humidity (also called water vapor density) is relatively easy to understand, because most people understand the concept of density. Higher absolute humidity means the density of water vapor is higher--more water vapor is in the unit volume of atmosphere. However, the biggest disadvantage of absolute humidity is that it depends on volume, which is variable. The volume of air changes rapidly as air parcels go up (expand) and down (contract) in the atmosphere, due to the decrease in atmospheric pressure with altitude. Absolute humidity, thus, changes as air parcels rise or sink through the atmosphere even though the water vapor amount in the parcel remains unchanged. Because of this, absolute humidity is seldom used in practice. VOLUME OF AIR CHANGES OFTEN DEPENDING ON PRESSURE

Hydrologic Cycle

The cycle of transforming water molecules from liquid to vapor, and then back from vapor to liquid. 1. Evaporation is the change of state of water (a liquid) to water vapor (a gas). On average, about 47 inches (120 centimeters) is evaporated into the atmosphere from the ocean each year. 2. Transpiration is evaporation of liquid water from plants and trees into the atmosphere. About 90% of all water that enters the roots transpires into the atmosphere. 3. Sublimation is the process where ice and snow (a solid) changes into water vapor (a gas) without moving through the liquid phase. 4. Condensation is the process where water vapor (a gas) changes back into a water droplets (a liquid). This is when we begin to see clouds. 5. Transportation is the movement of solid, liquid and gaseous water through the atmosphere. Without this movement, the water evaporated over the ocean would not precipitate over land. 6. Precipitation is water that falls to the earth. Most precipitation falls as rain but includes snow, sleet, drizzle, and hail. Around 313,000 mi3 (515,000 km3) of water falls each year, mainly over the ocean. 7. Runoff is the variety of ways of which water moves over the earth's surface. This comes from melting snow or rain. 8. Infiltration is the movement of water into the ground from the surface. 9. Groundwater flow is the flow of water underground in aquifers. The water may return to the surface in springs or eventually seep into the oceans. 10. Plant uptake is water taken from the groundwater flow and soil moisture.

Deposition

The process of water vapor directly becoming ice is called deposition. Water VAPOR --> ICE The heat released during deposition equals the summation of heat from condensation and freezing. L = ± 2.83 x 106 J / kg deposition or sublimation

Specific Humidity (q)

To overcome this shortcoming of absolute humidity, another variable, called specific humidity, was created. Specific humidity is the ratio of mass of water vapor to the total mass of air in a unit volume (the grams of water vapor per a kilogram of air). So, even though volume may change, because we are measuring the mass of water vapor and the mass of air, the ratio will remain the same. Specific humidity is invariant to changes in volume. Specific Humidity = mass of water vapor (g) / mass of air (kg) The higher the specific humidity, the more water vapor the atmosphere contains. For your reference, specific humidity measurements usually range between 1 and 20 grams of water vapor per kilogram of air. Specific humidity can vary with locations of different latitudes and altitudes. It is generally highest at locations over tropical oceans, lowest over the polar regions, and decreases with altitude.

A small dew point depression means the air is near saturation.

True

Humidity is the measure of water in the air at any given time.

True

Mixing Ratio (r)

Using the specific humidity ratio equation, we can replace the total mass of air in the denominator with the total mass of dry air. This new ratio is another concept known as the mixing ratio. Mixing ratio = mass of water vapor / mass of dry air As you have previously learned, the amount of water vapor in the air is quite small compared to other permanent gases. Therefore, when dividing the mass of water vapor by the total mass of air (in the case of specific humidity) or by the total mass of dry air (in the case of mixing ratio), the difference turns out to be insignificant. Therefore, the differences between specific humidity and mixing ratio are usually IGNORED. The advantage of both specific humidity and mixing ratio is that they are conserved as an air parcel ascends or descends in the atmosphere. They are also absolute measures of the amount of water vapor in the atmosphere.

Wet Bulb Temperature

Wet bulb temperature (Twb) is the lowest temperature that can be obtained by evaporating water into the air (which cools air). The difference between T and Twb is called wet bulb depression. Given a wet bulb depression, one can determine relative humidity and dew point temperature for a given air temperature by using a table called the Psychrometric Table.

Dew Point Temperature

When there is no change in the moisture content, the change in relative humidity is caused only by a change in temperature. So, for the example given earlier, the relative humidity on that cool morning in East Lansing was 100% when the air temperature was 10° C. Therefore, if there is no change in moisture content, the air will again be saturated when the temperature returns to 10° C. The temperature to which air must be cooled (at constant pressure and constant water vapor content) to become saturated is called the dew point temperature. In our example, 10° C is our dew point temperature. Unlike relative humidity, which is a measure of how close the air is to saturation, DEW POINT TEMPERATURE IS A MEASURE OF ITS ACTUAL MOISTURE CONTENT. The higher the dew point, the more water vapor in the air. When there is more water vapor in the air and the air is subsequently close to saturation, then a little bit of cooling would bring the air to saturation. Therefore, the dew point temperature would be close to temperature and the dew point depression would be small. On the other hand, when the amount of water vapor is low and the air is far from saturation, the air needs to be cooled quite a bit in order to reach saturation. In this case, the dew point temperature is much lower than the actual temperature and dew point depression is large. Dew point temperature is an absolute measure of the amount of water vapor in the air. Dew point depression = T- Td Key Point: Here is one quick rule of thumb: for every 10° C (18° F) increase in dew point, the amount of water vapor in the air is nearly doubled. Question: What does a large value for dew point depression signify? Answer: A large dew point depression would signify that the temperature and dew point are far apart, therefore indicating that the air is dry. Dew point temperature can be used to forecast minimum temperature at night. The lower the dew point temperature, the lower the expected minimum temperature. Question: Why does a lower dew point temperature result in a lower minimum temperature? (hint, when saturation occurs, water vapor would condense into liquid water. When condensation occurs, what would happen?) Answer: Lower dew point temperatures mean there is less water vapor content in the air. With higher amounts of water vapor in the air, condensation is more likely to occur as temperatures begin falling overnight. When condensation occurs, water is changing from its gaseous state to its liquid state, which results in the release of energy. It is this energy release that keeps minimum temperatures mild.

Dew point temperature is a measure of the air's _________ moisture content and the _______ the dew point, the more water vapor is in the air.

actual; higher

If you change the temperature, but keep the water vapor content the same the relative humidity:

changes

As the air temperature increases, with no addition of water vapor to the air, the relative humidity will:

decrease

If you decrease the temperature, but keep the water vapor content the same the relative humidity:

goes up

If you increase the water vapor content, but keep the temperature content the same the relative humidity:

goes up

Relative humidity is usually ______ in the tropics and ______ in the sub-tropical regions.

high, low

Suppose the dew point of cold outside air is the same as the dew point of the air indoors. If the door is opened and cold air replaces some of the warm air, then the new relative humidity indoors would be:

higher than before.

If very cold air is brought indoors and warmed with no change in its moisture content, the vapor pressure of this air will ______ and the relative humidity of this air will ______.

increase; decrease

As the air temperature increases, what happens to the air's capacity for water vapor?

it increases

The time of day when the relative humidity reaches a maximum value is usually:

just before sunrise.

In a blinding snowstorm in Vermont, the air temperature and dew-point temperature are both 30° F. Meanwhile, under clear skies in Arizona, the air temperature is 85 degrees F and the dew point temperature is 38 degrees F.

there is more water vapor in the air in Arizona.

The higher the air temperature, the higher its capacity for water vapor.

true

The higher the dew point in the air, the more water vapor is in the air.

true

Water being less dense when it is frozen means that:

water freezes from top to bottom.