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Moisture in the Atmosphere

Reading Assignment
The textbook for this course is entitled: eWeather and Climate by Netoff, Gillespie, Fujimoto-Strait and Tiller. Upon purchase of the Lab Manual for the Course GEO 1401/Lab, you will be provided a link to a downloadable pdf which will serve as the textbook for this course GEOG 1401/Lecture. The Lab Manual/cd text is only available for purchase online at: bearkatsonline.com.
 
Note:  This unit contains a great many graphics and photographs. The load time may vary depending upon your computer and web connection.
 
Click the radio button located on the left page margin opposite selected graphics for additional information. Be SURE and close the message box when you are done.

01. When the word Storms is used in relation to climatic controls, we are not talking about tornadoes, hurricanes and the like. Rather this control deals with the presence of moisture in the atmosphere, the various means by which moisture is removed from the atmosphere and the forms the moisture takes. Meteorologists are very interested in the amount of moisture in the air not only because the presence of water vapor is an indicator of the potential for precipitation, but the energy released as water vapor changes states provides the energy for storms.
 
Moisture, or humidity, is a general term we will use to describe the amount of water vapor in the air. There are a number of ways to express the amount of humidity in the air -- the most common being absolute humidity, relative humidity and mixing ratio. We will take up each of these shortly.

 
 

02. Let's begin our look at moisture by considering the ideas of saturation and vapor pressure. In the graphic below, suppose we have a sealed container that is half full of water. The temperature of both the water and the totally dry air (the air contains no water vapor) above the water is 70 degrees F. Assume that if we were to insert a barometer into the air overlying the water we would get a pressure reading of 29.92 inches of mercury.
 
 

03. In the graphic below the process of evaporation begins with the movement of one molecule of water vapor from the liquid water into the dry air above. As this first molecule makes its way into the air, we should expect to see an increase in the air pressure being registered on the barometer -- after all we have now added one additional gas (water vapor) molecule to the air above the liquid water. Now granted, in order to measure the pressure increase of one molecule, we would have to be dealing with a very sensitive barometer -- but the point is one additional molecule into the air would result in some increase in the air pressure within the sealed container. This portion of the total air pressure that is accounted for by the presence of water vapor (in this case just one molecule) is called vapor pressure.
 
As the process of condensation continues and the presence of water molecules grows greater, we will see an increase in vapor pressure within the sealed container.
 
 
 

04. In our final graphic in this sequence, we have reached the point where the air can not hold any additional water molecules. At this point, the air is said to be saturated -- it is at capacity. Any increase in vapor pressure (the result of any additional water molecules being moved into the air over the water) will cause a corresponding number of water vapor molecules to move from the air to (because the air temperature is above 32 degrees F) water.
 
 

05. Air is said to be saturated when a balance is achieved between those water vapor molecules leaving the water and those being returned to the water. The air can hold no more water vapor -- it is filled to capacity.
 
You will note that the capacity (ability) of the air to hold water vapor increases sharply with increases in temperature. In the graphic below observe that air at 30 degrees F is saturated when 3.3 grams of water vapor are present in a kilogram of otherwise dry air. Were you to heat the air to 60 degrees F, you can see that the capacity of the air increases to 10.7 g/kg (an increase of 7.4 grams). But when we increase the air temperature another 30 degrees F (to 90 degrees F), the capacity of the air increases 19.3 grams.
 
As you increase air temperature, the ability of the air to hold water vapor increases at an increasing rate.
 
And finally keep in mind that most of the moisture present in the atmosphere is found within the first few thousand feet. This is where the moisture is; this is where our weather is.
 
 
 

06. Take a little time to study the graphic below. I, of course, do not expect you to memorize the information presented here, but see what happens to the ability of the air to hold moisture as temperature is increased. Look at the capacity of the air at 90 degrees F (a not uncommon afternoon summer temperature in southeast Texas) -- 30.052 g/kg. Compare this to the capacity of air with a temperature of 50 degrees F (a fairly typical winter day temperature) -- 7.389 g/kg. The air in summer holds over 4 times the water vapor of a typical winter day. Can you begin to see why our summer storms tend to be more vigorous (more condensation taking place, thus more energy being released) and result in larger quantities of precipitation (not only more water vapor in the air, but the summer storm clouds may tower 30,000 to 40,000 feet compared to winter storm clouds of maybe a few thousand feet).
 
Too, how many of you have ever heard the saying: "It's too cold to snow." Well it's probably rarely too cold to snow, but a glance at the Capacity of Saturated Air table below will give you a good idea of where this saying may have come from. At temperatures below 0 degrees F, observe how little water vapor the air is able to hold if saturated. And after all, if it's not in the air, it's not going to be available to fall out.
 
 
 

07. Let's take a look at the various ways the moisture content of the air may be expressed. Keep in mind that meteorologists are interested in following the water vapor -- for the precipitation potential, and as the energy source for our storms.
 
Absolute Humidity. Absolute humidity does not have a lot of usefulness in the realm of the meteorologist. As you can see in the graphic below, as air moves about in the atmosphere, its volume changes dramatically. What is one cubic foot at the surface, may expand to become tens of cubic feet aloft.
 
In the example, a cubic foot at the surface contains 6 grams of water vapor. Moved aloft, the air expands to 3 cubic feet with each cubic foot containing 2 grams of water vapor. If you are trying to keep up with the water vapor (say in a neat one cubic foot package), you are going to have a major headache as the moving air expands and compresses with changes in height.
 
    
 

08. Mixing Ratio. Meteorologists make extensive use of the mixing ratio. Here atmospheric moisture is measured by comparing the weight of water vapor in the air to the weight of a unit of dry air. The mixing ratio is usually measured in grams of water vapor per kilogram of dry air. As depicted in the graphic to the right, the advantage for the meteorologist is that the measurement is not concerned with a volume of air (that changes dramatically with changes in altitude), but rather with a weight which remains the same regardless of the volume encompassed. If on a warm summer day a parcel of air is heated and rises and spreads out, the basic "container" remains constant -- a kilogram of air is a kilogram of air regardless of whether it takes up one cubic foot at the surface or 1000 cubic feet at 15,000 feet. Thus, it is relatively easy to follow the water vapor that is the focus of the meteorologist.
 
    

09. The graphic below depicts a sling psychrometer. This is an instrument used to measure the amount of water vapor in the air. As you can see, the instrument is comprised of two thermometers. On the right is the "dry bulb" thermometer. As the instrument is slung around in a circle, the dry bulb thermometer measures the temperature of the air. The "wet bulb" thermometer on the left is covered with a cotton "sock." Before use, the sock is dipped in water. As the instrument is being slung, water is evaporated off the sock. The amount of evaporation (a cooling process) is dependent upon the humidity of the air. If the humidity is low, a great deal of water can be evaporated into the dry air and the result will be a relatively low wet bulb reading. If the humidity is relatively high, then it is more difficult to evaporate much water off the sock and a relatively higher wet bulb reading will be obtained. The temperatures of the two thermometers are read and, with the use of a chart, the mixing ratio can be calculated.
 
 

10. Relative Humidity. Probably the best known measure of water vapor in the air is relative humidity. This measurement is commonly used on evening weather reports, and the term is frequently used by the general public. And while the concept of relative humidity has its place in our everyday life, and most of us can surely relate to the term, it is not the best measurement of the actual amount of water vapor in the air. And again, it is the amount of water vapor in the air that is important to the meteorologist.
 
Relative humidity can be defined as the amount of water vapor in air at a given temperature compared to what that air could hold at that temperature. For instance, in the example, air at 60 degrees F can hold (has a capacity/will be saturated when it holds) 10.7 g/kg. On a 60 degree F day, if the air holds only 5.35 g/kg of water vapor, the relative humidity of the air will be 50 percent. In other words the air is holding half of what it could hold.
 
    
 

11. We can change the amount of water vapor in a parcel of air by one of two means. In the graphic on the left we have two parcels of similar size. Both have a temperature of 60 degrees F and, if we were to consult the Capacity of Saturated Air table (see earlier graphic), we would see that both have a capacity of 10.699 g/kg. Neither is saturated. The parcel on the left, lying over a land surface, has a mixing ratio of 5.35 g/kg. Since the mixing ratio of this parcel is half of what it could hold, it has a relative humidity of 50 percent. The parcel on the right, lying over water, has, due to increased evaporation, a mixing ratio of 7.133 g/kg. The relative humidity of this air parcel is 66 percent. In this instance we have increased the relative humidity of the air mass by increasing the amount of water vapor in the air.
 
But adding (or removing) water vapor is not the only way we can alter the relative humidity of an air mass. Note the graphic on the right. Here we have two air parcels of similar size. The parcel on the left has a temperature of 60 degrees F. By consulting the Capacity of Saturated Air table we find that this air has a capacity of 10.699 g/kg. Because the mixing ratio of this parcel is 5.35 g/kg, the relative of this air is 50 percent. Notice what happens when we reduce the temperature of such an air parcel just 10 degrees F. The capacity is decreased from 10.699 g/kg to 7.389 g/kg (see the Capacity of Saturated Air table). While there has been no change in the mixing ratio (changing the temperature does not change the amount of water vapor in the air), the reduced capacity means that the mixing ratio of 5.35 g/kg is now a larger part of the capacity (now only 7.389 g/kg) and as a result the relative humidity rises to 72 percent.
 
We can thus change the relative humidity of a air parcel by either increasing or decreasing the amount of water vapor in the parcel (this changes the mixing ratio of the air mass) OR by raising or lowering the temperature of the air mass (this changes the capacity of the air mass). Because we are dealing with a ratio, any change in the relationship of the amount of water vapor (the mixing ratio) in the air to the capacity (the ability of the air to hold water vapor) of the air will change the relative humidity of the air parcel.
 
 
    
 

12. One must be careful when interpreting relative humidity. As you can see in the graphic below, if we had two parcels of air, both saturated, one with a temperature of 30 degrees F and the other of 60 degrees F, the one whose temperature was 60 degrees F would have over three times the water vapor in it. Again, this is because warm air holds more water vapor than colder air, and because relative humidity is a ratio between capacity (variable with temperature) and mixing ratio.
 
 
 
 
As an example of the kinds of problems relative humidity can create for the casual observer, let's consider the climates of western Europe and northern Africa. Most would associate the climate of western Europe with high humidities, clouds, drizzle, green landscapes and the like. Northern Africa (the Sahara Desert) is generally characterized by extreme heat and dryness.


13. Let's assume we have an air mass over western Europe with the characteristics indicated on the graphic on the left. At 60 degrees F the capacity of the air is 10.699 g/kg and the mixing ratio of the air mass is 10.699 g/kg. The air is saturated and the relative humidity is 100 percent. We are right at the point of condensation which, should it occur, would result in rain, drizzle and/or fog since the temperature is above 32 degrees F. Note especially that the air mass contains 10.699 g/kg.

 

 
Now, look at the graphic on the right. Here we have an air temperature of 115 degrees F -- almost twice the temperature of the air mass covering western Europe. Because of the higher temperature, the capacity of this air parcel is almost six times greater than that its European counterpart. However, upon measurement we find that the mixing ratio of this air parcel is only 13.370 g/kg -- resulting in a relative humidity of 20 percent. Now let's see -- almost twice the temperature, six times the capacity, but only 20 percent relative humidity compared to 100 percent relative humidity for the parcel overlying western Europe. But observe that the actual amount of water vapor in the air over north Africa is greater by almost a third when compared to that of western Europe. How is possible that a place with a 100 percent relative humidity reading can have less water vapor in the air than a place with a relative humidity?
 
It all comes back to the fact that with relative humidity you are dealing with relationships between capacity that varies with changes in temperature or mixing ratios.
 
 
    
 

14. Dew Point. As we have observed a number of times, if you are going to have condensation within an air mass, you are going to have to have moisture present and the temperature of the air mass will have to be falling. The temperature to which the air mass will have to be cooled is called the dew point. The capacity of air (see the Capacity of Saturated Air table) at 30 degrees F is precisely 3.368 g/kg; at 60 degrees F the capacity rises to 10.699 g/kg and at 90 degrees F the capacity is 30.052 g/kg. What these figures are saying is that if you have a parcel of air whose temperature is 60 degrees F, that air can hold (the most the mixing ratio can be is) a maximum 10.699 g/kg of water vapor.
 
If any more water vapor is added, or if the temperature is reduced below 60 degrees F, a change of state will occur. If you add any more water vapor, you will exceed the capacity of the air at that temperature to hold water vapor, thus for every additional water vapor molecule added, a molecule of liquid water will be condensed.
 
On the other hand, if you drop the temperature below 60 degrees F, the saturated air will be unable to retain all of the water vapor at the new lower temperature (its capacity will be exceeded) and a portion of the water vapor will be condensed.
 
This temperature where the change of state occurs is called the dew point. For air containing 3.368 g/kg of water vapor, the dew point is 30 degrees F; for air containing 10.699 g/kg of water vapor, the dew point is 60 degrees F; and for air containing 30.052 g/kg of water vapor, the dew point is 90 degrees F.
 
If you think about this for a moment, you can see that dew point is an excellent indicator of the actual amount of water vapor (the mixing ratio) in the air. Air with a high dew point will have a great deal of water vapor present; air with a low dew point will not have a lot of water vapor present. Too, you can see that the dew point temperature is directly tied to the amount of water vapor present. If you reduce the amount of water vapor present in the air, you reduce the dew point temperature (the point at which the air is saturated); if you add water vapor to the air, you increase the dew point temperature.
 
In the second graphic, we have four parcels of air of varying temperatures. Each air parcel is saturated (each has a 100 percent relative humidity reading). As the temperature of a parcel is reduced, its capacity is reduced. Too, since the capacity is less, the mixing ratio is also represented by a smaller number. Where is the water vapor going? While not indicated on the graphic (we will look at this idea in the following graphics), observe that, should what is being depicted actually be occurring over a specific geographic area, water vapor would be forced from the air as the capacity of the air is reduced and rain would be occurring.
 
Since the dew point is the temperature to which the air must be cooled in order for condensation to occur (saturation), as you remove water vapor from the air the mixing ratio changes as does the dew point temperature.
 
    
 

15. The following series of graphics is provided to demonstrate the relationships that exist between air temperature, capacity, mixing ratio, relative humidity and dew point. For purposes of example, let us assume that we are in a sealed room (no air in, no air out and no air conditioner or heater in operation -- and no breathing).
 
We pull out a thermometer and take a reading of the room's air temperature and find it to be 70 degrees F. Now, where to find the capacity of air with a temperature of 70 degrees F? Right -- the Capacity of Saturated Air table. We consult it and find that at 70 degrees F air has a capacity of (it will be saturated when it contains) 15.260 g/kg. We take out our trusty sling psychrometer, wet the sock down, give it a few slings, take the readings and upon consulting the sling table find that the air in the room is not at capacity -- it only contains (has a mixing ratio of) 10.699 g/kg. OK, it can hold 15.260 g/kg, but it actually holds 10.699 g/kg. It appears just from eyeballing the figures that the relative humidity -- what's there (10.699 g/kg) compared to what could be there at 70 degrees F (15.260 g/kg) -- is about two thirds. When we actually calculate it (10.699 divided by 15.260), we find the relative humidity to be 70 percent.
 
Now as to the dew point. We know we have 10.699 g/kg of water vapor present. To what temperature will we have to drop the air in order for what is actually present (the mixing ratio -- the 10.699 g/kg) to represent saturation? Well, we're once again going to need the Capacity of Saturated Air table. When we run down the table, we find that air with a water vapor content of 10.699 g/kg will be saturated when the temperature reaches 60 degrees F. Then the dew point of the air in this room is 60 degrees F.
 
 
 

16. Now, what happens if we turn the room thermostat up to 80 degrees F? Looking at your Capacity of Saturated Air table, you can see that as air temperatures rise, there is an increase in the ability of the air to hold moisture. At 80 degrees F the capacity of air increases to where it can hold 21.537 g/kg of water vapor. We have raised the air temperature, but in doing so we have not changed the actual amount of water vapor in the air, thus the mixing ratio must be the same as previously -- 10.699 g/kg.
 
However, the relationship between the air's capacity and the mixing ratio has been changed. This will have to cause changes in the relative humidity in the room. As you can see, the mixing ratio is now approximately half of the capacity. Increasing the air temperature has reduced the relative humidity. When we actually calculate the relative humidity, we find it to be 50 percent. Just as you should expect. If you have to drop the air temperature in order to have saturation/condensation, then an increase in temperature should be moving you away from saturation and any form of condensation.
 
And what about the dew point? Well, if we are going to saturate this air parcel (and we don't alter the mixing ratio), aren't we still going to have to drop the temperature of this air mass to 60 degrees F? Then the dew point must still be 60 degrees F. The dew point is tied to the amount of water vapor in the air. If you don't change the water vapor content of the air, you don't change the dew point.
 
 

17. We now drop the thermostat to 60 degrees F. Consulting the Capacity of Saturated Air table we find that the capacity of air at 60 degrees F is only 10.699 g/kg -- the same as the room's mixing ratio. If what's there (the mixing ratio) is the same as what could be there (the capacity), then the air must be saturated, the relative humidity must be 100 percent and we must be at dew point (60 degrees F). Now there is no thunderstorm taking place in the back of the room, and there is no fog nor condensation on the walls. The air is just saturated. That means the air can't hold one more molecule of water vapor. If any water vapor were to be added at this point (thereby placing more moisture into the air than it was able to hold), or if we were to drop the temperature any lower (thereby reducing the capacity of the air to hold the 10.699 g/kg), we would have a change of state (gas to liquid at this 60 degree F temperature). But right now the air is clear -- no clouds, fog or the like -- just saturated.
 
 

18. We now drop the thermostat to 50 degrees F and all kind of things begin to happen. Checking the Capacity of Saturated Air table, we find that air at 50 degrees F has a capacity of only 7.389 g/kg.
 
Well, if that's all the water vapor the air can hold -- then that's all it does hold. The mixing ratio drops to 7.389 g/kg. What happened to the excess moisture (the 3.310 g/kg)? It "fell out" of the air. Since the air temperature is above freezing, we have condensation, and most likely condensation has formed on room surfaces. We want to come back to this point in a moment, but for now let's complete the table.
 
Since we can assume a steady drop of temperature in the room as a result of the thermostat change, can you see that as the temperature hit 59 degrees F we condensed a little bit of moisture, a little more at 58 degrees F, and so on until we reached 50 degrees F at which point we had lost a total of 3.310 g/kg? As the capacity was steadily reduced, the mixing ratio was constantly changing downward to meet the downward trending capacity. The relative humidity remained constant at 100 percent since between 60 degrees F and 50 degrees F the capacity and the mixing ratio remained the same. And the dew point? Well, as we continued to lose water vapor (the mixing ratio) the dew point also followed a steadily downward trend so that we now find the dew point and the room temperature to be the same.
 
How many of you have ever asked yourself why rain sometimes comes down in buckets and other times we only get a slow, soaking drizzle? This line on the table ought to answer the question. We can see that dropping the temperature from 60 degrees F to 50 degrees F caused 3.310 g/kg of water to condense. If this temperature drop took five minutes to occur (like we might well have with a warm/high capacity summer day thunderstorm), then the 3.310 g/kg will condense out in five minutes. On the other hand if it took three days for the same temperature drop to occur (as you might get with a slow moving warm front), then it will take three days for the 3.310 g/kg to be condensed. The rate of precipitation is largely related to the capacity of the air, the rapidity with which the temperatures are dropped and the dew points hit.
 
 
 

19. Too cold for us, so let's run the thermostat back up to 80 degrees F. Consulting the Capacity of Saturated Air table we find that the capacity of the air in the room has risen to 21.537 g/kg. Again, since changing the temperature does not impact the actual amount of water vapor in the room (and assuming here no evaporation off the room's walls, floors, etc.), the mixing ratio in the room must be where we left it on the last graphic -- 7.389 g/kg). Eyeballing the capacity and mixing ratio we can see that the relative humidity has dropped to about 33 percent (actually to 34 percent) -- just as you might expect if you radically increase the temperature since you are moving away from condensation/precipitation. And the dew point? Well, look at the Capacity of Saturated Air table and find at what temperature would air with a mixing ratio of 7.389 g/kg become saturated. It looks like 50 degrees F.
 
 

20. Now let's throw a little wrinkle into the mix. Say someone opens the door and kicks in a tub of water. Evaporation begins to the point of adding (for purposes of example) 4.958 g/kg of water vapor to the air (sort of like an air mass moving over a large body of water, right?). What kind of impact will this have on our example?
 
 

21. We may have added water vapor to the air, but since there has been no temperature change, the capacity of the air remains the same at 21.537 g/kg. However, we must add the water vapor evaporated from the tub of water to the mixing ratio in the room prior to the time the tub of water was added. We did have 7.389 g/kg present. By adding 4.958 g/kg from the tub, we have increased the mixing ratio to 12.347 g/kg. This should increase the relative humidity (what's there compared to what could be there) -- and it does. Our relative humidity jumps to 57 percent.
 
Since we have changed the mixing ratio, we have changed the dew point. Consulting the Capacity of Saturated Air table, we find that in order to saturate the air presently in the room we would have to drop the temperature in the room to 64 degrees F -- not to 50 degrees F as was the situation before we added the moisture. And doesn't this make perfect sense -- if you add moisture, shouldn't it be easier to get condensation (by having to only drop the temperature to 64 degrees F instead of the lower temperature of 50 degrees F)?
 
 

22. Ok, a fast review. We have a kilogram of air whose temperature is 60 degrees F. How much water vapor can air at this temperature hold at saturation -- what is the air's capacity? We get out the Capacity of Saturated Air table, run the table to 60 degrees F and find that such air, when saturated, will hold 10.699 g/kg. The capacity of the air at 60 degrees F is 10.699 g/kg.
 
 

23. The air may hold 10.699 g/kg, but it feels fairly comfortable -- it just doesn't seem like it is actually holding 10.699 g/kg at the moment. Getting out our sling psychrometer and related tables, we find that the air is not saturated. It only holds 5.35 g/kg. The mixing ratio of the air is only 5.35 g/kg.
 
 

24. Well, if the air could hold (the capacity) 10.699 g/kg, but in fact it only contains (the mixing ratio) 5.35 g/kg, then the air is not saturated and the relative humidity must be something less than 100 percent. It looks like the relative humidity (and upon calculation we find that yes it is ...) 50 percent. The air is only holding half of what it could hold at 60 degrees F, thus the relative humidity of the air is 50 percent.
 
 

25. And the dew point? Well, by consulting the Capacity of Saturated Air table, we find that air that holds 5.35 g/kg of water vapor will be saturated when the temperature is reduced to a temperature between 41 and 42 degrees F. So if we want to condense moisture out of this air mass, we will have to drop the temperature to the 41/42 degree F area. That failing, we could increase the water vapor content (the mixing ratio) which would have the effect of raising the dew point to a temperature closer to 60 degrees F.
 
 

26. Like all gases in the atmosphere, water vapor can change states (gas/liquid/solid). But unlike many gases found in the atmosphere, water vapor changes state at temperatures found at the Earth's surface. As the change of state occurs, heat is either released or absorbed depending upon the change taking place.
 
Consider the example of ice cubes in a glass of tea. If we fill a glass with ice cube, then add tea and wait 10 to 15 minutes, we might find upon measurement the temperature of the tea to be 37 degrees F. Now, if we take the glass and place it over a flame, we will see the ice cubes begin to melt. Somewhat surprisingly the temperature of the liquid remains at 37 degrees F until all of the ice has been melted. The temperature of the liquid was 37 degrees F before we placed the glass over the flame; it remained 37 degrees F after all of the ice had been melted. Where did all of that heat from the flames go?
 
The heat energy was used to disrupt the crystalline structure of the ice cubes and cause them to melt. It is still present -- it now resides in the water molecules in the glass. Because the heat is not associated with temperature change, it is usually referred to as latent (or hidden) heat. This energy is not available until the liquid is returned to the solid state at which time it will be released into the atmosphere as heat.
 
Let's take a look at the various states water moves through and whether heat is being absorbed or released. Again, this heat is the source of the energy that drives our storms.
 
 
    
 

27. Melting. In the melting process, the solid ice is changed to liquid water. In this process, heat is absorbed from the atmosphere by the water molecule. For purposes of example, it takes approximately 80 calories of heat to produce one gram of water. This heat is often referred to as the latent heat of melting. The heat is retained in the water molecule to be released when the water is returned to the solid state.
 
 
    
 

28. Freezing. Freezing is the reverse of melting. Here liquid water is converted into ice. In this process heat is released into the atmosphere. How much heat? The same 80 calories absorbed during the melting process. This heat is referred to as the latent heat of fusion.
 
    
 

29. Evaporation. In the process of evaporation, approximately 600 calories are required to convert one gram of water to water vapor. This heat energy is absorbed by the water vapor molecule and will be available to be released as heat when the water vapor is returned to the liquid or solid state. The 600 calories required is called the latent heat of evaporation.
 
    
 
There are a number of factors that can aid in the process of evaporation. Some of the more important are touched on below.
 
Air Temperature. You will remember that warm air has a greater capacity to hold water vapor than does cold air. 
 
 
Degree of Saturation. Evaporation is also encouraged by the degree to which the air is saturated. Drier air can hold more water vapor than can more humid air.
 
 
Temperature of the Water. Evaporation is greater over warm water than over cold water. This is largely due to the fact that the molecules are more active in warm water and are thus more prone to break the surface tension of the water. When compared to warm water, the molecules comprising cold water move slower and have a reduced tendency to move from the liquid to gaseous state.
 
 
Wind. And finally we might note that evaporation is greater on windy days than on calm days. Think about hanging a wet shirt out on a clothes-line to dry. Very quickly the more active water molecules move to the surrounding air as a thin layer of water vapor surrounding the wet shirt. Now if there is little or no wind, the air around the shirt is quickly moved to the point of near-saturation -- it becomes more difficult for additional water molecules to move from the wet shirt to the surrounding air. Thus drying the shirt becomes an all day affair. On the other hand, if there is a wind, the wind removes the near-saturated layer surrounding the shirt and makes way for additional water molecules to make their move from the liquid to the gaseous state.
 
 
    
 

30. Condensation. In the process of condensation, water vapor is converted to liquid water. This process, which releases 600 calories of heat (the latent heat of condensation), is an important source of energy for our storms. Though less important than the movement of warm ocean water into higher latitudes, this process does assist in the transport of excess heat from the tropical regions to higher latitudes.
 
    
 

While melting, freezing, evaporation and condensation are well known to most, there are two other change of state processes whereby water vapor or its derivative makes a state change.
 
31. Sublimation. In one of these, sublimation, ice is converted directly to a gas without going through the liquid state. Probably the best known example of this process is dry ice. Another example would be where snow is evaporated (solid to a gas) without passing through the liquid state. In the process of sublimation, 680 calories of heat energy are absorbed by the gas molecule (80 calories coming from the latent heat of fusion and 600 from latent heat of evaporation).
 
    
 

32. Deposition. The opposite of sublimation is deposition. In this process water is moved directly from gas to the solid state (ice), again without passing through the liquid state. Snow and frost are probably the best known examples of deposition. In the process of deposition, 680 calories of heat energy are released into the atmosphere -- 600 calories from the latent heat of condensation, 80 calories from the latent heat of fusion.
 
    
 

33. The graphic below is derived from your textbook. In this summary of the various processes, note that melting, evaporation and sublimation result in heat energy being taken from the environment. Condensation, freezing and deposition result in heat energy being released into the environment.
 
 

34. When condensation occurs and water vapor is converted into a liquid, the results include dew, fog and the precursor to rain, snow, sleet and hail -- clouds. For condensation to occur, two conditions must first be met.
 
 

35. In the first condition, the air containing the water vapor must be saturated. As we have indicated earlier this can be brought about in either of two ways: (1) the air can be cooled below the dew point. In this instance, the ability of the air to hold moisture is exceeded and a change of state occurs. By far, this is the most typical means by which condensation is brought about. But there is a second way condensation occurs. (2) Condensation may also result when sufficient moisture is added to an air parcel to exceed the air's ability to hold the water vapor. Again, in such instances, a change of state occurs.
 
But there is a second condition that must also be met in order for condensation to occur, and that is there must be a surface present upon which the water vapor can condense. Such surfaces are called condensation nuclei and may include dust particles, spores or even such objects on the ground as leaves, grass and the like. As a rule, such particles are in abundance in the lower levels of the atmosphere.
 
A special kind of nuclei, one that actually attracts water vapor unto itself, is salt. Such water-seeking nuclei are called hygroscopic nuclei. When present in abundance, condensation may occur when relative humidities are in the 80 to 90 percent range.
 
 
    
 

36. Dew. One of the most common types of condensation is dew. In many respects dew is like the "sweat" that forms on that cold drink can or glass of iced tea you enjoy so much on a hot summer day. We all know that the "sweat" is not oozing out of the can or glass. Where does it come from? On the typical warm summer day, the air contains a great deal of water vapor. As this water vapor comes in contact with the cold beverage container, the cold container, via conduction, chills the air immediately adjacent to itself thus dropping the air temperature below the dew point. The excess moisture is condensed out of the air onto the container.
 
Dew is formed in much the same way. Dew is most likely to occur on clear, calm nights. Such conditions encourage rapid cooling of the surface, and the ground becomes much cooler than the overlying air. Conduction then cools the air to slightly below its dew point (which in order for dew to form must be above 32 degrees F) and the water vapor condenses on the nearest available surface which may be grass, leaves, the hood of your car or whatever. Dew does not "fall" (as does rain, snow and the like), but rather it condenses upon.
 
Dew can sometimes be very heavy -- say after a very warm, humid day (high mixing ratio and dew point), then followed by a relatively cool night (temperatures falling below the dew point on such a day may well produce large amounts of liquid since the very warm air is almost saturated to begin with).
 
At other times, the quantity of dew is small and soon evaporates as the morning temperatures begin to rise. These conditions would be most likely when the mixing ratio is relatively low (thus a low dew point). Night-time cooling may cause the temperatures to fall below the dew point, but the air, because it holds much less moisture at low temperatures, has less to give up as dew once the dew point is reached.
 
Let's see if we can take the following East Texas summer-time ditty apart.
 
When the dew is on the grass
Rain will never come to pass.
 
In the summer, one typically awakes (assuming you are up and moving before 10:00AM or so) to clear skies -- the result of high pressure (Continentality) over the land. Such conditions have encouraged night-time cooling and the formation of dew. Typically as the morning wears on, heating of the land results in the formation of cumulus clouds. These conditions intensify into the late afternoon hours. And while afternoon thunderstorms are often prevalent in the area in the late afternoon, your chance of receiving rain on any given summer afternoon is about 20 percent (how many times have you heard the weatherman say that during the summer months?). So clear skies in the morning often means not enough time before dark arrives for widespread thunderstorms to develop to the point of generating rain for you beyond about a 20 percent chance.
 
Compare this to the second stanza of the ditty.
 
When grass is dry at morning light
Look for rain before the night.
 
A couple of things come immediately to mind that could make the grass dry at morning light (air temperature didn't get to the dew point, not enough moisture in the air -- a relationship here?). Consider the following. You get up in the morning and the sky is cloudy. The clouds have prevented significant cooling of the land during the night -- no dew. Too, can you see that in this instance you have about a two to three hour head start (compared to most summer days) on cloud formation? Could it be that by the late afternoon we will have, not isolated thunderstorms, but instead widespread thunderstorms (thus a better chance for you to get rained on)?
 
 

 

37. Frozen Dew. Many of you have no doubt seen frozen dew -- sometimes called "Black Frost." Frozen dew is just what it sounds like it is. First dew forms (the dew point is above 32 degrees F). The temperature then continues to drop to points below 32 degrees F resulting in the dew droplets freezing. This can be a very striking early morning sight -- the ground is covered with small drops of frozen water glistening in the early morning light. Kick the grass and the droplets scatter like a bucket of small pearls. Something else (though not condensation as such) is typically found in association with frozen dew. That is frost.

 
 

38. Frost. Frost you will remember is actually a form of deposition, not condensation. Frost is formed when cooling air hits the dew point at a point below 32 degrees F. Water vapor is then converted directly to ice without passing through the liquid state. The result is a crystalline, frosty white covering of available surfaces. It is the crystalline structure visible to the eye that indicates that deposition has occurred. Had we moved from the gaseous water vapor to water and then to ice, we would have frozen water (droplets or sheets of ice) not crystals.
 
And now thinking back to comments about frozen dew, can you see why frozen dew is often accompanied by frost? First dew is formed (the dew point is above 32 degrees F), the temperature continues to drop (the dew/water droplets are frozen), and should the temperature continue to drop below 32 you will have deposition (the water vapor goes directly from water vapor to a solid) and the formation of frost.
 
 

    

39. Let's use frost and the danger it presents to agriculture crops and domestic vegetation to pull several themes together we have touched upon up to this point in the course. In the graphic below we have an apple orchard planted on a north-facing slope. What advantages can you see in such a location?
 
The presence of a water body that will have the temperature-moderating effect on its immediate surroundings should be one such advantage.
 
The north-facing slope is another. Think about when fruit trees are most vulnerable frost damage. It's not during the winter as you might initially think. During the winter, most trees are dormant and generally immune to such damage. Rather most are most susceptible during the early spring months -- when they are in flower. In early spring, late incursions of cold air are always a danger. If the orchard is planted on a south-facing slope, the Sun more readily warms the slope resulting in premature flowering of the trees. An orchard planted on a north-facing slope will not receive as direct Sun rays and the relative cooler surface will result in a later flowering of the trees (hopefully giving the tree another week or two of protection from a late cold snap).
 
And finally there is the slope itself. As you travel around the country, observe where most fruit trees are planted -- not on flat land, but rather on slopes. There is a good reason for this. It is inversions, not the cold air masses themselves, that present the greatest danger to many of our agricultural crops. These shallow (air is not a good conductor of heat) layers of cold air lay waste to millions of dollars of crops in the United States annually. By planting on slopes, the cold air as it forms will tend to move down slope and hopefully spare the crop/orchard.
 
Other frost-inhibiting practices would include:
 
The covering of sensitive plantings (rosebushes and the like) around homes. The covering is not there to keep the cold air off of the plant, but rather to keep the warm air rising from the Earth's surface trapped under the covering thus assisting the plant to ward off the cold temperatures. You may have seen urban "farmers" cover individual plants in their backyard gardens with cups and the like. Same factor is at work.
 
While more rare, some commercial growers, especially if they are producing valuable tree crops such as citrus, avocados and the like will employ orchard heaters. The heaters are not placed in the orchard to warm the trees, but rather each heater acts as a small low pressure cell and, if used in sufficient numbers, can effectively ward off freezing temperatures by stirring the air (acting as small wind generators) and reducing the damaging effects of the inversion threatening the crop/orchard.
 
A variant of the orchard heater would be permanently constructed wind machines (and in very rare instances the use of helicopters) to stir the air and break up the inversion.
 
And finally, how many of you have ever watched a television news report of efforts of farmers to ward off freezes by spraying their orchards with a light mist of water? Seems kind of unbelievable when you first see it. Here the farmers are spraying a fine mist of water over their groves and in effect covering everything in a sheet of ice. Looks like they are killing the plants, not trying to save them. While the crop itself may well be lost, remember the most valuable thing is the tree -- it may have taken five or more years to get to the point of producing a crop. Keep in mind that most such trees can stand temperatures of 27 to 28 degrees F for several hours. By covering the trees with a coat of ice (whose temperature is 32 degrees F) they are in effect insulating the tree from the colder/killing temperatures. But such an activity is tricky -- too little water and the trees will be killed for lack of insulation, too much water and you run the risk of breaking the limbs and even the main trunk itself.
 
 

 

40. Fog. Like dew (but unlike frost which is a type of deposition), fog is also a form of condensation. We can think of fog as a low cloud -- in that the two are almost identical in terms of their appearance. Clouds are usually defined as being above 50 feet, fog less than 50 feet above the surface. While they may look alike, in fact the two are formed very differently. Clouds, as we will see in the next section, are typically formed as a result of adiabatic cooling brought about by cooling of the air -- normally through uplift. Fog are typically formed through either radiational cooling of the surface which in turn then cools the air above, or by the movement of relatively warm air over a cooler surface. In both instances, the air is dropped to its dew point.
 
 

 

41. Advection Fog. Close to home here in southeast Texas, the widespread fogs of fall along the coast are generally advection fogs. Here, wind brings relatively warm and humid air inland from the Gulf. As the air passes over the cooler land surface, the air temperature is reduced to the dew point and fog forms.
 
You will often hear someone say that a fog "burns off." This statement implies that the Sun heats the fog and evaporates it. How many of you have ever heard someone say that a fog is "lifting?" This statement implies that a fog is evaporated from the bottom upward. Which is correct -- how does fog dissipate? Well, from our discussion to this point, we know that if cooling the air causes fog, then heating the air must cause it to dissipate.
 
Let's consider the "burn off" scenario first. The Sun striking the fog, heating the air with the resulting evaporation and dissipation of the fog. Sounds good, but think about that for a moment. What's going to heat the air? Won't the albedo of the air (the fog) be high? Not likely that a great deal of heat energy will be absorbed in this manner. Fogs do "burn off," but the tendency is to "burn off" from the ground -- up. In other words, we are going to have to heat the land, which in turn will heat the air above it, which will raise the capacity of the air to hold water -- the result being evaporation and dissipation of fog. Fogs do "burn off" or "lift," but they tend to do it from the ground up and around the edges first. Of the two, to say a fog "lifts" would probably be closer to describing the process.

 
 

42. Radiation Fog. A second type of fog, and one that is widespread in southeast Texas, is radiation (or ground) fog (Photo 1 below). We have all seen examples of radiation fog. Most typical on crisp fall days, radiation fog often looks like a low smoke covering area pastures. Radiation fogs are commonly associated with inversions where the cold surface cools the overlying air to the dew point. Because air is a relatively poor conductor of heat, most radiation fogs tend to be relatively shallow events. While some such fogs may extend 10 or even 20 feet (maybe to 50 or 100 feet with a light, stirring wind) off the ground, a more typical radiation fog is maybe chest-high. Ever seen a pasture where only the heads of cows were visible? Such fogs, especially when thick, make excellent playgrounds for little kids. Stand up -- now I see you. Crouch down -- where did they go?
 
Because the cold air associated with ground fog is heavy/dense, some of the more striking radiation fog events are to be seen when these fogs move downslope to lower elevations. Photos 4 and 5 below depict a ground fog in the process of moving from its place of formation in the uplands into a lower valley. Such events, if you can catch them as they begin, almost have the appearance of a "waterfall" of fog.

 
 

 

43. Valley Fog. Valley fogs represent a ground fog variant. As the name implies, these fogs are associated with valleys and are frequently caused by either cold air or ground fog draining into a lower elevation.

 

44. Steam or Evaporation Fog. Some "valley" fogs are caused by a completely different process. In valleys we often find creeks, rivers, lakes, ponds and the like. In the fall, the temperature of these water bodies tends to be warmer than the surrounding land. As the colder surrounding air drains down into these low-lying areas, the temperature of the moist air above the water is dropped to its dew point and fog frequently forms. Such fogs are more properly termed "steam" or "evaporation" fogs. This is the type of fog you may have seen overlying a swimming pool in the fall, or maybe a small pond in a pasture.
 
A fall Sunday morning a number of years ago when my son was maybe about four or five years old, I was relaxing reading the paper when he screams out, "Daddy, the man's on fire!" I jumped up out of my chair and rushed to the front door and looked out across the street to one of the local schoolyards where a bunch of boys were playing basketball. And sure enough not one, but several, of the boys were "on fire." Steam/smoke was rising off them in such quantities as to make a youngster think they were "on fire." In fact, it was an evaporation fog -- caused by the cold morning air condensing the evaporating moisture from the player's skin.

 

 

45. Upslope Fog. And finally, there is the upslope fog. As the name implies, this fog is formed as a body of air moves upslope (and cools). Imagine a parcel of air moving in off the Gulf. As it moves from Galveston to Dallas to Oklahoma City toward Denver, the land is gradually increasing in elevation. At some point the expanding air will cool to its dew point and as so often-times happens, a fog (a cloud if you will) just appears out of nowhere.
 
    
 

You have now completed Unit 9: Moisture in the Atmosphere. You might wish to check your knowledge of the material presented in this section by working through the Short Answer Review Questions, Multiple Choice/True-False Quiz Questions and the Drop-Down Statements available for Exam 3. To return to the top of the page.

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