|
|
|
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
- 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.
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.
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.
- When the dew is on the grass
- Rain will never come to pass.
- When grass is dry at morning light
- Look for rain before the night.
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.
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 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.
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?
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.
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.
Copyright 2014, The START Group, All Rights Reserved