Chapter 4
Humidity, Condensation and Clouds
Summary
The transformation of water from the gaseous to the liquid or solid state is an important source of energy in many meteorological processes and also makes weather phenomena visible to us. Some of the ways of measuring and quantifying atmospheric water vapor concentrations are discussed in this chapter.
The chapter begins with a short description of the hydrologic cycle. The important concept of saturation is developed next. Saturation represents an effective upper limit to the amount of water vapor that may be found in air and is a function of air temperature. Several different parameters can be used to express the air's humidity. Students are shown that the dew point temperature provides a better absolute measure of the air's water vapor content than a more commonly used parameter, relative humidity. The heat index is a practical measure of the effect that a combination of hot temperatures and high humidity have on our perception of temperature.
Condensation occurs when moist air is cooled below its dew point temperature. Dew or frost forms when this occurs at ground level. When air above the ground becomes saturated, water vapor will condense onto small condensation nuclei in the air and will form a cloud composed of small droplets of water. Fog is just a cloud that forms at ground level and can be produced either by cooling moist air to saturation or evaporating and mixing water vapor into air.
Finally, students are shown that a seemingly infinite variety of cloud forms may actually be classified into ten basic types according to their appearance and the altitude at which they form. Photographic illustrations of each of the basic cloud types are given and key characteristics that can be used to distinguish different kinds of clouds are discussed. This should allow students to develop a reasonable proficiency at cloud identification.
Teaching Suggestions, Demonstrations and Visual Aids
1. The dew point temperature can be determined by filling a metal cup or container with warm tap water (at least 50° F in winter and 75° F in summer). Place a thermometer in the water and begin to slowly add ice while continuously stirring the mixture. Read the temperature when the first sign of condensation begins to appear on the outside of thecontainer. This reading should be within about 3° F of the dew point temperature.
The reading above could be compared with a simultaneous measurement made using a sling psychrometer and with the current dew point temperature announced on the local weather radio broadcast. Students could be asked to measure the dew point temperature and the relative humidity indoors and outdoors and asked to account for any similarities or differences.
2.Saturation can be illustrated by filling a test tube with a few grams of solid iodine. The iodine will sublimate and fill the air in the test tube with iodine vapor. The iodine vapor has a purple-pink color which should just be visible at normal room temperature when viewed against a white background.
The effect of temperature on the saturation vapor pressure can be demonstrated by immersing the bottom of a second test tube in hot water. The gas in the test tube will have a noticeably darker color indicating a higher vapor concentration. If this warm test tube is cooled, iodine vapor will be deposited on the sides of the test tube. With some care two test tubes (warm and cool) may be passed around a small class for examination. The iodine vapor is also visible when the test tubes are placed on an overhead projector. Another good demonstration of the effect of temperature on vapor pressure has been given by W.S. Richardson and R.F. Jones (ref: "Demonstration of Vapor Pressure," J. Chem. Educ., 64, 968-969, 1987).
3.The strong dependence of saturation water vapor pressure on temperature can be demonstrated using several glass beakers with different volumes. The saturation vapor pressure doubles approximately with every 20°F increase in temperature. For example, 250 mL, 500 mL, 1000 mL and 2000 mL beakers can be used to represent the saturation vapor pressures at 30°, 50°, 70°, and 90° F (the saturation vapor pressures at these temperatures are approximately 6, 12, 25, and 50 mb). The concept of relative humidity can be shown by partially filling one of the beakers with water (meant to represent water vapor). By then transferring the water in the first beaker to another larger (warmer air) or smaller (cooler air) beaker, students can see how air temperature will change the RH even though the actual amount of water vapor in the air remains the same. Cooling the air to the dew point can be demonstrated by progressively transferring the water into smaller and smaller beakers (cooling theair) until a beaker is finally filled to capacity.
4.A dramatic demonstration of cloud formation in a bottle has been described by C.F. Bohren (ref: Cloudsin a Glass of Beer,pp. 8-14, John Wiley and Sons, New York, 1987). Place a small amount of water in a large thick-walled bottle or flask. Close the top of the bottle with a rubber stopper. Connect a pump (an ordinary bicycle pump works well) to the bottle with a tube through a hole in the stopper. Pump air into the bottle. If the stopper has not been inserted too firmly, it will suddenly pop as air is being pumped into the bottle and will allow the air to expand outward and cool. A faint cloud should be visible in the bottle.
This demonstration can be used to explain the formation of haze, fog, and clouds, as well as the role that condensation nuclei play in their development. The chapter by Bohren also includes a good demonstration of cloud droplet formation on grains of salt.
5.Dry ice offers a convenient way of producing thick clouds. Ask the students why a thicker cloud is formed when one blows on the piece of dry ice. Students should also understand why the cloud sinks.
6.Photographic images greatly enhance a discussion of clouds and cloud identification. Use a portion of the cloud image collection to illustrate the common cloud types and their characteristics. Then show additional cloud images and ask the students to identify the cloud type. If possible, include photographs of cloud patterns from the local area. This will give the students some experience identifying clouds they are likely to see outside during the course. In some areas, students should be made aware of local topography that can be used to judge low cloud base heights. In this way, interesting weather features or events can be photographed and then shown and discussed in class.
7.Measure relative humidity with a sling psychrometer and a psychometric table. Discuss the difference between dew point and wet bulb temperatures. Why are there different psychometric tables for different atmospheric pressures?
Ahrens Essentials of Meteorology, 5thChapter 4: Humidity, Condensation, and Clouds
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8.Using the Sky Identification/Name that Cloud section of the ThomsonNow web site, invite students to participate in a “Name that Cloud” competition during class.
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Student Projects
1.Have students plot daily high and low temperatures together with average dew point temperature and an estimate of cloud cover for a period of a week or two. Were the coldest nighttime temperatures observed with clear or cloudy skies, dry or humid conditions? Have students plot the daily temperature rangeagainst one of the moisture variables. How much variation is there in the dew point temperature from day to day? Can students explain sudden changes in the daily average dew point? Is there any correlation between apparent visibility and dew point temperature?
2.Students could build and test one of the simple hygrometers described in Chapter 4 (Moisture) of Hands-On Meteorology.
3.During certain times of the year, students can observe the formation of dew, frost or fog. Have the students record weather data on these days as well as days on which dew, frost and fog did not form.
Is frost ever observed when the measured overnight minimum temperature remains above freezing? Is the layer of dew or frost thicker on some objects than other objects? Does the color or composition of the object have any effect on the formation of dew or frost? Is the formation of dew or frost thicker at ground level or on objects above ground level? An excellent discussion of how environmental factors affect the formation of dew and frost can be found in What Light Through Yonder Window Breaksby C.F. Bohren. Many of Bohren's experiments and observations could serve as the foundation of a student project.
4.Students should be encouraged to observe, photograph, and try to identify different types of clouds. Students could observe and record the sequence of different types of clouds that are associated with the approach and passage of a low pressure center or front.
Ahrens Essentials of Meteorology, 5thChapter 4: Humidity, Condensation, and Clouds
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5.Use the Moisture and Stability/Moisture Graph activity on the ThomsonNow web site to demonstrate the relationship between temperature, vapor pressure and relative humidity during heating, cooling, evaporation and condensation. Describe these relationships in your own words.
Ahrens Essentials of Meteorology, 5thChapter 4: Humidity, Condensation, and Clouds
Instructor’s ManualPage 1 of 7
Ahrens Essentials of Meteorology, 5thChapter 4: Humidity, Condensation, and Clouds
Instructor’s ManualPage 1 of 7
6.Use the Adiabatic exercise on the ThomsonNow web site to answer the following questions.
(a) With a temperature of 10.0oC and a dew point temperature of 0.0oC on the upwind side of the Sierra Nevada mountain range, what will be the altitude of cloud formation on the upwind side, and the ground-level temperature on the downwind side, of air crossing the mountain range? (Answer: 1300 m; 17.0oC)
(b) Same as above, but use a temperature of 10oC and a dew point temperature of 5oC. (Answer: 700 m; 19.5oC).
Ahrens Essentials of Meteorology, 5thChapter 4: Humidity, Condensation, and Clouds
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Answers to Questions for Review
1.Within the atmosphere, there is an unending circulation of water. Since the oceans occupy over 70 percent of the earth’s surface, we can think of this circulation as beginning over the ocean. Here, the sun’s energy transforms enormous quantities of liquid water into water vapor in a process called evaporation. Winds then transport the moist air to other regions, where the water vapor changes back into liquid, forming clouds, in a process called condensation. Under certain conditions, the liquid (or solid) cloud particles may grow in size and fall to the surface as precipitation—rain, snow, or hail. If the precipitation falls into an ocean, the water is ready to begin its cycle again. If, on the other hand, the precipitation falls on a continent, a great deal of the water returns to the ocean in a complex journey.
2.Condensation is the transformation of water vapor into liquid, forming clouds. Under certain conditions, the liquid (or solid) cloud particles may grow in size and fall to the surface as precipitation—rain, snow, or hail.
3.Condensation nuclei are microscopic bits of dust, smoke, and salt from ocean spray that serve as surfaces on which water vapor may condense. These nuclei allow condensation to occur, clouds to form, and therefore assist in the generation of precipitation.
4Actual vapor pressure indicates the number of water vapor molecules in an air parcel, whereas saturation vapor pressure describes how much water vapor is necessary to make the air saturated at any given temperature. Actual vapor pressure would be the same as saturation vapor pressure if the air were saturated.
5.Temperature.
6.a. The ratio of the amount of water vapor actually in the air to the maximum amount of water vapor required for saturation at that particular temperature (and pressure). b. Because the amount of water vapor required for saturation depends strongly on the temperature. c. By changing the amount of water vapor in an air parcel, and by changing the temperature of the air.
7.When the air temperature is high and the relative humidity low, perspiration on the skin evaporates quickly, often making us feel that the air temperature is lower than it really is. However, when both the air temperature and relative humidity are high and the air is nearly saturated with water vapor, body moisture does not readily evaporate; instead, it collects on the skin as beads of perspiration. Less evaporation means less cooling, and so we usually feel warmer than we did with a similar air temperature, but a lower relative humidity.
8.Because the very cold temperatures of polar air mean that the amount of water vapor required for saturation is very small. Since this amount is the denominator of the fraction in the RH equation, a small value gives a large RH.
9.Because the wet-bulb temperature is the lowest temperature that can be reached by evaporating water into the air. On a hot day when the wet-bulb temperature is low, rapid evaporation (and, hence, cooling) takes place at the skin’s surface. As the wet-bulb temperature approaches the air temperature, less cooling occurs, and the skin temperature may begin to rise.When the wet-bulb temperature exceeds the skin’s temperature, no net evaporation occurs, and the body temperature can rise quite rapidly. Fortunately, most of the time, the wet-bulb temperature is considerably below the temperature of the skin.
10.a. the temperature to which air would have to be cooled (with no change in air pressure or moisture content) for saturation to occur. b. When the difference between the dew point and the air temperature is large, the relative humidity is low. When the difference between the dew point and the air temperature is small, the relative humidity is high.
11.With the use of psychometric tables such as those included in an appendix.
12.Dew: when the air cools to the dew point—the temperature at which saturation occurs. Frozen dew: If the air temperature should drop to freezing or below, the dew will freeze, becoming tiny beads of ice. Visible frost: forms on cold, clear, calm mornings when the dew-point temperature is at or below freezing. When the air temperature cools to the dew point (now called the frost point) and further cooling occurs, water vapor can change directly to ice without becoming a liquid first—a process called deposition.
13.Advection fog: Fog that forms when surface air is cooled to its saturation point by warm moist air moving over a cold surface. Radiation fog: Fog produced by the earth’s radiational cooling.
14.a. Radiation fog: Fog produced by the earth’s radiational cooling. b. Advection fog: Fog that forms when surface air is cooled to its saturation point by warm moist air moving over a cold surface.
15.When two unsaturated bodies of air mix, and the resulting mixture is saturated.
16.High: cirrus, cirrostratus, cirrocumulus. Middle: altostratus, altocumulus. Low: stratus, stratocumulus, nimbostratus. Vertical development: cumulus, cumulonimbus.
17.The sun shines brightly through cirrostratus, but appears as a dull yellow disk with altostratus.
18.a. altocumulus. b. cumulonimbus. c. cirrostratus. d. cumulonimbus. e. cirrus. f. cumulonimbus. g. nimbostratus. h. cumulonimbus.
Answers to Questions for Thought and Exploration
1.After you come indoors on a cold day, your glasses are cold but the air in contact with the lenses is warm. The warm air cools by conduction, become saturated, and the water vapor condenses onto the lenses.
2.Sure. Nighttime temperatures can get quite low in the desert, causing the relative humidity to riseabove 90%.
3.Yes. Air with a relative humidity greater than 100% is said to be supersaturated.
4.Higher.
5.The process of cooking the turkey, ham, potatoes and vegetables involves adding water vapor tothe air, raising the relative humidity. The air feels "cozy and comfortable" because of the added humidity.The air feels warmer because with the high relative humidity, perspiration is relatively ineffective atcooling the skin. In contrast, cooking pizza doesn't add very much water vapor to the air, keeping therelative humidity lower.
6.The ocean surface is cooler off the west coast, thus creating better conditions for advection fog.
7.Cirrus clouds. Cirrus clouds are less effective than stratocumulus clouds at absorbing and radiating infrared radiation toward the surface.
8.Icebergs chill the air in contact with them to below the dew-point temperature.
9.The windshield is quite cold. The car apparently travels into warmer air with a higher dew-point.The outside windshield cools the warmer air in contact with it to its dew-point temperature which is stillbelow freezing and frost forms.
10.In polluted air, condensation begins on hygroscopic particles when the relative humidity is lessthan 100 percent. Condensation removes water vapor from the air which lowers the dew-point