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 the container.
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.
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 4: Humidity, Condensation, and Clouds
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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 the air) 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: Clouds in 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
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 4: Humidity, Condensation, and Clouds
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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?
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.
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 range against 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 Breaks by 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.
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, 5th
Instructor’s Manual
Chapter 4: Humidity, Condensation, and Clouds
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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).
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.
4
Actual vapor pressure indicates the number of water vapor molecules in an air parcel,
whereas saturation vapor pressure describes how much water vapor is necessary to make the air
saturated at any given temperature. Actual vapor pressure would be the same as saturation vapor
pressure if the air were saturated.
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
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Instructor’s Manual
Chapter 4: Humidity, Condensation, and Clouds
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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.
cumulonimbus. g. nimbostratus. h. cumulonimbus.
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
d. cumulonimbus.
e. cirrus.
Chapter 4: Humidity, Condensation, and Clouds
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f.
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 rise above 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 to the 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 at cooling the skin. In contrast, cooking pizza doesn't add very much water
vapor to the air, keeping the relative humidity lower.
6.
fog.
The ocean surface is cooler off the west coast, thus creating better conditions for advection
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 dewpoint. The outside windshield cools the warmer air in contact with it to its dew-point temperature
which is still below freezing and frost forms.
10.
In polluted air, condensation begins on hygroscopic particles when the relative humidity is
less than 100 percent. Condensation removes water vapor from the air which lowers the dew-point
temperature. This increases the spread between air temperature and dew-point temperature, and
keeps the relative humidity at less than 100 percent.
11.
Fog droplets are so tiny that they settle very slowly. As long as the air remains saturated,
fog is maintained as water vapor condenses onto nuclei forming new fog droplets.
12.
Latent heat of condensation is released as fog droplets form. Also, fog absorbs infrared
energy radiated from the surface and re-radiates this energy back toward the surface.
13.
You see your breath because the warm, moist air from your mouth mixes with cooler air in
the atmosphere. The mixture is saturated and condensation occurs with the exhaled breath. The air
temperature does not have to be below freezing for this occur.
14.
If the rain is light or moderate and steady, the cloud is most likely nimbostratus. If the rain
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 4: Humidity, Condensation, and Clouds
Page 6 of 7
falls in heavy showers, and lightning, thunder, or hail accompany it, then the cloud is probably
cumulonimbus. (Careful: cumulus congestus may also produce heavy showers).
15.
Cumulus. Cumulus are individual-element clouds that can block the sun when passing
overhead. After they pass by, the sun will again be visible.
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 4: Humidity, Condensation, and Clouds
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