Clouds and Climate Change

(Full 21-page Text in HTML)

Glenn E. Shaw
Geophysical Institute
University of Alaska at Fairbanks



Chapter 1: Clouds and Cloud Formation
   Cloud Classification and Occurrence
   Cloud Composition
   The Underlying Physics of Cloud Formation
Chapter 2: Clouds and Climate
   Changes in Earth's Climate from Clouds and from CO2
   Cloud/Climate Feedbacks
Questions and Projects
Appendix I. Cloud Patterns on Other Planets
Appendix II. The Energy of Clouds
Appendix III. How to Form a Cloud in a Bottle
Additional Reading Material
Bibliographic Information

Index of Items
*Please note that the following links take you to the item's instance in the text
figure 1 Earth from Space
figure 2 The Major Cloud Types
figure 3 Global Cloud Maps
figure 4 Water Droplets on a Spider Web
figure 5 Six-Sided Ice Crystals
figure 6 John Aitken
figure 7 Wave Clouds
figure 8 Simple Feedback Control System
figure 9 Climate Feedback Loops
figure 10 Cumulonimbus
figure 11 Cloud in a Bottle
table 1 Vapor Pressure (in Millibars) of Water at Different Temperatures
table 2 Energy in a Cumulonimbus


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The color photograph of the full Earth taken by astronauts on their way to the moon ushered in a new age. Looking at Earth from space, we cannot see our political boundaries. Nor can we usually see the effects of our own activities.

What was and is striking about the image of the Earth from space is its exquisite and delicate beauty: the patterns of white clouds against the deep blue of the oceans. These patterns alter from moment to moment, from day to day, from season to season. They are probably quite different now than they were a billion or 4 billion years ago. Despite the fact that cloud patterns, amounts, and thicknesses change, the world's climate has remained remarkably constant, and no doubt clouds play an important role in determining and regulating that climate. Defining the role of clouds in climate is one of the trickiest problems for scientists developing computer models of climate. (The modelers might dearly love to have the clouds go away!) It is also essential to determining whether and how human activities are upsetting the Earth's delicate balance. To perceive, for example, how pollution may be affecting clouds, we must understand what a cloud is and what forces may shape or alter it.

These are questions that this module addresses. It introduces the basic features and classifications of clouds and cloud cover, and explains how clouds form, what they are made of, what roles they play in determining climate, and how they both cause and are affected by climate changes.

Glenn E. Shaw
University of Alaska at Fairbanks


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Looking at the Earth from space, we see that the planet is misnamed. The beautiful and fragile-looking sphere in Figure 1 shows very little earth. Perhaps it should have been named "ocean cloud." Clouds are not merely beautiful; they are a manifestly important part of our planet. Without clouds there would be no rain, no snow, no glaciers, no rivers, no lakes. In fact, the planet would probably be too hot for life. Even the most rudimentary attempt to understand the climate of the Earth must include a planetary cloud system. If the Earth had no clouds the oceans would boil away. If it were completely covered by clouds, there would be glaciers at the equator. Clouds are like white clothes in the sunshine. They reflect light, remain cool, and protect what is underneath from overheating.

Clouds can also heat up the Earth's surface. During the day clouds cool the Earth by reflecting sunlight to space. At night they act like a blanket to keep the Earth warm. Their warming effect is well known to farmers; crop-damaging frost is more likely on nights with clear skies. Clouds determine climate, but the climate, in turn, is instrumental in determining cloud cover.

Figure 1. Earth from Space
Planet "ocean cloud."
Photo courtesy of the National Oceanic and Atmospheric Administration.


Chapter 1: Clouds and Cloud Formation

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Cloud Classification and Occurrence

Clouds on Earth appear in seemingly endless variety, from wisps of fog or steam over lakes to huge and imposing thunderstorm anvils. To classify them may seem nearly impossible, but in 1803 the English naturalist Luke Howard (1772-1864) published a classification scheme based on appearance and height. It was by no means the first, but it had such logic and simplicity that it met with immediate and wide-ranging success and remains to this day the basis of cloud classification. Howard's contribution demonstrated that meteorological phenomena are not just random.

Some major cloud types retain the Latin names that Howard gave them (see Figure 2). Cirrus (Latin for "curl," or "hair") are generally thin and wispy, frequently exhibiting a feathery or filamentary appearance. They are the highest clouds. Cumulus ("heap") clouds have a piled-up look. They are cauliflower-like systems, often consisting of roughly spherical bubbles with flat bases. Stratus ("layer") clouds are sheet-like, with little or no internal structure; they are frequently associated with a dull grey sky.

Howard's classification system subdivides these cloud forms by height. Low-lying, sheet-like clouds that sometimes almost touch the ground are stratus. Those at midaltitude are altostratus. (They may show the Sun or Moon as a bright, diffuse glow.) The highest ones are cirrostratus.

Figure 2. The Major Cloud Types

In the 1960s, following the launch of the first meteorological satellites, views from space showed that clouds often congregate into huge systems thousands of kilometers in extent, and that some regions of the globe tend to be more cloudy than others. Even with the breadth provided by satellite views, though, compilation of atlases showing cloudiness patterns has proven extremely difficult. A major problem is that from any angle our view of clouds is incomplete. Looking down from high above the atmosphere we see only the top layer of clouds; those beneath are hidden from our view.

Figure 3 shows two kinds of global cloud map. Large-scale patterns are readily apparent in both. One notable feature is the persistent cloud bands in the middle and high latitudes. We see more or less the same pattern in both the Northern and Southern hemispheres. These great bands of cloudiness migrate poleward in summer and toward the equator in winter. The dearth of clouds at latitudes around 30 degrees, the belt where we find the great deserts, is associated with large-scale sinking motions of dry air in the atmosphere.

These are some of the main cloud patterns on our planet. Appendix I discusses cloud patterns on other planets.

Figure 3. Global Cloud Maps
Cloud coverage (in tenths of the sky covered with cloud).
A: Observed by satellite.
B: Calculated with a general circulation model.
From A. Slingo, R. C. Wilderspin, and R. N. B. Smith,
1989: Effect of improved physical parameterizations on simulations of cloudiness and the the Earth's radiation budget. Journal of Geophysical Research, 94. Reprinted by permission.

Cloud Composition

In 1672, the German physicist Otto von Guericke, perhaps inadvertently, put forward the idea that clouds were made of tiny suspended bubbles when he called the small particles he produced in a crude cloud chamber bullulae, or bubbles. The bubble misconception persisted for two centuries. The droplets, such as those caught on a spider web on a foggy morning (see Figure 4), do look like bubbles, but it's an illusion.

Figure 4. Water droplets on a spider web
From The Anatomy of Nature, by Andreas Feininger, Crown Publishers, 1956. Photograph by Andreas Feininger (copyrighted). Reprinted by permission.

In 1846, it was found that fog particles did not burst on impact, as bubbles would. And around 1884, cloud droplets were collected and studied under a microscope, finally settling the matter. Clouds consist of water droplets.

When clouds become very cold, the water vapor condenses to form tiny six-sided crystals of ice, many of them like the snowflakes in Figure 5. Some clouds contain both water droplets and ice crystals. Water droplets do not freeze instantly when the temperature drops below freezing but supercool to varying extents first. Supercooled liquid water is water at a temperature below freezing but still in the liquid state. As the temperature continues to drop, the fraction of frozen droplets increases until they all freeze at -40°C. The exact distribution of frozen to liquid droplets is not constant for all clouds but depends on the prevalence of trace atmospheric constituents called ice nuclei.

Figure 5. Six-Sided Ice Crystals
Snowflake forms showing hexagonal structure.
Snow Crystals, by W. A. Bentley and W. J. Humphreys, Dover Publications, 1962. Reprinted by permission.

The Underlying Physics of Cloud Formation

The effect of clouds on climate or climate on clouds depends on the physics of cloud formation. There are several concepts essential to understanding cloud-climate interactions.

Atmospheric Moisture: Evaporation and Condensation

If you pour some water into a jar and close it with a lid, the water begins to evaporate as water molecules leave the liquid surface and become airborne. (The level of liquid water in the jar will drop imperceptibly.) Molecules of liquid water are loosely bound to each other by electric forces, but they are mobile, being constantly knocked about and trading places. The airborne water molecules, in the gaseous or vapor phase, are essentially flying around loose in a state of frantic motion, colliding with each other and with air molecules billions of times a second and racing back and forth between collisions at a speed of about a kilometer a second (about the speed of a modern jet fighter).

If evaporation continues, the water dries up. But it is not a one-way process. The constant collisions of airborne water molecules exert a small but measurable force on the water surface and the interior of the jar: the gas or vapor pressure. As more and more water evaporates, the vapor pressure in the jar builds up, at first rapidly but later on more slowly. After a long while it approaches a constant value: the saturation vapor pressure. When this point is reached, the relative humidity is 100%. The gas (water vapor) and liquid are in equilibrium.

This does not mean that water molecules cease to evaporate, but that evaporation is balanced by another process: condensation.

Some of the myriads of molecules in the gas phase, speeding around above the water surface, smash into it and adhere. At equilibrium, equal numbers of molecules in a given surface area travel from water phase to gas phase as from gas phase to water phase in a set amount of time. In other words, evaporation and condensation rates are equal. If the vapor pressure builds up enough, more molecules on average enter the water than leave, and water condenses.

It takes energy to tear a molecule away from a body of water, so the water cools slightly each time that happens. Evaporation has a cooling effect. But if you add energy in the form of heat, the molecules move faster (the heat energy has become kinetic energy), and more get kicked out of the liquid into the air. So as the temperature rises, water evaporates faster. Because more gas molecules must be knocked down into the water to replenish the increased numbers that are bumped out, saturation vapor pressure (which we have defined as the pressure needed to maintain equilibrium) also increases. Remember, though, that the water vapor exerts this pressure in all directions. It presses not only against the water surface and the walls of the jar but against the surrounding air, which has its own pressure: atmospheric pressure. At room temperature saturation vapor pressure is only about 1/30 of atmospheric pressure. But at 100°C the molecules are moving so fast that saturation vapor pressure equals atmospheric pressure and the water boils. Table 1 lists the vapor pressure of water at different temperatures.

Table 1. Vapor Pressure (in Millibars) of Water at Different Temperatures
Temperature °C Temperature °F Pressure
-40 -40 0.189
-20 -4 1.25
0 32 6.10
20 68 23.4
40 104 73.8
100 212 1,000
Atmospheric pressure is approximately equal to 1,000 mb. Thus, water vapor pressure at 20°C (68°F) is about 1/40 that of the atmosphere.

To sum up this discussion:

  1. Heat dislodges molecules from a water surface and injects them into the atmosphere (evaporation).

  2. Water molecules in the gas (vapor) phase, in their chaotic thermal motion, occasionally strike the water surface and adhere (condensation).

  3. If the air is saturated, the evaporation rate (water to gas) and the condensation rate (gas to water) are equal, and increase with increasing temperature. The increase isn't linear with temperature; this subtlety is a key point in climate studies.

  4. At any given temperature there is a critical vapor pressure, called the saturation vapor pressure. If the vapor pressure exceeds this value, the condensation rate is greater than the evaporation rate and water vapor condenses; below this value the evaporation rate is faster than the condensation rate and water evaporates.

A familiar measure of moisture content in the atmosphere is relative humidity, the ratio of the existing vapor pressure to saturation vapor pressure. At a relative humidity of unity (sometimes expressed as 100%), the air is saturated with moisture, or, more precisely, the water vapor pressure equals the saturation vapor pressure.

The Supersaturated State

From the above, it should be clear that evaporation is the favored process when the air is unsaturated (the vapor pressure is below saturation) and that when the air is supersaturated (vapor pressure is higher than the equilibrium value), water will condense. So supersaturation is necessary for cloud formation. How does air become supersaturated? Standing water will humidify air, but only up to 100% relative humidity (saturation). Remember, though, that the saturation vapor pressure is lower for lower temperatures. We can supersaturate air by humidifying it (exposing it for a long time to a body of water), then quickly cooling it. This two-step process is the usual way of reaching a state of supersaturation.

At the turn of the century, two Scottish scientists, both inspired by the clouds that constantly form and evaporate on the moors, but working independently, conducted "cloud in a bottle" experiments that led the way to understanding how clouds form. C. T. R. Wilson, working at Cambridge University's Cavendish Laboratory in England, and John Aitken (Figure 6), an Aberdeen, Scotland, marine engineer, introduced water into a closed chamber, allowed it to humidify, and then cooled it by rapidly expanding it into another chamber. (Air cools when it expands and heats up when compressed; this is why a bicycle pump becomes warm when you're inflating a tire.) This brought the humid air into a state of supersaturation and, indeed, the water condensed out and formed a tiny cloud. (See Appendix III to learn how to perform a similar experiment.) Wilson and Aitken correctly surmised that this must be how clouds form in the atmosphere.

Figure 6. John Aitken
John Aitken, who from 1888 to 1892 performed a variety of experiments showing that clouds nucleate on motes of dust.
The Collected Papers of John Aitken, L. L. D., F. R. S. Edited by Cargill G. Knott, Cambridge University Press, 1923.

The Nuclei of Cloudy Condensation

Wilson and Aitken discovered something else: that it is more difficult to form a cloud after one or two expansion cycles. In fact, if they waited for the cloud droplets to settle between expansions, after several expansions no cloud formed at all.

They surmised that cloud droplets must form on invisible particles of dust in the air and that each cloud that settles out takes some of the dust with it. After several cloud-forming cycles, the dust would all be cleared out of the air and no more condensation could occur. We now know they were right. Some type of solid surface, called a cloud condensation nucleus, is necessary to initiate the condensation of water droplets. In subsequent experiments, Wilson demonstrated that clouds could still form in cleaned-out air, but only with extreme supersaturation corresponding to a relative humidity of 400%.

In the free atmosphere, although it is still necessary to exceed 100% relative humidity for clouds to form, such extreme supersaturations are not necessary. Atmospheric supersaturations are usually less than 1% (101% relative humidity). And there must be plenty of cloud condensation nuclei everywhere, because even in the extremely clean air over oceans or polar ice sheets, clouds seem to have no trouble forming.

Creation of the Supersaturated State in Nature

Supersaturation in nature happens the same way as in a bottle: through high humidity coupled with cooling. When warm, moist air blows over a cool surface it becomes chilled. When it is chilled below the dew point (the temperature at which water vapor begins to condense), a fog forms. On the west coast of the United States, warm, moist air from the Pacific blows over the cold California Current and then onshore to produce the familiar fogs of San Francisco.

When, on the other hand, cool air moves over warm water, it chills humid air rising from the water, condensing it into filamentary clouds that look like steam. Hunters, campers, and naturalists know that steam fog is rather commonly found over lakes in early morning.

When humidity is high and skies are clear, nighttime fogs may form at the bottom of valleys or on low-lying plains as the heat that the ground absorbed during the day radiates away. Radiation fogs usually dissipate within a few hours after sunrise as sunshine warms the air to above its dew point.

Air forced to flow over a mountain or hill expands - and cools - as it rises. Low-lying and very persistent clouds form in this way along windward coasts. A good example is the cloud bank over Hilo, Hawaii, when the warm, moist air of the trade winds encounters the rising terrain of the large volcanic mountains.

Clouds may form like a string of beads on the crests of atmospheric waves created when winds blow over mountain peaks. These clouds also form by cooling; the air is coldest at the top of the wave. Saucer- or lens-shaped wave clouds (Figure 7) are frequently seen just to the east of the Rocky Mountains and on the leeward sides of other mountain ranges.

Figure 7. Wave Clouds
Lenticular clouds form on the crests of atmospheric waves on the lee side of a mountain.
Photo courtesy of the National Center for Atmospheric Research.


Chapter 2: Clouds and Climate

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Earth's clouds are so deeply interwoven into the climate system that they cannot be ignored in even the most rudimentary climate models. But where they fit in is the question. We know they have both cooling and heating effects. How do these effects balance?

A glance at the cloud-covered planet from space illustrates how clouds brighten, and therefore cool, the planetary environment by reflecting sunlight. If we could somehow cleanse the atmosphere of all cloud condensation nuclei and prevent the formation of clouds, the reflectivity, or albedo, of the cloudless planet would drop from its present value of 30% to around 10%. That would mean an increase in the planet's temperature. It is even possible that the oceans might evaporate if clouds went away.

But clouds also warm the Earth by holding in outgoing long-wave (infrared) radiation. We have already noted how, as farmers know, a cloud layer is an excellent preventative for frost. In deserts, though solar heating during the day might be very extreme, loss of infrared radiation to the cloudless star-filled skies can lead to surprisingly cool nights.

Now consider the opposite: a hypothetical Earth cloaked everywhere with a uniform, thin, high-altitude cloud deck. Let us assume that these clouds have low reflectivity, so that the planetary albedo is unchanged from its present value of 30%. (The reflectivity of clouds is frequently higher than 30%, but here we wish to concentrate on their heating effect.) On this cloud-covered planet the surface temperature would be 27°C (49°F) higher than in the real world. It would be a hot planet indeed!

So the planetary system of clouds interacts strongly with the radiation passing both upward and downward through the atmosphere. Though the atmosphere itself lets sunlight pass through, clouds reflect significant portions of that light back to space. This reflective cooling, operating by itself, lowers the planet's surface temperature by more than 20°C. A blanket of clouds can also introduce warming at the surface by blocking the passage of infrared radiation (heat) from the ground. High, thin clouds are the most effective heaters.

Lack of reliable data on the heating and cooling by clouds has long hindered the study of climate and climate change. Recently, however, the heating and cooling by clouds was measured by V. Ramanathan (now at the Scripps Institution of Oceanography) and colleagues at the University of Chicago with instruments aboard the Earth Radiation Budget Satellite. They found that the reflective "white clothes" cooling of clouds was about 50% greater than their "blanket warming" effect. It appears, then, that the net effect of Earth's clouds is to cool the planet.

Changes in Earth's Climate from Clouds and from CO2

Changes in climate from pollution or natural causes are sometimes expressed in terms of the heating of the Earth system that would attend a doubling of carbon dioxide in the atmosphere. Our burning of wood, coal, gas, and oil will almost certainly cause a doubling of atmospheric CO2 over its 1950 levels sometime in the middle of the next century, and that will cause a change in climate.

We know that, on the average, clouds cool the Earth. Satellite measurements of cloud radiation indicate that the cloud cooling effect is about four times as large as the estimated heating introduced by the doubling of atmospheric CO2. So small alterations in the way clouds and radiation interact can play significant roles as feedback mechanisms.

Cloud/Climate Feedbacks

Scientists believe that during past ice ages cool oceanic currents flowing toward the equator caused a migration toward the tropics of cooling clouds like the oceanic clouds mentioned above and that this might well have amplified the general cooling trend. This is an example of positive feedback. There are numerous positive and negative feedbacks in the planetary climate system. This section discusses some of those involving clouds.

Feedback Loops in General

A feedback is a process that responds to a system change by enhancing or diminishing the change. When a fraction of the output from a system feeds back to the system, it either speeds it up (positive feedback) or slows it down (negative feedback). Feedback loops are ubiquitous in nature. The concept lies at the heart of all biological systems. Any time a living organism causes a change in its environment and then reacts to that change there is feedback. A dog is following a trail. If he deviates from the trail, the odor decreases (negative feedback), causing him to adjust his direction until the odor becomes stronger.

Feedback control systems also play important roles in such human enterprises as engineering, economics, sociology, and political science. A household heating system is a feedback system. The temperature drops to a certain point, the thermostat senses it and turns on the furnace, the temperature rises, and when the thermostat again senses a temperature threshold, the furnace shuts off. It has even been suggested by the British naturalist James Lovelock that the planet has evolved feedback loops to stabilize its climate system. This so-called Gaia Hypothesis is described in greater detail later.

Figure 8 is a wiring diagram for a simple feedback control system. Each block represents a component or machine in the system. Part of the output of the second block is fed back through block B to provide a transformed signal that is added to or subtracted from the system's input. The important point is that the system's output affects the input.

Figure 8. Simple Feedback Control System
General block diagram of a feedback loop. Box A is the forward, or main, transfer function; block B is the feedback transfer function. The feedback system's input is to the left, and its output is on the right.

Feedback systems have some interesting and frequently surprising characteristics. The way the system behaves depends on whether the output from Block B is added to (positive feedback) or subtracted from (negative feedback) the input. Systems with negative feedback tend to be sluggish and respond more slowly to input than do systems with positive feedback. Positive feedback, on the other hand, is associated with amplification, rapid response, even overshooting and instability. Some systems exhibit both negative and positive feedback and can be more responsive to some kinds of inputs than others.

Feedback Loops in Climate

Some of the feedback loops in the control system of Earth's climate machine are:

1. Ice/albedo feedback. As planetary temperature falls, ice sheets and oceanic pack ice build. The bright ice reflects more solar radiation back into space, and the cooling is amplified.

2. Water vapor/temperature feedback. This is another positive feedback loop. We learned earlier in this module that the saturation vapor pressure of water increases with temperature. Water vapor, like carbon dioxide, is a greenhouse gas, absorbing infrared radiation. Increasing global temperatures cause additional water to evaporate, and the water vapor content of the atmosphere rises. The greenhouse effect of the increased water vapor "feeds back," and amplifies, the warming, by holding heat.

3. Ocean temperature/cloud feedback. During the ice ages, low clouds and midlatitude cyclonic clouds over oceans may have been driven toward the equator by cool oceanic currents. During the height of the last ice age, the midlatitude Atlantic Ocean was 5° to 10°C cooler than at present. The new cloud banks over the cooler oceans would have amplified the cooling tendency (positive feedback) in a way similar to the ice/ albedo feedback. If the positive feedback is strong enough, there is the possibility of runaway; fortunately, in this case, there were apparently negative feedbacks that prevented the ice age from amplifying endlessly.

4. Drought/cloud feedback. Scientists carry out climate simulations on large computers like the CRAY Y-MP supercomputer at the National Center for Atmospheric Research in Boulder, Colorado, using global climate models called general circulation models (GCMs). These simulations have suggested that the central regions of North America may experience severe drying when atmospheric carbon dioxide doubles. This would decrease cloudiness, which would lead to increased solar heating on the soil of the Great Plains, amplifying the tendency toward drying.

5. Cloud height and amount/temperature feedback. Computer simulations show that increases in greenhouse gases cause clouds to be higher and deeper, especially in the tropics. Surprisingly, there tends also to be a reduction in cloud cover, leading to greater heating by sunshine and to higher clouds, which also heat the Earth. Some scientists have estimated that these two feedbacks together provide a slight net positive feedback, making the Earth slightly warmer than it would otherwise be.

6. Cloud wetness/temperature feedback. Global warming will lead to increased atmospheric water vapor, so one might expect the volume of clouds to increase accordingly. Larger clouds are brighter, so they would reflect more solar radiation and counter the warming trend, a negative feedback.

The cloud-climate control system for Earth, with some of the feedback loops mentioned above, is outlined in Figure 9. The strength of the feedbacks is uncertain. In a recent review paper on greenhouse warming, John Mitchell, of the United Kingdom Meteorological Office in Bracknell, England, says that the formation of clouds and their radiative properties depend on many small-scale processes that cannot be determined in large-scale models. Current models have only extremely crude representations of clouds, cloud water, and cloud radiative properties, and this is one of the largest sources of uncertainty in scientists' attempts to simulate Earth's climate. Resolving this uncertainty has become the number one priority for research in the area of global change.

Figure 9. Climate Feedback Loops
A diagram showing four climate feedback loops: ice/albedo, water vapor/temperature, cloud height/temperature, and cloud microphysics. "A" in the center represents transfer of solar radiation out of the system.

A Gaia Feedback Loop

Robert Charlson, a chemist and cloud physicist at the University of Washington, and colleagues have suggested a possible feedback loop that brings Earth's biosphere into the climate picture. Tiny organisms in the sea, phytoplankton, produce sulfur-containing gases as a byproduct of their physiological processes. The sulfur gases spread throughout the lower part of the atmosphere in the form of dimethylsulphide (DMS), which provides cloud condensation nuclei. Charlson and colleagues suggest that as the planetary temperature alters, the production of DMS and, therefore, cloud droplet concentration, would also change. A rising temperature means more DMS, which means more reflective clouds, which, according to Ramanathan, tends to cool the planet. The result would be to dampen out swings in planetary temperature.



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Earth is a remarkable and beautiful planet, apparently the only life-bearing one in the solar system. The British climatologist Ann Henderson-Sellers frequently refers to the fact that the water in the planet's oceans, atmosphere, ice caps, and clouds represents a great buffering agent. She reckons that in large measure we can attribute the great stability of the planet's climate over the past 4 billion years to the simultaneous presence of water in its three phases. The coexistence of water's three forms is also the prime agent allowing the evolution of the biosphere to proceed up such a long ladder over the past 3.5 billion years. It is critical that we take good care of our planet's unique water system.

To willfully change the Earth's climate, we might use our nuclear arsenal to destroy the forests or to melt the Antarctic and Greenland ice caps. Everyone would agree that such a drastic and dangerous action would have an immediate impact on the Earth's climate. Yet there are less spectacular but equally effective methods of perturbing climate; we are slowly and subtly modifying the global environment in ways that could have a longer, more potent impact on the Earth's climate. By introducing greenhouse gases (carbon dioxide, methane, etc.) we increase global warming. Chlorine compounds (in air conditioning systems and in the manufacture of foam and electronic products) deplete our protective ozone shield. Deforestation destroys an important natural regulator of atmospheric carbon dioxide. We know these are things that ultimately force climate to change, yet our ignorance of clouds' responses to such changes and the climate's response to the new cloud patterns represents the single largest obstacle to understanding how. It is sobering to think that the feedback might be positive and that the effect of clouds may be to amplify humanity's effects. On the other hand, the opposite might also be true.

To this writer, it is most remarkable that though clouds are variable and chaotic to an almost unimaginable degree, a change in cloudiness of only a few percent, or an alteration of only a few hundred meters in average cloud altitude, would result in highly significant climatic changes. I would like to end this module on clouds and climate with the so-far-unanswered but highly interesting question, "With so much variability in the Earth's climate system, with so many feedbacks, both positive and negative, how does the climate remain so stable?"


Questions and Projects

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  1. List the major cloud types in the Luke Howard cloud classification scheme.

  2. Find some photos or paintings with clouds in them and identify the cloud types using the Luke Howard system.

  3. Do the cloud-in-a-bottle experiment (Appendix III) using air from different locations and air that has been filtered through cotton. Relate the cloud formed to the cloud nuclei.

  4. Blow bubbles outside when it is cold. Sometimes an ice nucleus will land on a bubble, leading to the growth of a large, hexagonal ice crystal on the bubble's surface. Explain the relevance of this phenomenon to cloud processes.

  5. Provide a few examples of feedback control machines. Explain their operation briefly with the aid of diagrams, along the lines of Figure 8.

  6. Do a short research project on the role of the water vapor feedback loop.


Appendix I. Cloud Patterns on Other Planets

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Clouds are not unique to Earth. The multitude of cloud systems on the other planets of our solar system is what gives them their unique characteristic appearances.

Venus, the brilliant "morning" star, is as bright as it is because thick clouds of sulfuric acid are strewn throughout its atmosphere. Without its clouds, Venus would be pretty difficult to spot among the stars. The multicolored whorls, spots, and bands of Jupiter are clouds made of droplets and crystals of ammonia, water, and methane. Trace quantities of complex organic molecules provide the red and yellow coloration. The clouds on Jupiter form into large-scale systems. The bands and belts are analogous to cloud systems on Earth (see Figure 3), except there are many more bands on Jupiter. Some of these cloud systems mark the tops of rising parcels of air buoyed up by an internal heat source. The Great Red Spot is an extremely powerful and long-lived anticyclonic storm, similar to a super hurricane on Earth. The red color comes from organic compounds that normally lie beneath the ammonia clouds.

The dull red appearance of Mars is due to an absence of clouds. Of the bodies in the solar system large enough to retain atmospheres, Mars, with its tenuous atmosphere (less than one hundredth the pressure of Earth's) of carbon dioxide, is the only planet not endowed with an extensive cloud system. Even there, however, thin clouds, perhaps of dry ice, have been detected over the polar regions, and fog sometimes accumulates in cold, dry valleys. The reddish hue is iron oxide in the surface sands and rocks; similar rust-colored rocks are strewn around the southwestern United States. Harsh Martian winds kick up huge amounts of red dust into the thin, dry atmosphere. Something similar happens on Earth over deserts and the Arctic, where clouds and precipitation are rare. Evidently by some manner cloud systems suppress dust in planetary atmospheres.


Appendix II. The Energy of Clouds

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Clouds are more than simple modulators of climate. They absorb, store, release, and transport huge amounts of energy. One would hardly expect the soft and wispy cloud to possess much energy, but think of the power in a lightning storm. Clouds play a central role in Earth system energetics.

It is sometimes useful to consider the energy in systems in terms of units with which one is familiar. For example, we express an automobile's strength in horsepower. For large clouds and cloud systems, an appropriate energy unit is the "18-wheeler," defined as the kinetic energy of a fully loaded (80,000 lb) modern transport truck traveling at the current speed limit (65 mph) on U. S. highways. Some of the energies of a large thunderstorm-producing cumulonimbus (Figure 10) are displayed in those terms in Table 2 A big cloud has as much energy as several tens of millions of fully loaded 18-wheelers speeding down a highway!

Table 2. Energy in a Cumulonimbus
Energy Source Energy
(in 18-wheelers)
Heat of condensation 26,000,000
Gravitational potential energy
(energy available in falling water)
Updraft kinetic energy 9,000
Solar radiation intercepted by cloud over its lifetime 13,000,000


Figure 10. Cumulonimbus
Development of a cumulonimbus cloud over the Catalina Mountains in southern Arizona. Courtesy of the Institute of Atmospheric Physics, University of Arizona.


Appendix III. How to Form a Cloud in a Bottle

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Materials Needed


About a week before you plan to do the experiment, pour a small amount of water into the flask and plug it. Insert the copper tubing into the hole in the plug and fit one end of the plastic tubing over that. Plug the outer end of the plastic tubing and let the apparatus sit. The air will become saturated and any cloud condensation nuclei will have time to settle out, so the air inside will be very clean.

When it's time to do the experiment, dim the lights in the room, hold the black cloth behind the flask, and shine the laser, slide projector beam, or flashlight at the other side to make the cloud show up well (see Figure 11). Now suck on the plastic tube to cool the saturated air inside the flask. (You can also use a laboratory pump or a large medical syringe.) You will find that it is very hard to produce a cloud with clean air. In fact, no cloud will form unless the air is cooled extensively by creating quite a high vacuum in the flask.

Now inject some cloud condensation nuclei. Light a match and blow it out. Quickly suck on the tube to create a partial vacuum inside the flask and hold the smoking match next to the opening. The vacuum in the flask will draw smoke in through the tube. The cloud that you can form now will be dense and spectacular.

Figure 11. Cloud in a Bottle
Growing a cloud in a bottle.



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albedo - the percentage of incident radiation a body reflects. A completely black surface has an albedo of 0; 100 represents pure white.

atmospheric pressure - the force, or weight, of air. Atmospheric pressure varies both vertically and horizontally. Pressure decreases at higher altitudes above the Earth. Horizonal pressure varies from day to day; high pressure is associated with fine weather and low pressure with storms.

cloud condensation nucleus - an airborne particle, typically less than one micrometer in diameter and frequently composed of soluble material, on which liquid cloud droplets condense when an air mass is supersaturated.

dew point - the temperature to which air must be cooled for saturation to occur.

equilibrium - a stable, balanced system in which all influences are countered by others.

feedback control system - a system in which some fraction of the output is returned or "fed back" to the input. The climate system is full of feedback loops, some of which are stabilizing (negative feedback) and others destabilizing (positive feedback).

freezing nucleus, or ice nucleus - an airborne particle that initiates the process of freezing in supercooled clouds.

Gaia Hypothesis - the idea, conceived by James Lovelock, that the Earth, through its biosphere, has developed a feedback control system that compensates for changes in climate.

general circulation model - a computerized simulation of the large-scale, or general, wind systems on Earth, used to calculate climate and its changes.

kinetic energy - energy of motion. It is one of the basic forms energy assumes. A speeding car has kinetic energy; a car in the garage does not.

relative humidity - the ratio of vapor pressure to the saturation vapor pressure.

saturation - a state where relative humidity equals 100%.

saturation vapor pressure - the vapor pressure at equilibrium (when there is no net evaporation or condensation) at a liquid surface. The saturation vapor pressure rises with increasing temperature.

supercooling - cooling of a substance below the temperature at which a phase change (as from liquid water to ice) would normally occur, but doesn't. Supercooled water is below the freezing point but is still liquid.

supersaturation - the degree to which relative humidity exceeds 100%. A humidity of 150% would be supersaturation of 50%.

vapor pressure - the pressure exerted by molecules of water vapor.

Additional Reading Material

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Books on Basic Cloud Science

Battan, Louis J. Cloud Physics and Cloud Seeding. Garden City, N. Y.: Anchor Books, 1962.
In the mid-1950s, a group of physicists, high school teachers, and journalists initiated a series of popularly written books aimed at revising and improving the secondary school physics curriculum. This book, part of that series, covers some of the topics in this module, especially the formation of clouds, rain, and snow.
Schoenland, B. F. J.
The Flight of the Thunderbolts. Oxford: Clarendon Press, 1950.
Another entertaining, very well written book, jam-packed with interesting tidbits on the electrification of clouds.
Tricker, R. A. R.
The Science of Clouds. New York: American Elsevier, 1970.
A highly entertaining and instructive book written in lay terms. It should be enjoyed by anyone from a high school student to a practicing cloud scientist.

Field Guides and Cloud Atlases

There are quite a few popular atlases of clouds, but three are outstanding:

International Cloud Atlas, Vol. II. Geneva: World Meteorological Organization, 1987.
This 200-page color atlas shows examples of cloud forms. It is a gorgeous work to be savored by those who enjoy the poetry of clouds.
Schaefer, V. J., and J. A. Day.
Field Guide to the Atmosphere. Boston: Houghton Mifflin Co., 1981.
A Peterson Field Guide.
Scorer, R. S., and A. Verkaik.
Spacious Skies. London: David and Charles, 1989.
A beautiful book explaining clouds and showing satellite views of them.

Books on Cloud Physics at University Level

Mason, B. J. Clouds, Rain and Rainmaking, 2nd ed. Cambridge: Cambridge University Press, 1975.
An edited compilation of 16 lectures Mason gave to physics undergraduates at Imperial College, London. It is highly recommended to supplement the material in this module.
The Physics of Clouds, 2nd ed. Oxford: Oxford University Press, 1971.
The classic tome (650 pages) on cloud physics. Written in the mid-1950s, it is still a pleasure to peruse.
Pruppacher, H. R., and J. D. Klett.
Microphysics of Clouds and Precipitation. Hingham, Mass.: Reidel, 1978.
A more up-to-date account to complement Mason's.
Rogers, R. R., and M. K. Yau.
A Short Course in Cloud Physics, 3rd ed., Oxford: Pergamon Press, 1989.
For the student who's had some physics and wants an introductory general survey on cloud physics.

Books on Clouds Relating to Climate and Hydrology

Budyko, M. I. Climate Changes. Washington, D. C.: American Geophysical Union, 1977.
The classic book on climate and climate change for the college undergraduate.
Summer, Graham.
Precipitation Process and Analysis. New York: John Wiley and Sons, 1988
A readable up-to-date book discussing rain processes and mechanisms, especially from the point of view of geography and hydrology.

Miscellaneous Books

Bentley, W. A., and C. W. J. Humphreys. Snow Crystals. New York: Dover Publications, 1962.
A beautiful photographic compilation.
Lovelock, James.
The Ages of Gaia. New York: W. W. Norton, 1988.
This fascinating, highly readable book explains the possible role of clouds in a global-scale homeostatic control loop.
Proceedings of American Meteorological Society Conference on Cloud Physics. Boston: American Meterological Society, July 1990.
For the latest word from the research community.

Technical Papers

A few early papers on topics in cloud physics are recommended for students who like to read original material.

Aitken, John. On dusts, fogs and clouds. Transaction of the Royal Society at Edinburgh 30 (1880): 337.
Aitken's delightful accounts of his experiments using a pocket dust counter device to count cloud droplets that formed on invisible motes of dust shows how science is done by thinking, wondering, tinkering, guessing, then checking. You don't need expensive equipment.
Bigg, Keith. A new technique for counting ice-forming nuclei in aerosols.
Tellus 9 (1957): 394.
Another example of how science is best done with simple equipment.
Dessens, Henri. The use of spiders' threads in the study of condensation nuclei.
Quarterly Journal of Royal Meteorological Society 75 (1949): 23.
A lovely paper in the same spirit.
Twomey, Sean. The identification of individual hygroscopic particles in the atmosphere by a phase-transition method.
Journal of Applied Physics 24 (1954): 1988.
The Irish cloud physicist has come up with many new ideas during his investigations of clouds and rain. This paper demonstrates finesse in science.
Wilson, C. T. R. Condensation and water vapour in the presence of dust free air and other gases.
Philosophical Transactions of the Royal Society of London, A189 (1897): 265.
Wilson is famous for his invention of the cloud chamber, which not only demonstrated how clouds form but eventually provided numerous breakthroughs in atomic and nuclear physics. His research is another example of good science with simple apparatus.



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*Use your browser's Find feature to find these topics in the module.
Aitken, John
albedo, cloud
carbon dioxide heating
Charlson, Robert
cloud classification
cloud composition
cooling, by clouds
energy, of clouds
   cloud height
   ocean temperature-cloud
   water vapor-temperature
greenhouse effect
heating, by clouds
Henderson-Sellers, Ann
ice crystals
infrared radiation
Jupiter, clouds of
Luke, Howard
Mitchell, John
nuclei of cloudy condensation
ozone shield
pattern, cloud
Ramanathan, V.
relative humidity
saturation vapor pressure
sulfur gases
vapor pressure
Venus, clouds of
von Guericke, Otto
Wilson, C. T. R.


Bibliographic Information

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University Science Books
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Sausalito, CA 94965
Fax: (415) 332-5393

Managing editor: Lucy Warner
Editor: Louise Carroll
NCAR Graphics Team: Justin Kitsutaka, Lee Fortier, Wil Garcia, Barbara Mericle, David McNutt, and Michael Shibao
Cover Design and Photography: Irene Imfeld
Compositor: Archetype Typography, Berkeley, California

Copyright © 1996 by University Corporation for Atmospheric Research. All rights reserved.

Reproduction or translation of any part of this work beyond that permitted by Section 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful. Requests for permission or further information should be addressed to UCAR Communications, Box 3000, Boulder, CO 80307-3000.

Library of Congress Catalog Number: 95-061058
ISBN: 0-935702-87-3


This instructional module has been produced by the the Global Change Instruction Program of the Advanced Study Program of the National Center for Atmospheric Research, with support from the National Science Foundation. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author and do not necessarily reflect the views of the National Science Foundation.

Executive Editors: John W. Firor, John W. Winchester

Global Change Working Group
Louise Carroll, University Corporation for Atmospheric Research
Arthur A. Few, Rice University
John W. Firor, National Center for Atmospheric Research
David W. Fulker, University Corporation for Atmospheric Research
Judith Jacobsen, University of Denver
Lee Kump, Pennsylvania State University
Edward Laws, University of Hawaii
Nancy H. Marcus, Florida State University
Barbara McDonald, National Center for Atmospheric Research
Sharon E. Nicholson, Florida State University
J. Kenneth Osmond, Florida State University
Jozef Pacyna, Norwegian Institute for Air Research
William C. Parker, Florida State University
Glenn E. Shaw, University of Alaska
John L. Streete, Rhodes College
Stanley C. Tyler, University of California, Irvine
Lucy Warner, University Corporation for Atmospheric Research
John W. Winchester, Florida State University

This project was supported, in part, by the National Science Foundation.
Opinions expressed are those of the authors and not necessarily those of the Foundation.

A note on this series

This series has been designed by college professors to fill an urgent need for interdisciplinary materials on the emerging science of global change. These materials are aimed at undergraduate students not majoring in science. The modular materials can be integrated into a number of existing courses - in earth sciences, biology, physics, astronomy, chemistry, meteorology, and the social sciences. They are written to capture the interest of the student who has little grounding in math and the technical aspects of science but whose intellectual curiosity is piqued by concern for the environment. The material presented here should occupy about two weeks of classroom time.

For a complete list of modules available in the Global Change Instruction Program, contact University Science Books, Sausalito, California, Information about the Global Change Instruction Program is also available on the World Wide Web at or


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For billions of years, clouds and cloud patterns have affected the world's climate. Despite their constantly changing nature, they play an important role in determining and regulating a remarkably constant climate. This module introduces basic features and classifications of clouds and cloud cover, explains how clouds form and what they are made of, describes the role they play in determining climate, and considers how they both cause and are affected by climate changes.