Clouds are the sleeping giant of the climate system. Most of the variation in sensitivities of different climate models comes from clouds. Clouds interact and interfere with both visible and IR light. It turns out that according to visible light, clouds should cool the planet while with IR light clouds should warm the planet. It gets even more complicated; which of these two effects wins depends on the altitude of the clouds. High-altitude clouds don't reflect much visible light, but they are pure hell in the IR. Lower clouds are the reverse; less of an IR effect because they are warm, but a large change in albedo because they are thick and opaque.
You have already encountered the IR side of the story, in Chapter 3. Clouds are tolerably good blackbodies, emitting a nice clean blackbody spectrum of light at whatever temperature it finds itself. Light escapes to space from the tops of the clouds, so the crucial thing to know is how high the tops of the clouds are, and therefore what the cloud top temperature is. High clouds emit IR light at cold temperatures blocking the bright IR light from the warm ground. Clouds with low tops have a smaller effect on the outgoing IR light.
Clouds also reflect incoming solar radiation, increasing the albedo of the Earth. The electric field of incoming visible light stimulates the electric field in the water droplet to oscillate, and this oscillation emits light of its own, at the same frequency as the incoming light. The name for this process is scattering. This interaction of light with matter is not the same as light absorption because the energy never gets converted to thermal energy of the water. Absorbed light would be reradiated in the IR, but scattered light in effect bounces off the water droplets as photons at the same frequency they rode in on. Some of the light energy is absorbed also, and the fraction of scattered versus absorbed light depends on many things. As a result, the albedo of clouds varies widely between different types of clouds. Scientists are struggling to figure out how real clouds work and how they might be changing already. Modeling them perfectly is currently out of our reach. The best we can do is a very crude approximation.
One important factor in determining the albedo of a cloud is the amount of water it contains. Low clouds contain more water than high clouds, in general. Clouds that are about to rain have more water in them than nonraining clouds. Another factor is the size of the cloud drops. Scattering is more efficient in smaller drops if the drop size is smaller. A third factor is the directionality of the scattered light. Most of the light scattered by spherical liquid drops continue in a more or less forward direction, which, if the light is coming down vertically, would mean to continue downward toward the Earth. Ice crystals are better at actually reversing the direction of light, sending it back up into space, increasing the albedo.
As air rises it cools and the saturation vapor pressure decreases (Fig. 5.6). We heard about this process in Chapter 5, but let's look at the cloud formation process in a little more detail. As soon as the relative humidity reaches 100%, there would be an energy payback if the water were to condense. But it might not happen right away. The water can get "stuck" supersaturated, that is, humidity can exceed 100%, by quite a bit, if the air is very clean with no particles in it for the droplets to form around. Liquid water has the property that it abhors the gas/liquid surface. Abhors may be a strong word, but the water molecules have a definite preference to be immersed, with other liquid water molecules on all sides, rather than sitting on the surface of the drop exposed to the air. Small particles have a greater proportion of these unhappy, unfulfilled surface molecules. Therefore, the water in small droplets has a slightly higher tendency to evaporate into the gas phase. When we report relative humidity, we are reporting it relative to water that has a normal, flat surface, or maybe large droplets. The saturation vapor pressure in equilibrium with small droplets is higher than it is for lower-energy water molecules with a flat surface at the same temperature. We will require a relative humidity higher than 100% if we wish to form small cloud droplets.
Most air in the troposphere has particles in it before any cloud drops start to form, around which the water droplets may begin to grow. These particles are known as cloud condensation nuclei. Clouds begin to form at lower relative humidity if condensation nuclei are present. The condensation nuclei are of the order of 0.1 ¡xm in size. Once the droplets form, the large droplets in a cloud tend to grow at the expense of the small ones because the latter have a greater tendency to evaporate. Urbanization of cloud droplets. The droplets in clouds tend to grow fairly quickly to sizes of around 5 ¡ m, larger in clouds that are about to rain. At high altitude, ice has a tendency to form, either by freezing liquid or by forming directly from the vapor. Vapor deposition releases latent heat on a growing ice crystal as it grows, resulting in the amazing symmetric patterns that snowflakes are formed in. No two alike, they say. That would be because no two snowflakes have exactly the same temperature and water vapor supersaturation histories.
There are three main types of clouds (Fig. 7.4). Cirrus clouds are located up at 8-12 km altitude. These are the only types of clouds up that high. Between cirrus clouds and low-altitude clouds there are several kilometers of air space where you typically do not find clouds. Cirrus clouds contain 10 or 100 times less water per volume than lower altitude clouds typically hold. Because they are so thin, cirrus clouds do not block incoming solar radiation as effectively as lower clouds do. You can often see blue sky right through cirrus clouds. The albedo impact of cirrus clouds is therefore weaker than it is for lower, thicker clouds. The IR effect of cirrus clouds is strong because the cloud is effective enough as a blackbody to block intense IR from the warm ground, substituting for it weaker IR from the cold upper atmosphere.
50-100 ^m diameter ice
50-100 ^m diameter ice
5 ^m diameter
The two main types of clouds at low altitude are called cumulus and stratus. Cumulus clouds are towers, the result of a focused blast of convection. Thunderstorms come from cumulus clouds. Stratus clouds are layered, formed by broad diffuse upward motion spread out over a large geographical area. Both types of fluid motion are difficult to simulate in climate models; the focused motions because they occur on spatial scales that are smaller than the grid sizes of the models, and the broad diffusive upward velocities because they are so slow and are difficult to predict accurately. Stratus clouds are particularly important to get right for the climate forecast because they affect the albedo of such a large area of the Earth's surface. Two-thirds of the albedo of the Earth derives from clouds, stratus clouds in large measure.
There is plenty of scope for human activity to impact the climate by altering clouds. Anthropogenic condensation nuclei are mostly combustion products. The abundance of cirrus clouds on Earth may be augmented by the contrails left by jet airplanes (Fig 7.5). In clean air, in the absence of cloud condensation nuclei, the air can be supersaturated indefinitely without forming clouds. An airplane passes through and creates a trail of air with higher humidity, from combusting the hydrogen in the jet fuel and with exhaust particles that can serve as condensation nuclei. Droplets form very quickly behind the airplane and then continue to grow and to spread out. Most of the moisture in the contrail comes from the ambient air, not from the airplane. The cloud particles spread out and eventually become indistinguishable from natural cirrus particles. For this reason it is difficult to know how much impact aircraft have on the cloudiness of the upper atmosphere. Contrails tend to warm the planet, by perhaps a few percentage of the total human-induced warming. A difference between contrail warming and CO2 warming is of course that CO2 accumulates in the atmosphere whereas contrails would dissipate in a few days, if we stopped creating them.
Low-altitude clouds can be affected by smoke particles from coal power plants, in particular, sulfur in coal. Sulfur is emitted in flue gas as SO2 and it oxidizes in a week or so in the atmosphere to sulfuric acid, H2SO4. The sulfuric acid condenses into small droplets less than 1 ¡xm in size called sulfate aerosols. Eventually, the sulfuric acid
Fig. 7.5 Contrails.
rains out and is called acid rain. Sulfate aerosols are extremely good cloud condensation nucleibecause the strong acid tends to pull water out of the air like those silica gel packets that are shipped with electronics equipment. Natural condensation nuclei include sea salt, dust, pollen, smoke, and sulfur compounds emitted by phytoplankton. Most parts of the world have enough natural condensation nuclei so the issue is not whether to form a droplet or not, the way it is for cirrus clouds and contrails. Rather, the issue is the size of the droplets. If there are plenty of condensation nuclei around, more droplets will form, and the average drop will be smaller. This is a big deal because smaller drops scatter more efficiently and therefore have a higher albedo. Get this: a change in droplet size from 10 ¡xm down to 8 ¡xm has the same effect on the radiation balance of the Earth as doubling the CO2 concentration. That's a big deal from such tiny little droplets! The potential for sulfate aerosols to change the albedo of the Earth by changing the average size of cloud droplets is called the sulfate aerosol indirect effect. This is a potentially huge effect, but the uncertainty is as large as the estimated effect (Fig. 10.2). One indication that this might be a real effect comes from ship tracks in the ocean, which are followed by clouds of lower-atmosphere droplets just like contrails (Fig 7.6).
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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.