Clouds reflect or absorb light

Clouds in the climate system

The multifaceted function of clouds

Clouds are one of the most important components of the climate system that interact with numerous other processes. Clouds influence the radiation budget of the atmosphere; they reflect the short-wave solar radiation and absorb and emit long-wave thermal radiation. Clouds are also an important factor in the atmospheric water cycle. They arise from condensation from water vapor. They cause precipitation that gets into the rivers, lakes and oceans, from which evaporation then comes to the water vapor from which the clouds arise. This process also affects the energy balance of the atmosphere: When it evaporates, heat is extracted from the atmosphere, which is released again as so-called latent heat during condensation.

On the other hand, clouds are heavily influenced by other factors. If the air cools down, it can lead to condensation of water vapor and the formation of clouds. If the air warms up, clouds can dissolve through evaporation. The rise of air masses - e.g. due to warming, at mountains or at air mass boundaries - in which the air cools down, and the decrease in air masses in which the air warms up, play a central role. However, in order for cloud droplets to form from water vapor, so-called condensation nuclei, the aerosols, are required. If there are few aerosols in the atmosphere, clouds with rather large droplets form; if the aerosol density is high, clouds with many small droplets form. Clouds are in constant motion. Wind currents can move clouds far away from their areas of origin and lead to precipitation hundreds or thousands of kilometers from the point of origin of the cloud water.

Since clouds can change very quickly in the smallest of spaces, it is very difficult to depict them adequately in computer models. Because of their central importance in the climate system, clouds are one of the biggest, if not the biggest, problems in climate prediction.

Cloud cover

Clouds cover about two-thirds of the globe: The areas of the low-pressure train tracks over the oceans in the mid-latitudes and the tropical rain belts are heavily cloudy, while the continental desert regions and the central subtropical oceans are relatively low in clouds.1

The type of cloud cover is not the same in all geographic regions and not at all altitudes. In most areas it also changes very rapidly over time. One can distinguish the clouds on the one hand by the height at which they occur and on the other hand by their shape. With the exception of ice crystal clouds in the polar regions, clouds only occur in the troposphere, the lower layer of the atmosphere in which the weather takes place.

Most of the high clouds are near the equator and over the tropical continents. But they can also be found in the low pressure zones and in summer over the continents of the middle latitudes. Clouds of medium height are typical of the low pressure areas of the west wind zones of the middle latitudes.

There are low clouds over almost all oceans, but mainly over the subtropical oceans, which are cooler (due to upwelling water), and also in the polar regions. Depending on the altitude, different cloud shapes or types emerge, which are designated with cirrus for high, altus for medium and stratus for low clouds. The cloud genera Nimbostratus, Cumulus and Cumulonimbus extend over several altitude ranges.

Clouds and radiation

The earth's climate system receives short-wave radiation from the sun, which heats the atmosphere and the earth's surface. The earth's surface and atmosphere in turn reflect part of the short-wave solar radiation and emit long-wave heat radiation back into space. The energy of short-wave solar radiation is significantly higher than that of long-wave thermal radiation. It depends on the temperature of the radiating bodies, which in the case of the surface of the sun is over 5500 ° C, while the atmosphere of the earth close to the ground is only 15 ° C warm on average.

There are numerous interactions between clouds and the short-wave and long-wave radiation flows in the atmosphere. Clouds reflect the short-wave solar radiation, absorb the long-wave thermal radiation and emit it again and are themselves changed by radiation. This function of the clouds can be experienced on a daily basis: a cloud cover that is pushed in front of the sun leads directly to a lowering of the temperature on the ground. On a winter night, a cloud cover prevents heat from being radiated into space; Compared to a clear, cloudless winter night, it is much less cold. Strong solar radiation can also lead to the dissolution of a cloud cover in summer.

Cooling effect of clouds

When viewed from space, clouds are light in color. This is due to the fact that the tiny water and ice particles of the clouds reflect 30 to 60% of the sun rays hitting them.2 Clouds therefore have a relatively high albedo. Compared to a cloud-free sky, the effect of the cloud albedo is greater, the stronger the solar radiation and the darker the ground under the clouds. Clouds over a tropical rainforest, over which the sun is more or less vertical, have a particularly high albedo. The same applies to clouds over the dark surfaces of the tropical oceans. Over snow-covered regions at high latitudes, however, the cloud albedo can be lower than that of the earth's surface. In the winter polar night, when the sun no longer rises above the horizon, the albedo no longer plays a role. Nor does the albedo of the clouds play a role at night.

Compared to a cloud-free atmosphere, clouds significantly increase the albedo of planet Earth. Their total radiation effect through the reflection of short-wave solar radiation is approx. -50 W / m2.1 NASA's two earth observation satellites, CloudSat and CALIPSO3 determined the albedo effect of clouds more precisely over the period from 2000 to 2010. During this period it was 47.5 W / m2 and is mainly caused by the reflection of sunlight by clouds in the middle latitudes of the respective summer hemisphere.4 Overall, clouds reflect more than twice as much sunlight as the earth's surface.

The individual cloud shapes reflect the solar radiation to different degrees. The tall and thin cirrus clouds allow most of the solar radiation to pass through and reflect only a small amount of it back into space. So their cooling effect is small. In contrast, the thick and low Stratus and Stratocumulus clouds have a strong cooling effect. They are not very transparent to the short-wave rays of the sun and to a large extent reflect them back into space. Low clouds, often stretching over large areas, are the main contributors to the cooling cloud albedo. The cumulonimbus clouds, which are several kilometers high, also reflect the solar radiation very strongly due to their thickness, but are usually not as extensive as the stratus clouds.5

Warming effect of clouds

But clouds do not only have a cooling effect. They also hinder the long-wave heat radiation from the earth's surface and the lower layers of the atmosphere into space by absorbing it and emitting it again in all directions. The strength of the radiation emitted by a cloud depends primarily on its temperature, but also on other factors such as the thickness of the cloud and the particles from which the cloud is formed. The surface of the clouds is usually colder than the surface of the earth and therefore emits less heat radiation in the direction of space than this. The result is that thermal energy is trapped beneath the cloud and increases the temperature of the atmosphere below the cloud and the surface of the earth. Clouds thus have a greenhouse effect similar to that of greenhouse gases. In the entire radiation budget of the atmosphere, long-wave radiation directed downwards is 344-350 W / m2. Clouds contribute to this with 24-34 W / m2 at.6 The Intergovernmental Panel on Climate Change (IPCC) gives an average value of 30 W / m2 at.1 Clouds are thus responsible for almost a tenth of the planet's greenhouse effect.

How strong the contribution of the clouds is in the individual case depends above all on the height of the clouds. The high and thin cirrus clouds allow the short-wave solar radiation to pass through, but they absorb the long-wave heat radiation and emit it both towards space and back to the earth's surface. Because cirrus clouds are high and therefore very cold, their radiation into space is significantly less than the radiation from the warmer earth's surface and atmosphere would be without clouds. Since cirrus clouds on the one hand prevent this radiation, but on the other hand partly reflect the received thermal radiation back towards the lower atmosphere and the earth's surface, they heat them up significantly. So they have a relatively strong greenhouse effect.

The temperature of low clouds such as stratocumulus clouds, on the other hand, differs only slightly from that of the earth's surface. They therefore emit a similarly strong radiation towards space as the earth's surface. On the other hand, the thin layer of air between the underside of the cloud and the ground itself is heated by the long-wave radiation that the clouds radiate downwards.

The extent to which the radiation emitted downwards reaches the air near the ground and the earth's surface depends essentially on the amount of water vapor below the clouds, which absorbs the radiation emitted by the clouds and re-emits it again. The radiation is lowest in the tropical latitudes, because here the water vapor content is highest below the clouds, especially above the warm oceans. The long-wave radiation emitted directly from the clouds towards the earth's surface is strongest in the middle to high latitudes, because there is less water vapor in the atmosphere and the downward heat radiation is therefore less absorbed and reaches the earth's surface.5

Overall effect

Because clouds increase the planetary albedo, they cause a radiation effect of approx. -50 W / m2 through the reflection of short-wave solar radiation. At the same time, clouds also contribute to the greenhouse effect with around +30 W / m2 by absorbing and emitting long-wave thermal radiation. The net radiation effect is around -20 W / m2, meaning a significant cooling of the current climate due to the effect of clouds.1 Expressed in degrees Celsius, clouds cool the earth by 12 ° C through reflection and warm it by 7 ° C through the greenhouse effect. The net effect of clouds on the Earth's current climate is -5 ° C.2

In the case of high clouds, the warming effect dominates. They are very transparent to the short-wave solar radiation, but absorb the long-wave radiation from the ground. Because of their low temperature, they emit only a small amount of the absorbed radiation in the direction of space. The downwardly directed long-wave radiation and the solar radiation passing through the thin cirrus clouds together clearly outweigh the radiation into space.

In the case of low clouds, the radiation balance is exactly the opposite. They reflect a large part of the solar radiation. Although they also absorb long-wave radiation from the earth's surface or the atmosphere, they emit almost as much of it towards space as towards the ground. The net radiation effect is therefore negative over most of the globe, especially where extensive stratus and strato-cumulus clouds predominate, such as in the mid-latitudes and over the eastern subtropical oceans.1
Individual evidence
1. IPCC (2013): Climate Change 2013, Working Group I: The Science of Climate Change, 7.2.1
2. NASA: Cloud Climatology, Last updated: 2013: 03: 11
3. Wikipedia: CloudSat and CALIPSO
4. Stephens, G.L., et al. (2012): An update on Earth’s energy balance in light of the latest global observations, Nature Geoscience 5, DOI: 10.1038 / NGEO1580
5. NASA Facts: Clouds and the Energy Cycle
6. Stephens, G.L. (2012): The Global Character of the Flux of Downward Longwave Radiation, Journal of Climate 25, 2329-2340