Clouds and Albedo

The fraction of the total incoming solar energy that is reflected back to space is called the albedo. Ocean surfaces and rain forests have low albedos, reflecting only a small portion of the sun's energy. Deserts and clouds, however, reflect a large portion of the sun's energy, and have high albedos. The global surface albedo over the solar spectrum is approximately 0.1. However, the presence of clouds means that the overall figure is considerably higher, as is seen by comparing the image in Figure 1 below, which has been processed to remove cloud, with the untreated image in Figure 2.

Figure 1: Untreated Meteosat image (photo ESA)
Figure 2: Meteosat image without cloud (photo ESA)

Whether a particular cloud will heat or cool the Earth's surface depends on several factors which affect the reflection of incoming solar radiation and absorption of Earth's outgoing infrared radiation. Try to think what some of these factors may be.

The following table shows the albedo of some cloud types.

Cloud type Albedo
stratus 0.3 - 0.6
altostratus 0.2 - 0.5
cirrus 0.1 - 0.3
cumulonimbus 0.7 - 0.9

  How do you think different clouds affect the climate. Try the drag and drop activity below to find out.

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These results are explained in more detail below.

  High Clouds
High thin cirrus clouds are transparent to shortwave radiation and transmit most of the incoming solar radiation (their albedo is low). However, they absorb the Earth's outgoing radiation and then emit longwave, infrared radiation both out to space and back to the Earth's surface. The portion of the radiation radiated back towards the Earth adds to the energy already reaching the surface. This additional energy causes a warming of the surface and atmosphere. Therefore, overall, high thin cirrus clouds have a warming effect which enhances atmospheric greenhouse warming.

  Low Clouds
Because low clouds are optically thick, they do not let as much solar energy reach the Earth's surface as would be the case if they weren't present. Instead, they reflect much of the solar energy back to space (their albedo is high) and in this way have a cooling effect. These clouds also emit longwave radiation out to space and towards the Earth's surface. The longwave radiation emitted downward towards the Earth from the clouds' base tends to warm the surface. However, this effect is small and the net effect of these clouds is to cool the surface.

Last updated and validated 23/06/99
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