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Factors
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Clouds are factors in climate that influence the radiation budget and therefore temperature. Clouds reflect a large fraction of solar radiation, resulting in surface cooling. On the other hand, clouds inhibit longwave radiation loss from the surface, which can lead to higher surface temperatures. The dominant process depends on many factors including cloud type and thickness, the magnitude of the solar radiation, and the albedo of the underlying surface.
Clouds are composed of minute water droplets, ice crystals or a combination of the two that have condensed on such atmospheric particles as airborne dust, smoke, sea salt, chemical compounds, and meteoric fragments. Condensation on nuclei occurs at relative humidities near 100 percent. Many condensation nuclei such as salts are hygroscopic, that is, they have a special chemical affinity for water molecules and promote condensation at relative humidities under 100 percent.
The most important characteristic of clouds in the Arctic is the summer stratus. From about mid-June to mid-September, the ocean area covered by sea ice is 80 to 90 percent covered with this cloud type. Summer stratus has important effects on the radiation balance of the surface.
The three main cloud layers, with approximate heights, are:
In general, much of the arctic sky is covered by low stratus and stratocumulus clouds. Total cloud cover is least extensive in December and January. Starting in May, cloudiness increases. Warm air over the water adjacent to ice, frequent temperature inversions, and fog, cause low level stratus clouds to form and persist through the entire warm period.
The following description of the seasonal cycle of cloud cover based on gridded cloud cover fields has been drawn from the data section of the Arctic Climatology Project Arctic Meteorology and Climate Atlas.
The most striking feature of low cloud cover during the winter months is the relative maximum over the northern North Atlantic. This manifests the uplift of air masses by the frequent cyclone activity in this area associated with the Icelandic Low. Low cloud cover is rather limited over central and eastern Siberia because of the general subsidence (downward motion) of air in the area of the strong Siberian high. During summer, the Icelandic Low and Siberian High weaken. Low cloud cover becomes more uniform, but with a distinct increase over the Arctic Ocean. The increase reflects the dominance of low-level stratus, which form as warm air masses moving over the ocean are chilled by the cold, melting sea ice cover. Then autumn months illustrate the transition back to the winter pattern. Total cloud cover combines low, middle and high clouds. While amounts of total cloud are hence greater than for only low cloud, it can be seen that most cloud cover is of the low variety.
