Clouds
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.
Types of Clouds
The three main cloud layers, with approximate heights, are:
Clouds in the Arctic
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. Long term averages of total cloud cover in the "Gridded Fields" section of this Atlas illustrate the seasonal cycle.
A description of the seasonal cycle of cloud cover based on gridded cloud cover fields has been drawn from the data section of the Atlas.