Stratus and Nimbostratus Clouds

The stratus clouds are layer clouds and are more common in the winter prior to the passage of warm fronts. They are also common in winter over large bodies of water after a cold front passage. These clouds blanket the sky over a large region, and unlike cumulus clouds, do not have a defined shape. As an example, fog is a stratus cloud that forms at or near the surface. Stratus clouds aloft form slowly by gradual lifting as opposed to the turbulent convection of cumulus clouds. As a result, the cloud droplets form more slowly and are generally smaller and more numerous than the cloud droplets of a cumulus cloud. Stratus clouds are also non-precipitating. The nimbostratus, somewhat analogous to the cumulonimbus, is a layer cloud that does produce precipitation. This is the cloud that we associate with the continuous rains and snows we receive in the winters, in contrast with the very temporary heavy downpours we may experience on summer afternoons from the cumulonimbus cloud.

These layer clouds can persist for days, making for a moist, chemically active atmosphere. In addition, they are not associated with good vertical mixing of pollutants since their low tops usually mark a capping, frontal inversion. Rather, pollutant emissions are restricted within a relatively low mixing layer where we can be exposed to higher concentrations. Because nimbostratus clouds are associated with extended periods of precipitation, they are effective at cleansing the atmosphere by wet deposition.

Stratus clouds play a major role in the heat budget of the earth. Not only do they reflect much of the sun's radiation away from the earth, they absorb much of the long wave radiation emitted by the earth. The net result is cooler days and warmer nights, reducing the differential heating of earth between night and day.

(More cloud photographs can be accessed on-line at Michael Bath and Jimmy Deguara's Weather Photography Library and the Plymouth State College Meteorology Program Cloud Boutique.)