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Name: Bob
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
What holds a cumulous cloud together as it travels with the air currents? I seem to recall that it is formed by rising warm air that has reached the dew point, starting at a particular altitude. I live in Cleveland, Ohio, and can often see Lake Erie "lake-effect" creating clouds as winds (especially NW winds) pass over the lake. Why doesn't the rising, moist air create one long sheet of cloud - more like a stratus-type? I think I have read that a cumulous cloud has rising air at its center and falling air currents at its sides. If that is so, then how can it possibly maintain that circulation pattern while passing over so many different types of surface terrain? Is the circulation pattern really that persistent in spite of attempts at disruption? Is there a different force at work that is keeping the clouds "together" and creating cumulous clouds with clear (sometimes predictably consistent) distances between adjacent, neighboring clouds? Shouldn't some other types of rising, falling, etc. air currents disrupt the cumulous cloud and cause it to run into neighboring clouds or pull apart as its circulation or the forces that maintain its integrity are disrupted?


Replies:
Bob,

A cumulous cloud is a "convective" cloud, meaning that it forms from rising air that is initially warmer than the surrounding air. This air rises rapidly and cools as it rises, condenses, and forms a cloud. These are often called cell clouds as they do not normally form a sheet (except in stratocumulus, altocumulous, and cirrocumulous) like stratus. Stratus clouds are formed by the lifting of an entire layer of air. The three cloud types that I named in parentheses combine the two processes (the layer is lifting and individual parcels of air are being lifted in the layer). Sometimes rising air dominates and the cumulous clouds fill in and result in stratocumulous. As the cumulous cloud rises, the air around it must descend (assuming that the entire layer of air is not being lifted) to make way for it. Thus, cumulous clouds are often well spaced.

SODAR (SOund Detection And Ranging, using acoustics) and LIDAR (LIght Detection And Ranging, using lasers) remote sensing instrumentation show that on a cumulous cloud day, such as a nice day in summer) rising air from a large area of the surface is interspersed with falling air. These rising and falling air volumes can be amazingly persistent and can move with the cloud. A very small difference in temperature change with height can create either of these preferential areas.

Terrain can indeed disrupt cumulous cell health. Once formed, the cumulous cloud is it's own environment (a wet one) and will not dissipate unless the air that surrounds it drags it downwards (so that the air can warm above the dew point) or it is eroded by drier air from around it (called entrainment).

As for the Lake Erie clouds, the different areas of warmer and cooler water (for different depths of water, etc.) act in the same way as the land, with preferential warm and cooler areas. Cumulous cell development over the water usually is not as strong because the differences in water surface temperatures are not as great as those over land areas. During winter, when water temperatures are low, you will see more stratus over any of the Great Lakes than during the summer; layer lifting is more common in winter.

David Cook
Argonne National Laboratory





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