Captain Meteo II

Q: Why do (some) clouds keep their shape for a long time and do not disperse quickly?
[asks Wolfgang]

If we watch clouds, one thing we notice is that most clouds keep changing their shapes. They grow and shrink, form and evaporate. A few, however, don't. Both processes are driven by motions in the atmosphere. I will consider one process of each type, though there are many more.

The most common shape changer might be the fair weather cumulus. These are the kind of clouds that appear especially on warm and sunny summer days with little wind. Typically they form in the early to mid-afternoon and have a flat-bottomed, roundish shape and a puffy top, without great thickness. Individual clouds are rather short lived, usually less than an hour. After the sun warms the ground and the air in contact with it, puddles of warm and hence lighter air form near the ground, begin rising, and eventually form columns of rising air. Typically, this near ground air is both warm and moist, and as the air rises, it cools, and the moisture in the cloud eventually reaches a level and temperature at which it begins to condense. This level forms the base of the cloud. See, for example, Wikipedia on the Lifted Condensation Level.

Such clouds tend to be transitory, because the pool of warm air that fed the rising column of air runs out, and its moisture is mixed away and its heat radiated away and it falls again. If there are turbulent winds aloft, the pool of warm air is torn apart, mixed with local dry air, and the cloud fragments and vanishes.

One shape maintaining cloud is the so-called orographic cloud. This kind of cloud forms especially over mountains when there is a steady wind aloft. What happens in such clouds is that a stream of air is forced upward as the wind blows over the mountain and that upward motion cools the air and causes it to condense as it rises. As the air flows beyond the mountains, it falls again and the moisture evaporates. The orographic cloud is like the wave in a fast moving stream that forms over a shallowly submerged rock - the water keeps moving past, but the wave, and the orographic cloud, stays in one place.

Here is a link to a very nice orographic cloud: My part of New Mexico often has rather spectacular orographic clouds. Sometimes the air flowing over the mountains will have several moist layers with drier layers in between - in such cases orographic clouds can stack up like pancakes, one on top of another.

Other cloud formations maintain their shapes to a greater or lesser degree - thunderclouds, frontal clouds, hurricanes. In each case, the key to stability or instability is the underlying dynamical process. Is the stream of energy feeding the cloud steady or unsteady, transitory or lasting?

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