One of our readers awoke to some beautiful clouds in the summer sky recently and two excellent questions popped into her mind: What holds the clouds up in the air and what makes some clouds appear to be fluffy on top but flat on the bottom?
Clouds are composed of tiny liquid water droplets — whose diameters are about the width of a human hair — and tiny shards of ice in a variety of shapes, or habits. Whether a cloud is mostly liquid water droplets or ice particles depends, as you might guess, on the ambient temperature of the air in the cloud.
Tiny cloud liquid water droplets can remain in the liquid state to temperatures as low as about 14 degrees and when they do they are known as supercooled liquid water droplets. These droplets feel the downward directed force of gravity just like a baseball or a watermelon would. Because the droplets are so small, and therefore have small masses, the gravitational force can easily be balanced by an upward directed friction force resulting from the interaction of the droplets with the air molecules around them and so the droplets remain suspended — this is what holds clouds up in the air.
When these droplets grow, by a variety of interesting processes, they gain water mass, and eventually the gravitational force overwhelms the friction force, and the now larger droplets fall to the surface.
The fluffy appearance of the tops of some clouds are evidence of convection — that is, the air parcels within the cloud are buoyant and literally bubble to the top. As the air rises, it cools by expansion as it encounters environments with lower and lower pressure the higher it goes. This cooling increases the relative humidity of the air, and once the relative humidity gets to 100% condensation of the invisible water, vapor begins to produce the cloud liquid water droplets.
The bottom of clouds often appears flat because the first level at which rising air parcels begin to condense is usually rather uniform over a given region. This level is known as the lifted condensation level — that is, the level at which lifted air parcels first begin to experience condensation.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at email@example.com or firstname.lastname@example.org.