One of our readers awoke to some beautiful clouds in the summer sky recently, and those two excellent questions popped into her mind.
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.
Whether a cloud is mostly liquid water droplets or ice particles depends, as you might guess, on the temperature of the air in the cloud.
Tiny cloud liquid water droplets can remain in the liquid state to temperatures as low as about -10 degrees Celsius (14 degrees Fahrenheit), and when they do they are known as supercooled liquid water droplets.
These droplets feel the downward force of gravity just like a baseball or a watermelon would. But because the droplets are so small, and therefore have small masses, the gravitational force can easily be balanced by an upward friction force resulting from the interaction of the droplets with the air molecules around them. The droplets remain suspended, and that’s what holds clouds up in the air.
When these droplets grow, they gain 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, when buoyant air parcels within the cloud literally bubble to the top. As the air rises, it encounters environments with lower and lower pressure and cools by expansion.
This cooling increases the relative humidity of the air. Once that relative humidity gets to 100%, condensation of the invisible water vapor begins to produce 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 stevea @ssec.wisc.edu or jemarti1 @wisc.edu.