Dear Tom,
I often see cumulus clouds with billowing tops and seemingly flat bottoms. What causes this?
Roger Kleist, Valparaiso, Ind.

Dear Roger,

It has to do with the way cumulus clouds develop. A cumulus cloud exists in an atmospheric updraft–a column of rising air that, at any given height above the ground, is warmer than the surrounding air at the same height. The fact that the column of rising air is warmer than its surroundings what gives the updraft air its upward buoyancy. An updraft often starts in the lower levels of the atmosphere and then builds upward, ascending into progressively colder air aloft. The air in the updraft stays warmer than surrounding air even though it cools with ascent, eventually chilling to a temperature at which its water vapor begins to condense into visible cloud droplets.  The height above ground at which cooling allows condensation to begin varies from day to day, but is approximately uniform within any given air mass. This means the base of a cumulus cloud is nearly flat and lies parallel to the ground, and the bases of all cumulus clouds in the sky are at the same height. As the ascent continues above the condensation level, the cloud takes on its characteristic billowing appearance like any other thermally driven rising motion such as with volcanic ash clouds.