Dear Tom,
I have seen cumulonimbus clouds (thunderheads) with classic anvil formations at the top. What air mass is above the anvil causing it to flatten?
Scott, Bloomingdale, Ill.

Dear Scott,

The troposphere, the Earth’s lowest layer, extends from the earth’s surface upward about seven miles (but this varies with season and latitude), and in this layer the temperature decreases with increasing height at an average of about 3.5 degrees per thousand feet of altitude. Since the air cools with altitude warm, buoyant air in a thunderstorm can continue to accelerate upward. Above the troposphere lies the stratosphere, beginning at about 40,000 feet (higher in summer, lower in winter), where temperatures increase with altitude and it is at this level that thunderheads top out. Warm air surging upward in the storm encounters a warmer environment and decelerates causing the cloud to spread outward into an anvil-shaped top.