The appearance of a flat anvil top indicates that the developing thunderstorm cloud (cumulonimbus) has reached a(n) extremely stable portion of the atmosphere.
Strong winds at high altitudes help create anvils in thunderclouds. The flat top of the anvil forms when rising cloudy air meets the highly stable air of the tropopause and lower stratosphere.
This marks the beginning of the second stage (maturation). The term downdraft is used to describe rain and cold air that begins to descend from a thunderstorm. Downdrafts cumulonimbus (as opposed to updrafts) can be thought of as cold air masses in clouds heading toward the Earth's surface.
