Structure of a thunderstormWhen the atmosphere becomes unstable enough to form large, powerful updrafts and downdrafts (as indicated by the red and blue arrows), a towering thundercloud is built up. At times the updrafts are strong enough to extend the top of the cloud into the tropopause, the boundary between the troposphere (or lowest layer of the atmosphere) and the stratosphere.Click on the icons along the left-hand side of the figure to view illustrations of other phenomena associated with thunderstorms.