A thunderhead is a tall, towering cloud that signals a thunderstorm is forming or approaching. These massive clouds build upward like enormous castles in the sky, sometimes reaching heights of 40,000 feet or more. Their official name is cumulonimbus, but people call them thunderheads because they can look like giant heads or anvils looming on the horizon.

You can often spot a thunderhead building on a hot summer afternoon. The cloud starts small but grows rapidly, billowing upward as warm, moist air rises. The top often flattens out into an anvil shape when it hits the upper atmosphere. Thunderheads have a distinctive dark, sometimes greenish-gray base where rain is falling, while their tops remain brilliantly white in the sunlight.

Inside a thunderhead, powerful updrafts and downdrafts create the conditions for lightning, thunder, and heavy rain. Pilots avoid flying through thunderheads because the turbulence inside can be violent and dangerous.

When you see a thunderhead approaching across the prairie or forming over the mountains on a summer day, it is a strong sign that the peaceful afternoon is about to turn stormy.