Thunderstorm cloud anvil

Thunderstorm cloud with anvil-shaped top, typical of mature cumulonimbus.

About this subject

The anvil shape, or incus, is the flattened top of a cumulonimbus cloud, formed when the updraft hits the tropopause. This stable layer prevents further vertical growth, forcing air to spread horizontally. The anvil can extend for tens of kilometers downwind, indicating high-level wind shear and jet stream direction. These clouds reach altitudes between 12 and 18 km, up to 20 km in the tropics. A well-defined anvil is associated with severe thunderstorms, producing lightning, hail, and even tornadoes. In aviation, anvils are avoided due to strong turbulence and ice crystals that can damage engines. Meteorologists use anvil width to estimate updraft strength: broader anvils typically indicate more intense storms.

Frequently Asked Questions

What causes the anvil shape on top of thunderstorm clouds?

The anvil shape forms when the updraft reaches the tropopause, the boundary between the troposphere and stratosphere. Because the air there is more stable, the cloud cannot rise further and spreads horizontally.

How tall is a typical cumulonimbus cloud with an anvil?

Mature cumulonimbus clouds usually reach between 12 and 18 km in height, but in the tropics they can reach 20 km. The exact height depends on the storm's intensity and the local tropopause temperature.

Why are clouds with anvils often associated with severe weather?

A well-developed anvil indicates a strong, persistent updraft that sustains the storm. These conditions favor the formation of lightning, hail, strong winds, and in extreme cases, tornadoes.