Anvil Cloud, Western Africa Under specific...

Anvil Cloud, Western Africa

Under specific conditions, the towering, fluffy white clouds known as cumulonimbus can become flattened into the shape of an anvil. The anvil in the image above was captured by astronauts aboard the International Space Station as it crossed over western Africa in February 2008.

Cumulonimbus clouds form when air warmed by sun-heated ground rises. If the warm air contains water vapor and it encounters cooler air, the moisture condenses into water droplets. The air continues to rise, expand and cool as atmospheric pressure and temperature decrease. At the same time, heat released from the phase transition between water vapor and liquid water warms the air. The cooler air wants to fall, while the warmed air wants to rise, which sets up convection cells that feed the tall cloud towers and often result in thunderstorms.

In the tropics, these towers can grow to be 12 miles tall. At this point, they hit the tropopause, which is the boundary between the troposphere and stratosphere layers of the atmosphere. Beyond the tropopause, air no longer cools as it rises, which stops the cloud top, which may then spread and flatten along the boundary.

(Source: Wired)