What Are Mammatus Clouds?

What do mammatus clouds look like?

Mammatus clouds appear as rows of rounded, pouch-like protrusions hanging from the underside of larger clouds. Each pouch is typically a few hundred feet to over a mile across, and they often appear in large groups covering the underside of a parent cloud. The shapes can be quite uniform, like rows of inflated balloons, or more irregular. They're often most visible during dramatic sunset lighting, when the warm light reaches the underside of the clouds and emphasizes the three-dimensional texture, producing some of the most photographed cloud images.

How do mammatus clouds form?

Mammatus clouds form when air containing droplets or ice crystals sinks downward into clearer, drier air below. Normally, sinking air would warm and the cloud droplets would evaporate, but in mammatus formation the air sinks faster than it can warm enough to fully evaporate the droplets. The result is pockets of cloud sinking below the main cloud layer in the characteristic pouch shapes. The process requires specific atmospheric conditions: an existing thick cloud layer with the right droplet content, plus instability that allows downward motion.

What weather do they indicate?

Mammatus clouds usually indicate severe weather has occurred or is occurring nearby. They commonly appear on the underside of cumulonimbus clouds during or after severe thunderstorms, often hanging beneath the anvil. Their presence doesn't necessarily mean severe weather is imminent at your location, but indicates that strong convective activity has been recent or nearby. Mammatus clouds can also appear under other cloud types occasionally (volcanic ash clouds, contrails), but the storm association is by far the most common.

Why are mammatus clouds shaped like pouches?

The pouch shape comes from the physics of cool, dense air sinking through warmer, less dense air. The boundary between the sinking cloud and the warmer surrounding air becomes unstable, producing rounded shapes similar to other interfaces where dense fluid sinks through less dense fluid. The size of each pouch reflects the scale of the instability, which depends on the temperature difference and atmospheric conditions. The relatively uniform size of mammatus pouches within a single cloud reflects roughly uniform conditions across that section of the underside.