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What Are Mammatus Cloud?

Mammatus (pronounced Ma-Ma-Tuss) clouds do not produce severe weather. However, they are associated with big towering cumulonimbus clouds because they are usually found on the underside of an anvil cloud. The reason some people think Mammatus clouds mean severe weather is coming, is because they often appear before a strong storm moves in.

However, they are often found after a storm rolls through. They also don't always appear with severe storms. So they can't be used alone as a reliable signal that severe weather is approaching, but they certainly signal the possibility.

To understand the process of how mammatus clouds form, let's follow a water molecule through a thunderstorm.

At first the water is near the surface of earth and in the gas form called water vapor . In fact, you may have just exhaled that water into the air. Then it gets drawn upwards and becomes part of a strong updraft within a thunderstorm. As it ascends it condenses and becomes part of the towering thunderstorm cloud. But this water droplet is still in the updraft and goes higher and higher, eventually freezing into an ice crystal as it reaches 57,000 feet above the surface.

At that point the upward motion of the storm is weak and the water spreads out horizontally, now forming part of the Anvil cloud (for more on anvil clouds, click here).

At this point, the water is out ahead of where the main storm is and part of the Anvil cloud like the one pictured to the right. The ice crystal is part of a group of ice crystals, or a moist air parcel, which now starts to sink because it is more dense then the surrounding air. As the ice crystal sinks it reaches the anvil base, where the cloud stops and below this point it's all clear air. The flat cloud base forms because below that point all moisture evaporates.

However, because this ice crystal and others in the moist air parcel are so large, they aren't evaporating at that level. They continue to sink a little farther, creating the mammatus pouch. Eventually, our ice crystal evaporates, but not before forming that cool looking cloud, mammatus.




Pictures Courtesy of National Oceanic and Atmospheric Administration (NOAA)