NOAA's National Weather Service Weather Forecast Office

   A relatively rare cloud phenomenon paid a visit to much of south central Nebraska late Saturday afternoon and evening (June 12) as mammatus clouds blanketed a large portion of the area. Mammatus clouds are the product of sinking air in the atmosphere, and are most commonly viewed when thunderstorms are in the area. Thunderstorms are born when air begins to rise rapidly, creating a thunderstorm updraft that leads to cumulonimbus cloud development. Eventually, this rising air reaches a point where it can no longer rise, and the rising air spreads out, forming the anvil of the thunderstorm. It is in this anvil that the air eventually begins to cool and becomes colder than the surrounding air, forcing it to sink. If the moisture content of the sinking air is high (meaning that the cloud drops are larger in size), mammatus clouds are often formed. This process occurs because, if the cloud drops are large, they take much longer to evaporate as they sink. Therefore, the cloud is able to maintain itself for a longer period of time - long enough to produce a unique pattern of pouch-shaped clouds in the sky known as mammatus.

    Mammatus clouds are a good indicator of the intensity of a thunderstorm updraft. If the clouds are rather small and do not extend downward much from the cloud base, the updraft, while intense enough to create a thunderstorm, is likely not very strong. On the other hand, if the mammatus clouds extend well below the anvil base (like those on Saturday), it is likely that the thunderstorm updraft is quite intense. In this case, the thunderstorm updrafts at the time were located over north central Kansas, and these storms were producing very large hail at the time (indicative of a very strong thunderstorm updraft). Although some people may believe it to be true, mammatus clouds are in no way associated with tornado activity.

Story and pictures by David Lawrence.