Clouds are visual indicators of what is going on in the atmosphere.  They are also aesthetically interesting.

Mammatus clouds are one of the more striking sights in the sky. Technically, they are a supplementary feature of a variety of other large clouds. However, they are most dramatic when they line the underside of a cumulonimbus. Shaped like giant udders, they form when parts of the anvil cool and sink into warmer air.  In general, the more plump the udders, the more severe the recent or nearby thunderstorm.

Below is a photo of the remnants of the first Mammatus cloud I ever saw in person. It was taken in New York City shortly after a violent summer thunderstorm a few years ago.

Photo Credit: MF at the Weather Gamut