Mammatus clouds are pouches of clouds that hang underneath the base of a cloud. They are most often associated with cumulonimbus clouds that produce very strong storms. These clouds usually form during warm months, and are formed by descending air in the cloud. Mammatus clouds are sometimes described as looking like a field of tennis balls or melons, or like female human breasts. In fact, the name "mammatus" comes from the Latin word mamma, or breast.