A mammatus is round and smooth like many round balls. When there is obvious sinking or swirling airflow at the bottom of the cloud, a mamma cloud may be formed at the bottom of the cloud. When this kind of cloud is on developing stage, it usually does not rain. However, if the cloud starts to dissipate, the water droplets in the cloud will fall and the chance of heavy rainfall will be high. When the upper part of anvil cools due to radiant heat, the air will sink to bottom of the cloud anvil, then produce mamma cloud. If the cumulonimbus cloud is strong, the mammatus will becomes very impressive.