Mammatus clouds often make an appearance during thunderstorms, extending as a series of lobes that protrude from the bottom of cumulonimbus clouds. These pouches of ice and liquid water can be visible anywhere from 15 minutes to a few hours before they dissipate and are often a preeminent warning of an oncoming storm or severe weather system. If mammatus clouds are to form, there must be sinking air that is considerably cooler than the air around it as well as high liquid water or ice content.