Aside from the basic cloud types and subcategories, a number of specialized clouds have been given there very own names. Certain flat-topped cumulonimbus clouds are described as Anvil Clouds. This towering cloud has grown so tall that strong winds above the surface are shearing off its top. An anvil cloud is actually a sign of a very strong thunderstorm, as only thunderstorms produce clouds of this towering nature. A Wall Cloud appears at the base of a strong thunderstorm, and looks like a round curved wall. A Funnel Cloud is a tornado that hasn't yet made contact with the surface. Lenticular clouds form east of mountains and look like flat, round lenses in an otherwise clear sky. Mammatus Clouds hang beneath a cloud layer like the udders of cows.