A Cumulus cloud, often termed a "fair weather cloud", is commonly seen in midsummer. When in the cumulus humilis and cumulus mediocris forms, cumulus clouds rarely produce any precipitation. However, these clouds may grow into cumulus congestus, which sometimes produces light rain. They can also grow into cumulonimbus clouds, which are often called thunderheads. A very rare phenomenon occurs when a dust devil latches onto the base of a cumulus congestus cloud, forming a weak tornado.