When the cumulus cloud becomes very large, the water molecules in it
become large and heavy. They start to fall through the cloud when the
rising air can no longer hold them up. Meanwhile, cool dry air starts to
enter the cloud. This is known as entrainment. Because cool air is heavier
warm air, it starts to descend in the cloud (known as a downdraft). The
downdraft pulls the heavy water molecules downward, making rain.
This cloud has become a cumulonimbus
cloud because it has an updraft, a
downdraft, and precipitation. Lightning and
thunder start to occur, as
well as heavy rain. The cumulonimbus is now a thunderstorm cell.
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