On a hot summer day with a clear sky and no wind, a large mass of humid air may sit almost motionless over a wide area. People will feel sticky because they cannot cool themselves - their perspiration evaporates slowly into the almost saturated air. Air begins to rise as it is heated by contact with the warm ground. The air forms separate convection cells, with warm air rising through the centres of the cells and cooler air sinking at their sides. The cloud that forms is called cumulonimbus. It resembles cumulus but towers much higher, with a base at 1,000-5,000 ft (300-1,500 m). Rain starts to fall. Inside this very violent cloud, air rises rapidly in the convection cells and sinks between them. Lightning is, produced, causing the rumble of thunder as it strikes.

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