Thunderstorms have three stages: cumulus, mature, and dissipation. In the cumulus stage, warm, humid air near the ground is pushed upwards by strong thermals, or by the collision of air masses coming in from several directions at once. As this moist air rises, it cools and the heat that is released enters the surrounding air, causing convection that, in turn, causes more air to rise. A loop that feeds off itself causes a rapid increase in cloud formation, temperature differences between lower and higher altitude, and rainfall. The storm reaches its second stage (the mature stage) when air has reached its cap and is not rising any farther. At this point, cumulonimbus clouds become cumulonimbus incus (thunderheads), with water droplets freezing at the top, thawing into rain as they descend. As the mixture of water, ice, and wind becomes more turbulent, an electrical charge builds up, aligning the ice crystals within the clouds until a bolt of lightning is discharged. This happens repeatedly until the storm weakens in the dissipation stage.