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Atmospheric convection, like other convection, is caused by temperature differences. This may be because the atmosphere is unstable. When parts of the Earth's surface become warmer this can also cause convection. The first type (unstable state of atmosphere) can be called spontaneous. The second type (convection caused by unequal soaking) can be called contact convection. Spontaneous convection starts at any level. Contact convection starts near the terrain.
The most frequent is contact convection. The sun is shining on the surface. It gets warmer and warmer. This leads to vertical draft, that is spontaneous rising and downward motions. This warm air has higher temperature than its neighborhood. Warmer air has lower density than cold air. Warm air rises. Through this rising motion the air element decelerates because of friction. This trend stops when the temperature of this element is equal to the temperature of surrounding air. Next rising continues due to inertia
These thermal motions are used for example in gliding. The Thermal flow has enough energy to bring gliders to higher altitude.