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Atmospheric convection is caused by one of two possibilities. One is instable state of atmosphere. The second is different ability of Earth's surface to get warmer. 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 origination of spontaneous rising and downward motions. This warm air has higher temperature than its neighborhood. Warmer air has lower thickness than cold air. Warm air is getting high. Through this rising motion the air element decelerates because of the fiction. This trend stops when the temperature of this element is equal to the temperature of surrounding air. Next rising caused just the inertia.
These thermal motions are used for example in gliding. The Thermal flow has enough energy to bring gliders to higher altitude.