Atmospheric convection
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.