thermal

A local-scale rising air current that occurs over a surface that is warmer than its surroundings. Different types of terrain (sand, rocks, grass...) transfer different amounts of heat to the air in contact with them. In the areas of terrain that heat up the most, the air acquires a higher temperature and becomes lighter (lower density). It is then subject to a greater upward hydrostatic push, which culminates in the formation of thermals. Their presence is revealed by the circling flights of birds of prey, which, by doing so, manage to stay aloft with barely any effort. It is the same technique used by glider pilots with their sailplanes, or hang-glider and paraglider pilots. If the air rising in a thermal reaches the condensation level, a small cumulus begins to form, whose subsequent vertical development will largely be dictated by the degree of atmospheric stability/instability.