Reverse thermal
As reverse thermals is called uplift processes in ground-level to medium high layers of the atmosphere, which in contrast to the thermals are not caused by sunlight and the air mass associated warming, but by cooling of air masses in the absence of light. Since this situation often occurs at sunset, one also speaks of evening thermals . The air, which cools down faster on mountain slopes than in the valley, for example, flows into the valley due to the increase in density and displaces the warmer, less dense air above the valley upwards. Reversal thermal that can be used for gliding is a rare phenomenon compared to daytime thermal. In the technical jargon of the paraglider and hang-glider, it is therefore often referred to as the Magiclift.
The development of reverse thermal is strongly related to the topography. In a longitudinal valley , the cooler mountain wind can quickly flow away near the ground without releasing the warmer air mass in the valley over a large area. In closed valley basins , the displacement effect is more pronounced due to the outflowing air, which favors the emergence of reverse thermal effects.
In contrast to thermals, the reversal thermals, which are usually only weakly pronounced, are important, especially for model , hang- gliding and paragliding pilots , as they allow longer flights even after the thermal processes have subsided in the evening. The onset of mountain winds is an indication of the occurrence of reverse thermal .