Thermal Low - Formation

Formation

In deserts, lack of ground and plant moisture that would normally provide evaporative cooling can lead to intense, rapid solar heating of the lower layers of air. The hot air is less dense than surrounding cooler air. This, combined with the rising of the hot air, results in a low pressure area called a thermal low. Over elevated surfaces, heating of the ground exceeds the heating of the surrounding air at the same altitude above sea level, which creates an associated heat low over the terrain and enhances any thermal lows which would have otherwise existed. During the cold season, (winter), warm water bodies such as the Great Lakes can induce a thermal low. Thermal lows which develop near sea level can build in height during the warm season, or summer, to the elevation of the 700 hPa pressure surface, which lies near 3,100 metres (10,200 ft) above sea level. Heat lows normally are stationary and have a weak cyclonic circulation. As they are strongest at the surface and warm near their center, and weaker aloft where the air is more stable, the thermal low is considered warm core. The strongest versions of these features globally are over Arabia, the northern portion of the Indian subcontinent, Arizona, Mexican plateau, northwest Argentina, southwestern Spain, Australia, and northern Africa. The formation of the heat low over northern Africa leads to a low-level westerly jet stream from June into October.