A heat burst occurs when hot dry air from a dying thunderstorm accelerates quickly toward the surface causing strong winds and a sudden significant temperature increase. It is similar to other strong thunderstorm outflows, such as downbursts and microbursts, but these are usually accompanied by rain and cooler temperatures.
Temperature curve recorded at Donna Nook, Lincolnshire, UK on evening of 25 July 2019, associated with collapsing thunderstorm, showing 10C spike in surface temperature - source: UK Met Office
When a thunderstorm begins to dissipate, most of the air within it is moving downward (downdraft). If dry air is present in the middle levels of the storm, any rainfall will evaporate immediately (see Virga). While this cools the air down somewhat, the air continues to descend toward the surface which warms it quickly because of the increasing pressure (Adiabatic processes). Even after the downdraft air becomes warmer than the outside air, thus losing its negative buoyancy, it continues to sink due to downward momentum and increased density (dry air is heavier than moist air). The rapidly descending warm dry air pushes air out from below causing strong winds outwards from beneath the thunderstorm. Temperatures can increase 20 degrees F or more in minutes. Readings over 100 degrees and even 120 degrees F have been recorded.
Since Heat bursts are associated with a decaying thunderstorm, this rare phenomenon occurs most commonly in the late evening or at night. They are also more common in the summer months.