A heat burst is a meteorological phenomenon in which air descending from a decaying thunderstorm causes a rapid temperature increase and strong straight-line winds at the surface. A heat burst typically begins with a dying thunderstorm, which often takes on a “serpentine” shape on radar. As in many decaying thunderstorms, air high in the storm is cooled by evaporation of some of the water in it (just as evaporation of sweat cools your skin). As the air cools, it becomes denser than the air around it and begins to sink.
Normally, sinking air will be compressed by the weight of the air above it and will warm as it sinks. In a dying thunderstorm, however, the cooling of the air by evaporation offsets the warming caused by compression. End result: As long as there is still evaporation going on, the air inside the thunderstorm stays cooler than the air around it and keeps on sinking.
If the air around the thunderstorm is very dry, then rapid evaporation can cause large amounts of cooling, causing the air inside the storm to sink at very high speed. A downburst, characterized by strong winds and cool, moist air, occurs when this rapidly sinking air hits the ground while evaporational cooling is still occurring (i.e. while it still contains liquid water).
In a heat burst, all of the water in the sinking air is evaporated before it reaches the ground. At this point, the air begins to warm due to compression without any evaporation to counter it. This warming slows the descent of the downdraft. However, if it has sufficient momentum built up, the hot, bone-dry air will still push its way down to the surface, hitting and spreading out as a sudden burst of hot, gusty wind.
Below is an example of an event in Kansas in 2013.
Probably the most extreme event recorded was in Kopperl Texas on June 15, 1960 which is still known as Satan’s storm.
Kopperl as a town on the edge of Lake Whitney in Bosque County, Texas, about fifty miles southwest of Fort Worth. It was founded in 1881 and named for a banker, Moritz Kopperl.
It was a typical June night in Kopperl. Skies were mostly clear. Some heat lightning was visible on the horizon. One clump of clouds rolled toward the town after midnight. The temperature was about 70F. Suddenly, a tremendous wind arose. It gusted to over 75 mph over a wide area. A store was unroofed. Trees were knocked over. The temperature shot up with an incredible momentum. In just a few minutes, it rose to over 100F. There are reports that thermometers designed to register temperatures up to 140F actually broke as the alcohol expanded so rapidly with the dramatic heat.
People awakened when their air conditioners went out as power failed. Suddenly, their houses were sweltering saunas. They rushed outside, thinking their houses must be on fire. They found that the air outside was scorching. It was hard to breathe. Lightning flashed. They thought the world was coming to an end. Parents wrapped their terrified children in wet sheets to keep them cool.
The next morning, farmers found that their corn that had been green the day before was cooked on the stalk. Ranchers found their young cotton fields burned to a crisp. Leaves on trees, shrubs, and plants were burned as if there had been a freeze.
The event was unexplained back in those days as heat bursts are so rare. You can imagine how the people may have been scared by this….
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