Shortly after 3am central time Wednesday, a heat burst occurred at Eppley airfield in Omaha, Nebraska. This caused the temperature to rise six degrees in an hours time, and caused winds to gust at 43-mph. What is a heat burst?
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.