Last Thursday, around midnight, the temperature in Wichita, Kansas, jumped 20 degrees in 20 minutes. Needless to say, this is not normal—both for the speed of the temperature increase, and the time of day at which it happened. (There's a reason we normally talk about overnight lows.) So, what's the deal?
It's called a "heat burst," says John Rennie at PLoS Blogs. Heat bursts are weather phenomena related to stuff you've probably heard of before—downbursts and microbursts. In those events, rain-cooled air suddenly becomes denser than the air beneath it and plummets quickly to Earth. But, for obvious reasons, that doesn't fully explain what happens in a heat burst.
The missing piece involves the very particular conditions that produce heat bursts and the relation between the pressure and temperature of a gas.
First, heat bursts almost invariably form at night on the trailing edge of thunderstorm systems. Presumably, the energy of the storm system helps to make sure that enough moisture can be raised to a great height, while the absence of sunlight and surface evaporation helps to ensure that the air column beneath it remains very dry. The virga that initiates the burst begins by falling into particularly high, arid air. The Wichita heat burst probably started about 3,000 feet (roughly 900 meters) up–quite a bit higher than many downbursts.
As in any downburst, the air made cold and dense by the evaporating moisture plummets. But in the heat burst, that descending air mass also becomes even more dense as it falls because the higher atmospheric pressure at lower altitudes squeezes it. Compressing a gas raises its temperature, a process called adiabatic heating. If the air contained significant moisture, its temperature would change less because the water molecules could absorb a lot of the latent heat energy, but the air is instead bone dry. So as the air of the burst descends, it becomes almost 1 degree C hotter with every 100 meters it falls.
Of course, heating the air also makes it expand, which in principle ought to cool it off and make it less dense, so one might think the descending air mass would settle into a comfortable equilibrium at some altitude, like a hot air balloon, and radiate away its excess heat. But it never gets that chance. The air is falling with so much velocity that it overshoots whatever equilibrium altitude it might achieve and plunges on into the earth. And that–at least in broad outline–is the source of the 102 degree F air and the 60 mile per hour winds that briefly wracked Wichita so early on Thursday morning.
Image: Hot Hot Hot, a Creative Commons Attribution (2.0) image from rcbodden's photostream
