Shortly after 12 a.m. this past Thursday, night owls in Wichita, Kansas, felt a midday swelter suddenly intrude on the darkness. The temperature had been an already sultry 85 degrees Fahrenheit, but then the winds unexpectedly lashed up to gale-force gusts and for the next 20 minutes the mercury soared almost one degree a minute, peaking at 102 degrees F.
As subsequent news stories described, Wichita had experienced the rare meteorological event called a heat burst—a downward blast of unusually warm air produced by the collapse of an air pocket at high altitude. Most of the reported explanations of what caused the incident (and the temperature surge) struck me as a bit sketchy, however. After all, isn’t air from the upper atmosphere much colder than that at ground level? Where would all that extra heat come from? For those who are curious, here is a bit more detail.
Heat bursts are one extreme form of the more general phenomena called downbursts, all of which involve rain-cooled air masses plunging vertically. Some downbursts are wet, because they still carry falling rain, but others are dry; they can be big but they can also be small (in which case they are called microbursts, notorious for the wind-shear problems they can pose to aircraft during takeoffs and landings). The heat burst in Wichita was thus both big and dry.
Downbursts of all these types begin with rain falling through some mass of dry high-altitude air. Passing through that layer of low humidity, some or all of the precipitation evaporates (if it all disappears, meteorologists refer to it as virga rather than rain). In the process, the evaporating water pulls heat out of the surrounding air. The air then becomes both cooler and denser—notably, denser than the air beneath it. And so it drops earthward and splashes outward in all directions on contact with the ground.
That explanation can’t be complete in the case of heat bursts, however, because it fails to account for the extra heat. 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 Celsius 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. Meteorologists acknowledge that many of the details about how heat bursts form are still mysterious, as are those for microbursts and other types of downbursts. Expect such bursts to remain the focus of considerable research interest into the future, if only because of the hazards they pose to aviation and the clues they may offer into the prediction of other extreme weather events such as tornadoes.