Very Rare "Heat Bursts" Cook Parts Of South Dakota Overnight

A graph illustrates a 2008 heat burst in
Sioux Falls, South Dakota. Note the abrupt temperature
rise (in red) and sharp drop in humidity (in blue) 

A rare weather phenomenon surely had people scratching their heads overnight in parts of South Dakota.

The communities of Chamberlain and Huron, and I'm sure other South Dakota towns experienced an overnight heat burst.

Between about 3 and 4 a.m. local time, the temperature in Huron rose abruptly from 73 to 91 degrees, before dropping back to 74 degrees an hour later. Chamberlain had a similar, brief heat wave at about 2 a.m. local time.

So how the heck does it suddenly get so hot in the middle of the night?

Blame some big thunderstorms north of the two South Dakota towns overnight.

Many strong storms have big downdrafts. Those are the big gusts of wind you often see as a thunderstorm approaches. These gusts are very common in storms.  Precipitation in the storm cools the air. Cool air sinks, so you get that burst of air. That's why the wind from a thunderstorm almost always feels cool.

But a special set of circumstances involving the downdrafts cause heat bursts. They originate high up in the storm and start out as the precipitation cooling the air, and sinking. But if the upper atmosphere is very dry, the rain evaporates completely on the way down.

With the rain gone, the downdraft is still on it's way toward the surface. But with no rain to cool the downdraft, compression causes the air to heat up. Normally, when the air heats up, it stops sinking, since warm air tends to rise.

But if the original downdraft was extremely strong and had a lot of momentum,  the downdraft, now heating up dramatically, hits the surface. Hence the kind of heat burst you got in Huron, South Dakota last night.

Since the upper air was very dry, the heat burst is dry. In Huron before the heat burst, it was a very muggy night. During the heat burst, the mugginess disappeared in an instant, and it felt more like desert heat.

Heat bursts usually don't last long and temperatures quickly return to close to where they were before the burst started.

There have been heat bursts more extreme than those in South Dakota last night, according to weather historian Christopher Burt.

In Sioux Falls, South Dakota early in the morning on August 8, 2008, the temperature rose from 73 to 101 degrees within 10 minutes.

Perhaps the worst heat burst in U.S. history hit a small area northwest of Waco, Texas on June 15, 1960, Burt said. The temperature in an intense late night wind gust rose from about 70 degrees to 140 degrees. Cotton fields in the area were carbonized.