Residents of Cedar Rapids experienced an unusual meteorological phenomenon prior to daybreak on Monday August 3rd, 2009. Dying thunderstorms and a very dry, warm lower and middle atmosphere led to the occurrence of a heat burst.
A heat burst is characterized by a dramatic, almost instantaneous, rise in air temperature and fall in dew point temperature. Most, but not all, heat bursts are also accompanied by a drop in surface pressure, little to no precipitation, and gusty, rapidly shifting winds. The most dramatic heat bursts can even cause severe wind gusts that result in property damage. Winds associated with this heat burst in Cedar Rapids reached speeds of 60 mph to 70 mph, producing damage to trees and knocking out power to thousands.
What causes a heat burst?
When thunderstorms are growing, they draw warm, moist air up and into the cloud where moisture condenses and falls out on the other side. But when a thunderstorm surpasses maturity and a lot of moist air is held high in the cloud, it begins to drop as the thunderstorm loses its updraft. As this heavy, rain-cooled air begins to fall, it compresses due to higher pressure at the surface. As the air compresses, it heats up. This heating can be significant resulting in a rapid and large spike in air temperature. In addition, as the air plummets from beneath the thunderstorm and hits the ground, it has no where to go except outward. In much of the same dynamic principle as a thunderstorm microburst, the air crashes into the ground and spreads out in all directions, resulting in strong to severe wind gusts that may produce damage.
Heat bursts are typically a late spring and summer, as well as a late evening and nighttime, phenomenon. Although many regard the heat burst as a fairly rare phenomenon, heat bursts actually occur more often than many may think. Areas that have a dense observation network, such as the state of Oklahoma, typically detect multiple heat burst occurrences per year. In other areas of the Plains, where observation networks are less dense, heat bursts are difficult to detect and are not reported to the National Weather Service unless they result in wind damage or happen to affect one of the few observing stations.
Below is some data and observations from the event.
NWUS53 KDVN 031200
PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE QUAD CITIES IA IL
700 AM CDT MON AUG 03 2009
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
0528 AM HIGH SUST WINDS 1 SSE CEDAR RAPIDS 41.95N 91.66W
08/03/2009 E60.00 MPH LINN IA TRAINED SPOTTER
WIND GUSTS ESTIMATED OVER 60 MPH. TEMPERATURE SHOT UP TO
0528 AM NON-TSTM WND GST CEDAR RAPIDS 41.78N 91.32W
08/03/2009 M61.00 MPH CEDAR IA BROADCAST MEDIA
AT KCRG STUDIOS IN DOWNTOWN CEDAR RAPIDS. TEMPERATURE
SPIKED TO 84 DEGREES.
0530 AM NON-TSTM WND DMG CEDAR RAPIDS 41.97N 91.67W
08/03/2009 LINN IA BROADCAST MEDIA
TIME ESTIMATED. A NUMBER OF KCRG VIEWERS REPORT POWER OUT
ON THE NORTHWEST AND SOUTHWEST SIDES OF CEDAR RAPIDS AS
WELL AS A NUMBER OF TREES BLOWN DOWN.
0532 AM HIGH SUST WINDS 1 SSE CEDAR RAPIDS 41.95N 91.66W
08/03/2009 E70.00 MPH LINN IA TRAINED SPOTTER
WIND GUSTS ESTIMATED OVER 70 MPH. POWER OUT. TREE
BRANCHES COMING DOWN. LIGHTNING OFF TO THE SOUTHWEST.