June 21, 2013: Another Night of Heat Bursts

Heat bursts occurred during the pre-dawn hours on Friday, June 21, 2013, for the second time in the last two weeks, although they were much more localized this time. Showers and thunderstorms, with very high cloud bases (around 12,000 feet), repeatedly formed and dissipated from north-central Kansas into south-central Nebraska, primarily along and west of Highway 183. This resulted in some heat bursts, with rapidly rising temperatures, rapidly lowering humidity, and gusty winds.The table below reveals the most impressive changes in weather observations. Other locations experienced heat burst activity, but the changes were not quite as extreme.

 

Location Time Temperature Before/After Relative Humidity Before/After Peak Wind Speed
Holdrege 355 am 79° / 97° 56% / 11% 41 mph
Gothenburg 233 am 80° / 94° 60% / 14% 44 mph

 

Definition/Explanation of A Heat Burst: A heat burst is caused when a shower or thunderstorm weakens over a layer of very dry air. As the last of the precipitation from the weakening shower or thunderstorm falls through the layer of dry air, the precipitation begins evaporating thus causing the air to cool. As this air cools it will become more dense, eventually more dense when compared to the surrounding warmer air and as a result, begins descending to the surface at a high rate of speed. Eventually, all of the precipitation within the descending air evaporates. At this point the air is completely dry and because no more evaporation can occur, the air can no longer cool. The air however continues to descend toward the surface due to the momentum it has already acquired. As dry air descends through the atmosphere, compression due to increasing atmospheric pressure causes the the air to warm. It is important to note that the density of this air is now going to begin decreasing because of the increasing temperature. However, because the descending air already has a great deal of momentum carrying it to the surface, the increase in temperature and resultant decrease in density does little to slow the descending air.  So, the dry air continues to descend, all the while warming more and more due to the aforementioned compressional heating. Eventually, this descending air reaches the surface and the momentum, which was moving downward towards the surface, is now moving horizontally along the surface in all different directions, thus resulting in a strong wind! In addition, the intrusion of the very warm and very dry airmass from aloft, will cause the temperature at the surface to increase very quickly, and the dewpoint at the surface to decrease very quickly.  Acquiring all the needed ingredients for a heat burst can be difficult, thus making the development and observance of a heat burst rare.



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