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Heatbursts Question: Last year it was reported on the local weather news that there had been an unusual weather phenomenon occuring in the area. During the middle of the night at about 2AM, the temperature would rise to the mid-90s Fahrenheit or higher and be accompanied by gusts of 20-30 mph, seemingly out of the blue. Is there a name for this phenomenon and what is its cause? Answer: What you likely heard about was a heatburst. They are rather rare but I did find a reference that seems to fit from August 14. Here is the NWS summary on it: A rare heatburst was observed in the San Angelo, Texas area shortly after midnight on the 14th. The heatburst developed when a decaying thunderstorm complex slowly dissipated over the region. Thunderstorms created downdrafts of dry air which warm by compression as the air descends aloft to the surface. The resulting heatburst occurred near the instrument sensors which was able to pick up the event which is usually seen as a rapid temperature rise with strong winds and low humidity, followed by a decrease in temperature as the event moves away. The temperature observed at 1235 AM on the 14th was 75 degrees. F, with a south wind at 31 mph gusting to 40 with a relative humidity of 62 percent. The observation at 105 AM reported the temperature jumped to 94 degrees F, winds northwest at 15 gusts to 40 with a relative humidity of 19%. The event ended by 130 AM, the temperature fell back to 73 degrees F, winds south at 31 mph, relative humidity at 66 percent. I wrote a piece on heatbursts for The Weather Notebook, the description below of the heatburst is derived from that piece. The exact mechanism behind the heatburst is still unclear. However, storm researchers believe they start in the highest levels of tall thunderstorm clouds. Usually after sunset, the warm, moist air fuelling the thunderstorm ceases, shutting off the main updraft. This causes the storm to collapse. Without the updraft, raindrops in the storm's upper levels are no longer held aloft and fall into the cool, dry air entering the storm from its rear. There they evaporate, further cooling the air in the process. This cold air is now quite heavy and rapidly plunges toward the ground. Usually, this produces an outburst of cold air from the storm. However, if the descending air falls from a very high altitude, over 20,000 feet, (6000 m) it warms significantly by compression during its descent. Such warming makes the air more buoyant which generally stops its descent. But in the case of the heatburst, the downdraft's momentum is too great to be halted, and the air slams into the surface, spreading outward as a hot, dry gust of wind. Any tall, dying storm can produce a heatburst if sufficient evaporation takes place high in the cloud. A heatburst's intensity depends on the initial size of the collapsing storm and the degree of warming before the air hits the ground. The surface temperature rise can be dramatic. One heatburst observed in 1996 drove the temperature from 88 to 102o Fahrenheit (31 to 39o Celsius) within 25 minutes. Wind gusts reached 105 mph (170 km/h) within the ten-county area. Learn More From These Books Chosen by The Weather Doctor
The Weather Doctor's Weather Almanac Heatbursts
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