Heatburst
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Cold gusts dropping from a thunderstorm often bring welcome relief from oppressive summer heat. But, on occasion, a very hot blast of air may emerge, an event known as a heatburst.

Hi, I'm Bryan Yeaton and this is the Weather Notebook.

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, 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 in1996 drove the temperature from 88 to 102 Fahrenheit within 25 minutes. Wind gusts reached 105 mph within the ten-county area.

Thanks today to contributing writer Keith Heidorn. The Weather Notebook is a production of the Mount Washington Observatory. The program is supported by the National Science Foundation and Subaru.