WRAL WeatherCenter Blog

When living in AZ years back, I witnessed a weather phenomenon (can't recall the term) where the nighttime air temps around collapsing(?) thunderstorms can quickly rise 10 or more degrees for short periods. I remember one such night where the temp in Phoenix shot from ~95 to 107 at 10pm. Are you familiar with this? And what is the term for it?

Posted July 3, 2007

MIKE MOSS SAYS:      Corbett,     You are probably thinking of an even called a "heat burst" in which a thunderstorm produces a downward rushing pocket of air that usually starts at a rather high altitude and is somewhat lacking in moisture so that it vaporizes most if not all of the cloud and precipitation droplets within it. If the downward moving air picks up enough momentum to continue to ground level, it may arrive there at a considerably higher temperature that its surroundings, and at tempertaures considerably warmer than the pre-existing air at ground level. Since most downbursts that warm considerably lose their downward momentum due to becoming buoyant relative to the air around them before reaching the surface, such heat bursts are rather rare. On occasion, there have been bursts that push surface temperatures up to around 120 degrees. These usually occur on the periphery of thunderstorms where there isn't a lot of precipitation and where there are some relatively dry pockets of air aloft that can be entrained into the downward directed air. They are also most noticeable when surface temperatures have already had a chance to cool off some relative to daytime highs. In these cases, there is a temperature inversion present in which a shallow layer of cooler air near the surface is overlain with somewhat warmer air a few hundred to a few thousand feet up, making it easier for the downburst air to punch through to the surface.

While these heat bursts can cause rapid and occasionally extreme warmups, it is not so unusual for a simpler mechanism to cause a rather rapid 5-10 degree warming at night. This too is associated with the development of a strong temperature inversion at the surface. In these cases, the ground-level temperature may fall well below the temperature not far above the ground, usually in the presence of mainly clear skies and light winds. If some atmospheric feature, be it a frontal boundary, outflow from a nearby storm, the arrival of clouds aloft, or simply a buildup of wind a thousand or two feet above the ground to the point that a turbulent "overturning" of the air near the surface occurs, there can be an increase in wind at the ground, and vertical mixing through the lower atmosphere can bring warmer air down to the surface leading to a quick increase in temperatures. A mild version of this effect occurs from time to time in our state, wherein the temperature at night falls in kind of a stair-step pattern, with the wind dying off to near calm at the surface and a rapid cooling at ground level setting in, then a few minutes in which wind shear between the ground and a couple thousand feet up produces a turbulent eddy that brings a surge of wind and warmer temperatures to the ground, another period of rapid cooling and calm winds, and then another mixing event, and so on for several hours.


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