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All About Heat Bursts
A heat burst is a meteorological phenomenon in which air descending from a decaying thunderstorm causes a rapid temperature increase and strong straight-line winds at the surface. This page explains how heat bursts form, shows radar and mesonet observations from a recent event, and answers some common questions about heat bursts.
How Heat Bursts Work
A heat burst typically begins with a dying thunderstorm, which often takes on a “serpentine” shape on radar (see image). As in many decaying thunderstorms, air high in the storm is cooled by evaporation of some of the water in it (just as evaporation of sweat cools your skin). As the air cools, it becomes denser than the air around it and begins to sink.
“Serpentine” radar echo from May 23, 1996 heat burst
It Begins Like a Downburst…
Normally, sinking air will be compressed by the weight of the air above it and will warm as it sinks. In a dying thunderstorm, however, the cooling of the air by evaporation offsets the warming caused by compression. End result: As long as there is still evaporation going on, the air inside the thunderstorm stays cooler than the air around it and keeps on sinking.
If the air around the thunderstorm is very dry, then rapid evaporation can cause large amounts of cooling, causing the air inside the storm to sink at very high speed. A downburst, characterized by strong winds and cool, moist air, occurs when this rapidly sinking air hits the ground while evaporational cooling is still occurring (i.e. while it still contains liquid water).
A Trick Up Nature’s Sleeve
In a heat burst, all of the water in the sinking air is evaporated before it reaches the ground. At this point, the air begins to warm due to compression without any evaporation to counter it. This warming slows the descent of the downdraft. However, if it has sufficient momentum built up, the hot, bone-dry air will still push its way down to the surface, hitting and spreading out as a sudden burst of hot, gusty wind.
An Example: June 9, 2011 near Enid, OK
Below are some images from a heat burst event that occurred in late spring 2011 near Enid, OK.
A weakening area of convection moved through North Central Oklahoma. A typical "serpentine" radar signature is visible, with the tip of the radar echo located near Breckenridge, where large wind and temperature increases were measured.
Doppler radar radial velocities of up to 64 mph were detected near Breckenridge in association with the end of the serpentine radar echo. At this location, the radar is detecting wind speeds approximately 2300 ft above ground level.
Another example can be found here, which occurred early on June 11, 2010.
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When do heat bursts typically occur in our area?
Most heat bursts happen at night or during the early morning, when surface temperatures are cooler. The most common times of year for a heat burst to occur are late spring and summer. During those seasons, we frequently see strong thunderstorms move into our area and die off overnight – a sequence of events that sets the stage for heat bursts.
Just as “rain” encompasses everything from light sprinkles to a heavy downpour, heat bursts vary in intensity from very weak to very strong. Heat bursts that produce 10° F temperature increases and 50-plus mph winds are not uncommon.
On May 22, 1996, one of the strongest heat bursts ever recorded in our area occurred between Lawton and Chickasha, OK. Temperatures in Chickasha increased from 87 to 102 degrees, and winds in Tipton, OK gusted to 105 mph. The Lawton area alone suffered $15 million in damage from the straight-line winds produced by the event.
Once thought to be fairly rare, dense observation networks such as the Oklahoma Mesonet have shown that, while they are strange, heat bursts are actually fairly common in late spring and summer in Oklahoma and Western North Texas. An Oklahoma Climatological Survey study found that more than a dozen heat bursts typically occur every year in Oklahoma alone.
How does a heat burst compare to a downburst?
You are probably familiar with downbursts – strong wind events caused by cold, wet air flowing out of a thunderstorm. Downbursts and heat bursts have some similar characteristics. In fact, both typically begin in exactly the same way – with cold, moist air sinking inside a thunderstorm. However, by the time they reach the surface, they are very different events.
- Both are caused by thunderstorms.
- Both involve downward-rushing air hitting the surface and spreading out.
- Both originate as an area of cool, moist air falling downward through a thunderstorm.
- Both can cause damaging winds at the surface.
- When downbursts reach the surface, they contain cool, moist air, frequently with rain or even hail.
- When heat bursts reach the surface, they contain hot, dry air.
What ingredients are necessary for a heat burst?
These are the basic ingredients of a heat burst. Even with all of these ingredients, a heat burst will not occur unless the local environment is just right.
- Time: Late night or early morning
- Decaying thunderstorm
- Deep layer of very dry air aloft
- Low level inversion (temperatures at the surface are cooler than just above the surface)