Heat Bursts - A Detailed Explanation

On June 8, 2011, a heat burst affected parts of Green, Rock, and Jefferson Counties.

Not many people know what heat bursts are, and most folks probably have never heard such a term.

In early June 2011, we composed a Top News Story about the June 8th heat burst - it can be found HERE.

So, we decided to give you a detailed explanation of heat bursts and a simple diagram showing how it evolves.  Please continue reading below.

In some summer-time weather situations the lower part of the atmosphere is drier than normal.  However, if there is enough “lift” in the atmosphere eventually invisible water vapour  eventually condenses into a visible cloud.  In these situations the cloud base (known as the Lifting Condensation Level, or LCL) is rather high, usually above 8,000 feet above the ground.  This is illustrated at level 1 in the image below.

heat burst image  

If the cloud is tall enough the temperature at the top of the cloud can be colder than -10C (14F) which allows ice crystals to form.  These ice crystals merge into snow-flakes which fall and melt into rain drops.  The rain falls below cloud base toward the ground and drags with it some of the drier air just below cloud base.  This is illustrated at level 2.

Since the atmosphere is drier than normal below cloud base, most or all of the rain evaporates.   The evaporation process cools the air surrounding the rain drops resulting in the production of a large bubble of rain-cooled air.  Since this bubble of rain-cooled air is colder than the air surrounding it, the bubble of air sinks and accelerates due to the effects of gravity.  Remember – cold air is heavier than warm air.  This is illustrated at level 3.

The temperature inside our bubble of air starts to increase as it sinks due to the compressional effects of gravity and denser air in the lower part of the atmosphere.  Eventually, the temperature of our bubble is higher than the surrounding, undisturbed air.  At this point, the bubble is lighter and more buoyant than the surrounding air so it wants to rise up in the atmosphere.  However, if the downward momentum of the bubble is strong enough the bubble will continue to hurdle toward the ground against buoyance effects.   This is illustrated at level 4.

Once the bubble of air reaches the ground there is a localized, significant increase in temperature and wind speed and a decrease in humidity at the surface, illustrated at level 5.  We now have a heat burst.

Below, there is listed temperature, dew-point, relative humidity and wind gust data recorded during a heat burst on June 8, 2011 in South-central Wisconsin.  Weather conditions before and after the heat burst are given.  Here is the data format:   Air Temp/Dewpoint/Relative Humidty

                                                    BEFORE         AFTER        MAX WIND GUSTS
MILTON                                     76/65/69       92/46/21      42 MPH
WHITEWATER                         78/66/67       89/49/26      55 MPH
FT. ATKINSON                         76/64/67        88/43/21      35 MPH
SULLIVAN WEATHER            75/67/76        82/59/46      32 MPH

One more note....heat bursts are similar to dry micro-bursts, in the sense that precipiation evaporates below cloud base which generates a "bubble" of heavier, rain-cooled air below cloud base that accelerates down to the earth's surface in the form of gusty winds.  The main difference is that heat bursts have warmer temperatures. 

Scott Trevorrow - Student Volunteer

Rusty Kapela - WCM

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