Heat Bursts

Objectives

Upon completion of this web page you will be able to:

1. Define what a heat burst is.
2. Explain the physical process that produces a heat burst.
3. Identify a heat burst in a series of observation.

What is a Heat Burst?

According to the Glossary of Meteorology (Glickman 2000), a heat burst is defined as follows:

Heat bursts are sometimes called "heat thunderstorms" and the temperature jump that this phenomenon causes are, at times, referred to as a "hot blast of air".

Physical Process

The examples below and several studies including Johnson (1983) show that heat bursts are characterized by:

These features are typical of strong dry adiabatic descent reaching the earth's surface. Indications are that this subsidence occurs near the dissipating or rear edge of storms. Lapse rates of sounding taken near heat bursts are strongly dry adiabatic.

These soundings have features similar to High Plains dry microburst soundings identified by Wakimoto (1985). His soundings were characterized by mid-level moisture around 500 mb (near cloud level), a sub-cloud layer with a dry adiabatic lapse rate, and mixing ratio values of 3 to 5 k/kg.

For heat bursts, the physical mechanism usually associated with dry adiabatic descent is evaporational cooling of precipitation beneath a trailing storm anvil. The denser evaporationally cooled air accelerates downward, and if it can penetrate the boundary layer, produces the warming and gusty winds observed in a heat burst.

Examples

Presented below are five examples of heat bursts as depicted in surface weather observations and, in some cases, documented in local statements. In all cases the temperature rises, the dew point drops, and the winds gust strongly during the heat burst event.

Example 1: Lincoln, Nebraska, May 6, 2004

The table below lists the weather conditions at Lincoln, Nebraska, during the late evening of May 6, 2004. At 0319 UTC winds picked up from the southwest and then shifted to the northwest with gusts as high as 60 knots. During this period there were thunderstorms in the vicinity, the temperature rose from 27.2°C (81.0°F) to 31.7°C (89.1°F), while the dew point dropped. These conditions are typical of heat bursts.

Example 2: Fort Riley, Kansas, September 10, 1990

The table below lists the weather conditions during the early morning of September 10, 1990 at Fort Riley, Kansas. A period of showers started around 0438 UTC with thundershowers starting around 0510 UTC. The temperature rose a few degrees when the showers began, but it was not until the 0930-0955 UTC time period when heavy thunder was reported that the temperature rose dramatically and the wind gusted to 61 knots. The peak temperature during this period was 91°F, an 11 degree rise in temperature. Note that the dew point dropped during this temperature rise.

This example is different from most heat burst events in that precipitation occurred just prior the heat burst itself.

Example 3: North Platte, Nebraska, June 28, 1990

The table below lists the weather conditions at North Platte, Nebraska, just after midnight on June 28, 1990. Quoting the statement by the National Weather Service office in North Platte:

Note that as the temperature rose the dew point dropped from the prevailing mid-60 values to the mid-40s. The "hot bubble" persisted for approximate 2 hours before pre-heat burst conditions returned.

Example 4: Pierre, South Dakota, June 20, 1989

The table below lists the weather conditions at Pierre, South Dakota, during the night of June 19-20, 1989. Quoting the statement in the National Weather Summary on the morning of June 20th:

The dew point dropped some 25 to 30 degree Fahrenheit during the downburst.

Example 5: Dodge City, Kansas, April 3, 1989

The table below lists the weather conditions during the evening of April 3, 1989 at Dodge City, Kansas. Although not as dramatic a case as some of the other examples, there is a sudden wind shift from south to southwest and strong gust to 51 knots at 0345 UTC. Accompanying this wind change is a 10 degree Fahrenheit temperature rise from 65°F to 75°F. The dew point also drops some 8 degrees Fahrenheit during this period. The "heat bubble" dissipates within a hour as conditions return to pre-heat burst levels.

Review Questions

Instructions: Place the cursor over the answer of your choice. If you are correct, it will be highlighted in green; if you are incorrect, it will be highlighted in red.

When a heat burst occurs, what weather is observed at the Earth's surface

1.  rapidly rising surface temperatures
   
   
2.  falling dew points
   
   
3.  strong gusty winds
   
   
4.  all of the above

The vertical temperature sounding associated with a heat burst is usually characterize by:

1.  an inversion
   
   
2.  a dry adiabatic lapse rate through a deep layer
   
   
3.  a saturated layer near the surface
   
   
4.  an absolutely stable lapse rate

References

Glickman, T.S., 2000: Glossary of Meteorology. American Meteorological Society, Boston, 855 pp.

Johnson, B.C., 1983: The heat burst of 29 May 1976. Monthly Weather Review, 111, 1776-1797.

Wakimoto, R.M., 1985: Forecasting dry microburst activity over the High Plains. Monthly Weather Review, 113, 7, 1131-1143.