METEOROLOGIST JEFF HABY
A heat burst is a downdraft of hot and dry air that typically occurs in the evening or overnight hours.
A heatburst is much more rare than a
severe thunderstorm. Thus, the
atmospheric conditions have to be just right for a heat burst to occur. Heat bursts usually happen in the evening
or at night after thunderstorms are ending. Thus, a thunderstorm has a vital role. Two other
characteristics are that the air must start its descent from fairly high up and the environmental air
aloft needs to be very dry.
Precipitation falling into very dry air aloft will cause the air to cool
latent heat absorption. If the air is very high aloft and the air is much more dense than
the surrounding air then it will accelerate toward the surface. In the case of a heat burst, all
the precipitation that cooled the air aloft has been vaporized. Therefore, the precipitation can
no longer absorb latent heat. As this dense air accelerates toward the surface it rapidly warms
at a dry adiabatic lapse rate compression. What must make a heat burst so rare is that this
downdraft must reach extremely high velocities. Velocities must be high enough so that the momentum
of the sinking air offsets the fact that it is becoming warmer in temperature and thus
less dense than the environmental air it is falling into.
Not only is the air in a heat
burst anomalously hot but it is also extremely dry. The extremely high temperatures
combined with extremely dry air plus the wind can remove all the moisture out of
vegetation. Just like any atmospheric phenomena, heat bursts have differing
intensities. The worst heat bursts persist for a significant amount of time
and have temperatures that go over 120 F, even in the middle of the
night. This extremely hot and dry air can remain in place for several hours before
temperatures return to normal.
A microburst occurs through the same process except that the origin of the microburst is not as high aloft
as a heatburst. Thus, the microburst is not able to get as much adiabatic compression as a
heatburst. Often a microburst will not lose all of it's precipitation before hitting the
ground. If precipitation is within sinking air, evaporative cooling will partially offset
the adiabatic compressional warming.