Outflow Phenomena

This section is on visual identification of macrobursts, microbursts, gust fronts and other outflow phenomena. Damaging thunderstorm winds have been termed downbursts by renowned severe storm researcher Dr. Ted Fujita. Dr. Fujita further classifies these events as macrobursts (greater than 2.5 miles in diameter) and microbursts (less than 2.5 miles in diameter).

The problems that aircraft have had with thunderstorm-induced wind shear, particularly microbursts, indicate that the spotting and reporting of microbursts is of paramount importance. Although some spotters will think that events such as microbursts and flash floods are less dramatic than tornadoes, in reality, they are just as lethal, if not more so, in some circumstances. Hopefully, pilots will find these slides beneficial in identifying outflow structures that could result in dangerous approach or take-off conditions, and delay their subsequent actions until danger has passed.

Photograph by: Moller

A downburst is a strong downdraft which includes an outburst of potentially damaging winds on or near the ground and if the diameter of the downburst is greater than 2.5 miles, then it is called a macroburst. As a macroburst or a non-severe gust front passes overhead, the ragged, concave-shaped underside of the shelf cloud accompanies the onset of cold outflow winds at the ground. Although some rotation may be visible in these clouds, it is likely to be short-lived and without vertical continuity, precluding a major tornado. Another clue for spotters as to the potential of any observed rotation would be the lack of warm and moist surface-based inflow to the feature.

Photograph by: Moller
Occasionally a cloud hole will appear behind, or in some cases immediately ahead of a gust front. The cause is frequently a small scale downdraft, possibly a microburst, which is resulting in rapid cloud dissipation.

There is little doubt about a small downdraft being the culprit in this particular case, as evidenced by the amount of blowing dust that has been kicked up beneath the cloud hole.

Wind Shear
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Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.

gust fronts