Downburst
Primary reference(s)
WMO, 1992. International Meteorological Vocabulary. WMO-No. 182, 2nd edition. World Meteorological Organization (WMO). Accessed 13 October 2020.
Additional scientific description
Downbursts are powerful winds that descend from a thunderstorm and spread out quickly once they hit the ground. These winds can easily cause damage similar to that of an EF0 (65–85 mph winds) or even EF1 (86–110 mph winds) tornado, and are sometimes misinterpreted as tornadoes. However, downbursts are a completely separate phenomenon (NOAA, 2019). The key differences between a downburst and a tornado are expressed by two words – IN and OUT (NOAA, 2019):
- IN - all wind flows INTO a tornado. Debris is often lying at angles due to the curving of the inflow winds.
- OUT - all wind flows OUT from a downburst. Debris is often lying in straight lines (hence the term ‘straight line winds’) parallel to the outward wind flow.
Downbursts are also far more frequent than tornadoes – in fact, for every one tornado there are approximately ten downburst damage reports. Tornadoes average about 800 per year in the United States, in contrast to an average of 100,000 thunderstorms (NOAA, 2019).
Metrics and numeric limits
| Microburst | Macroburst |
|---|---|
| Damaging winds extending up to 2.5 miles (4 km) | Damaging winds extending more than 2.5 miles (4 km) |
| Lasts 5 to 15 minutes | Lasts 5 to 30 minutes |
| Can cause damaging winds up to 168 mph (270 kph) | Damaging winds, causing widespread, tornado-like damage, up to 134 mph (216 kph) |
Key relevant UN convention / multilateral treaty
Not applicable.
Examples of drivers, outcomes and risk management
Downbursts are a particular hazard to aircraft at low level, especially on take-off or landing. An aircraft approaching a downburst will first encounter a strong headwind, which will lead to an increase in indicated airspeed. When trying to fly a set airspeed on approach, a pilot might therefore be tempted to reduce power. This would be very dangerous because, as the aircraft passes thorough the downburst, the wind becomes a tailwind and the indicated airspeed and lift drop. The significant downward force of air in the downburst may be enough to force the aircraft into the ground or at least cause it to lose a significant amount of height. The subsequent loss of performance, as the aircraft encounters tailwinds, may cause further loss of height and be enough to cause the aircraft to stall (SKYbrary, no date).
Many lives have been saved because of the reduction, if not elimination, of potential airline crashes caused by dangerous wind shear conditions on take-off and landing. These saved lives are the result of training pilots on the dangers of microbursts and the installation of Doppler radars at major airports across the United States to warn pilots when microbursts are present (Wilson and Wakimoto, 2001).
References
NOAA, 2019. How do downbursts form? National Oceanic and Atmospheric Administration (NOAA) National Weather Service. Accessed 19 November 2019.
SKYbrary, no date. Microburst. Accessed 19 November 2019.
Wilson J.W. and R.M. Wakimoto, 2001. The discovery of the downburst: T. T. Fujita’s contribution. Bulletin of the American Meteorological Society, 82:49-62.