Microburst

A microburst is a localized, powerful column of descending air (downdraft) within a thunderstorm, less than or equal to 4 km in diameter and typically lasting less than 5 minutes. Despite its small size and brief duration, a microburst can generate winds exceeding 160 km/h and create a life-threatening shear environment for aircraft on approach or departure.

Diagram of a microburst showing the central downdraft and diverging outflow winds at the surface, with a typical aircraft encounter showing airspeed increase then dramatic decrease
Microburst structure. Source: NAV CANADA Aviation Meteorology Reference

How a Microburst Affects an Aircraft

An aircraft flying through a microburst on approach encounters a characteristic sequence: first a headwind increase (causing an apparent performance gain — the aircraft gains airspeed and goes above the glidepath), then the core of the downdraft (pushing the aircraft down), then a tailwind as the aircraft exits on the far side (causing a sudden, severe airspeed loss and glidepath undershoot). This sequence is extremely dangerous at low altitude.

Formation

Microbursts are associated with thunderstorms. Conditions that promote intense downdrafts include: dry air at mid-levels that evaporates falling precipitation (evaporative cooling drives the downdraft faster), high freezing levels (precipitation falls farther and evaporates more), and instability at low levels. High-based thunderstorms over dry terrain are particularly likely to produce microbursts.

Detection

  • PIREPs from preceding aircraft are the most timely warning
  • Low-Level Windshear Alert System (LLWAS) at equipped airports detects outflow wind changes
  • Doppler weather radar can detect the diverging outflow from a microburst
  • Visual indicators: rain shaft beneath dark cloud base, virga (rain that evaporates before reaching the ground)

Response if Encountered

  • Immediately execute a go-around — do not attempt to land through a microburst
  • Apply maximum thrust
  • Rotate to a go-around attitude and climb
  • Fly through the shear rather than trying to correct back to the glidepath
  • Declare an emergency if necessary

On the TC Written Exam

Microburst is a core topic in the meteorology section. Know: it’s a localized downdraft from a thunderstorm; the headwind-downdraft-tailwind sequence; maximum duration (5 minutes); maximum diameter (4 km); that it’s the most dangerous form of low-level wind shear.

Related Terms

See also: Low-Level Wind Shear, Thunderstorm, Low-Level Jet.