University of Illinois

> online guides
  educational cd-rom
  current weather
  about ww2010

Online Guides
> meteorology
  remote sensing
  reading maps
  projects, activities

  air masses, fronts
  clouds, precipitation
  el nino
  forces, winds
  hydrologic cycle
  light, optics
  midlatitude cyclones
> severe storms
  weather forecasting

Severe Storms
  dangers of t-storms
  types of t-storms
> tstorm components

Tstorm Components
  wind shear
> outflow phenomena
  wall clouds

Outflow Phenomena
  gust fronts
> microbursts
  scud clouds, virga
  rain foot, dust foot

> introduction
  developing rain shaft
  extreme microburst

User Interface
> text

NOTE: We've guessed that you're not using a client that supports colored tables and have tried to compensate. Low graphics mode looks much better on clients that do... we recommend switching to Netscape 3.0 or Microsoft Internet Explorer.
downburst less than 2.5 miles in diameter

A downburst is a strong downdraft which includes an outburst of potentially damaging winds on or near the ground. If the diameter of the downburst is less than 2.5 miles, it is called a microburst. The diagrams below depict the evolution of a microburst.

A microburst initially develops as the downdraft begins its descent from cloud base. The downdraft accelerates and within minutes, reaches the ground (contact stage). It is during the contact stage that the highest winds are observed. During the outburst stage (above), the wind "curls" as the cold air of the microburst moves away from the point of impact with the ground. During the cushion stage, winds about the curl continue to accelerate, posing a great threat to nearby aircraft.

These are very weak, high based showers without thunder, but with microbursts. Studies have shown that they predominantly occur in the High Plains and western U.S.: particularly in unstable, very dry low level environments with surface temperature-dew point spreads of 30 to 50 degrees and an area of mid-level moisture as a source for the weak showers.

[Image: virga trail indicating microburst (63K)]
Photograph by: Moller

The cloud on the left is developing, whereas the fuzzy anvil on the right has matured and is producing a trail of virga. Microbursts would be most likely to occur beneath the virga, when the downdraft reaches the ground. Several of these virga showers did produce microbursts in the Lubbock, Texas area.

[Image: blowing dust resulting from microburst (55K)]
Photograph by: Moller

The same day, near the Lubbock Airport, we see several of the small microbursts which emanated from the virga patch in the upper right corner of the photograph. Wind shifts of 35 to 40 MPH were noted shortly after this time, with a rapid onset and cessation of the wind gusts.

gust fronts
Terms for using data resources. CD-ROM available.
Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.