17th April 2006

Microburst

Source: Excerpt from The Book " Weather "

Distribution: Dry Form common in arid areas, Wet Form common in wet climates. Height: Cloud base around 5000 feet for dry; 2500 feet for wet. Cause: Powerful downdrafts. Associated Weather: Brief often powerful gusts of wind and heavy showers. Hazard Warning: Extreme aviation hazard. A microburst is a brief, powerful gust of wind that appears to radiate from a central point on the ground. It is caused by strong downdrafts that form in the central part of a congestus or cumulonimbus cloud. There are two distinct types of microburst -- Dry and Wet. The dry microburst occurs in dry conditions, when a column of rain falls into a layer of dry air beneath the cloud, and immediately begins to evaporate. Since evaporation produces cooling, this accelerates the downward motion of the air column, producing a powerful gust of wind that spreads in all directions. Where there is warm air near the ground, this will tend to rise and counter the downdraft. However, the descending air may still reach the surface with some velocity. Because the precipitation usually evaporates completely, the only visible sign of a dry microburst will be raised dust. The wet microburst is usually associated with heavy rain and, again, evaporation is the vital ingredient producing strong surface winds. However, in this case, the precipitation reaches the land below. Often, the wind and rain meet the ground with such force that they spread outward and upward, forming a distinctive curl. Microbursts are a major hazard for aviation as they can destabilize an aircraft that is taking off or coming in to land. This has been the cause of a number of serious accidents in the industry.

---

Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.