Microburst at Mathis Field
in San Angelo
July 9, 2012
|Wind damage at Mathis Field.|
A damaging wind event occurred at Mathis Field during the evening hours of Monday, July 9, 2012. Around 5:00 PM CDT, an ordinary thunderstorm developed near Lake Nasworthy, moving slowly to the south southwest. This thunderstorm quickly collapsed as it moved across the airport grounds, resulting in damaging straight line winds in excess of 70 mph. Widespread damage to large mesquite trees was noted on the northwest side of the airport between the Customs building and Knickerbocker Road. On the airport grounds, damage was found at several hangars. One large hangar door was pushed in and another was blown completely off of the building.
A third hangar suffered minor roof damage and had several north facing windows blown in. No planes were damaged during this event but several were reportedly turned 90 degrees by the strong winds. The Automated Surface Observing System (ASOS) on the far side of the runway measured a wind gust of 68 mph at 5:18 PM CDT. Slightly stronger winds of near 75 mph were estimated in the areas that received the most extensive damage. Fortunately no injuries were caused by this storm.
How do thunderstorms create damaging winds?
As air rises, it will cool to the point of condensation where water vapor forms tiny water droplets, comprising the cumulus cloud we see. As the air continues to rise, further condensation occurs and the cloud grows. Near the center of the updraft, the particles begin to collide and coalesce forming larger droplets. This continues until the rising air can no longer support the ever increasing size of water drops. This is called precipitation loading.
Once the rain drops begin to fall friction causes the rising air to begin to fall towards the surface itself. Also, some of the falling rain will evaporate. Heat energy is removed from the atmosphere by evaporation which results in a net cooling of the air associated with the precipitation. Dry air intrusion into the middle layers of the storm will enhance this cooling effect and is commonly observed with severe thunderstorms.
As a result of the cooling, the density of the air increases causing it to sink toward the earth. The downdraft also signifies the end of the mature cycle of the thunderstorm and represents its transition to the dissipating phase.
When this dense rained-cooled air reaches the surface it spreads out horizontally (sometimes in all directions) with the leading edge of the cool air forming a gust front. The gust front marks the boundary of a sharp temperature decrease and increase in wind speed. The gust front can act as a point of lift for the development of new thunderstorm cells or cut off the supply of moist unstable air for older cells.
Downbursts are defined as strong winds produced by a downdraft over a horizontal area up to 6 miles (10 kilometers). Downbursts are further subdivided into microbursts and macrobursts.
|Figure 1. Microburst Schematic Diagram|
What is a microburst?
A microburst is defined as a small downburst with horizontal dimensions of less than 2.5 miles (4 kilometers) that typically last on the order of 2 to 5 minutes. Microbursts can be very strong, despite their short life cycles. Winds within a microburst can exceed 100 miles per hour. They can be especially dangerous to the aviation community and are responsible for numerous aviation mishaps throughout the years. A macroburst is a larger phenomenon that typically produces damaging wind gusts over a horizontal range of greater than 2.5 miles (4 kilometers).