Microbursts-Green

**Microbursts** Drew H. & Andrew M. Green Class Introduction  Microburst: Strong, damaging winds caused by downdrafts which can spread out about 5 km upon surface impact 1+4 . Microbursts generally appear from cumulonimbus clouds and come in two varieties: wet and dry 4 . A wet microburst is accompanied with a downpour, but during a dry microburst no rain reaches the ground 1. Microbursts are most recognizable because of their effect on trees. During a microbursts, intense winds (sometimes up to 100 mph 1 ) cause trees to bend far over and many times snap right in half. Dangers Microbursts are a major hazard for aviation as they can destabilize an aircraft that is taking off or coming in to land 1 . The strong downward force of microbursts often causes them to crash as shown in the image to the right. <span style="font-family: 'Comic Sans MS',cursive; line-height: 21px;">Also, microbursts tend to be large-scale. Thankfully they can then be easily distinguished as harmful to an aircraft and are avoided<span style="font-family: 'Comic Sans MS',cursive; line-height: 21px;">.<span style="font-family: 'Comic Sans MS',cursive; line-height: 21px;"> But unfortunately they are isolated, small and quick to develop <span style="font-family: 'Comic Sans MS',cursive; line-height: 21px; vertical-align: super;">2 <span style="font-family: 'Comic Sans MS',cursive; line-height: 21px;">, making them unpredictable<span style="font-family: 'Comic Sans MS',cursive; line-height: 21px;">. The fatalities per year attributed to microbursts vary greatly, but some years it is well over 100 <span style="color: #008080; font-family: 'Comic Sans MS',cursive; font-size: 110%; line-height: 21px; vertical-align: super;">3 <span style="font-family: 'Comic Sans MS',cursive; line-height: 21px;">. Most often planes crash because the tailwind of the microburst is so strong that it drives the ascending plane nose-down toward the ground. Many microbursts cause trees to fall as well (see video below). This could result in damaged homes and power lines. <span style="color: #404040; font-family: 'Comic Sans MS',cursive; font-size: 24px; line-height: 36px;">Conditions of Formation <span style="font-family: 'Comic Sans MS',cursive;">As the name suggests, dry microbursts occur in dry conditions. Rain falls into a layer of dry air under the cloud causing it to evaporate, this evaporated rain is considered virga since it never reaches the ground as a liquid <span style="font-family: 'Comic Sans MS',cursive; vertical-align: super;">2 <span style="font-family: 'Comic Sans MS',cursive;">. Since evaporation is an endothermic state change, heat is lost from the dry layer of air. The cool temperatures of the outflow of a microburst are evidence <span style="font-family: 'Comic Sans MS',cursive;">of this evaporation <span style="font-family: 'Comic Sans MS',cursive; vertical-align: super;">3 <span style="font-family: 'Comic Sans MS',cursive;">. This lower temperature, combined with the fact that drier air is more dense than humid air due to <span style="color: #000000; font-family: arial,helvetica,sans-serif; font-size: 13px; line-height: 19px;"><span style="color: #008080; font-family: 'Comic Sans MS',cursive; font-size: 16px; line-height: 23px;">water vapor's low density, causes the dry, cooler air to accelerate downward. Occasionally the air will meet with warmer surface <span style="font-family: 'Comic Sans MS',cursive;">air that rises to counter the downdraft, but the descending air can still reach the ground with some velocity 2. Since the precipitation usually evaporates completely, there is little to no precipitation of any kind.

<span style="color: #008080; font-family: 'Comic Sans MS',cursive; font-size: 120%;">Wet microbursts form in the same circumstances, however the layer of air is more humid and often thicker, so only some rain evaporates. These microbursts are associated with cumulonimbus clouds. The remaining rain droplets fall to the ground and, because of friction, carry some air with them 2. The combined descent of the air coming with the rain and air sinking because of some heat loss forms a wet microburst. <span style="color: #008080; font-family: 'Comic Sans MS',cursive; font-size: 120%;">To see an animation of a microburst click here (click on the images to continue the animation). <span style="color: #404040; font-family: 'Comic Sans MS',cursive; font-size: 22px;">Historical Examples <span style="color: #008080; font-family: 'Comic Sans MS',cursive; font-size: 120%;">The first significant microburst-related aviation accident occurred on June 24, 1956 in Nigeria. 32 people died, 11 more were injured, and just 2 people went by unscathed <span style="color: #008080; font-family: 'Comic Sans MS',cursive; font-size: 120%; vertical-align: super;">3 <span style="color: #008080; font-family: 'Comic Sans MS',cursive; font-size: 120%;">. However, almost four times as many people died in another tragic accident in New York in 1975, which was an eye-opening experience for s cientists, and the first notable microburst-related accident in the United States 5. Following the accident, the Federal Aviation Administration (FAA) was encouraged to build what is now referred to as the LLWAS by scientists at the National Severe Storms Laboratory (see below under safety) 3. Interestingly enough, a meteorologist named John McCarthy had pleaded several years earlier to the FAA about how microburst winds are strong enough to take down a plane and that they should make precautions, but the the FAA, "walked out of his presentation" 5. But in 1982, the most fatal microburst aviation accident on record occurred at Louis Armstrong Airport in New Orleans, Louisiana. Heavy rain and wind pounded the aircraft, killing over 150 people, 94% of the people on board the aircraft. No one was left uninjured. Shockingly, the microburst only covered a 2 km area and winds never reached more than 35 mph <span style="color: #008080; font-family: 'Comic Sans MS',cursive; font-size: 120%; vertical-align: super;">3+5 <span style="color: #008080; font-family: 'Comic Sans MS',cursive; font-size: 120%;">. There have been cases of winds over 80 mph from a microburst. If we aren't prepared, there could be many worse aviation accidents 3. media type="youtube" key="q6idRlyyq7o" height="259" width="320" align="right" <span style="color: #404040; font-family: 'Comic Sans MS',cursive; font-size: 24px;">Safety <span style="color: #008080; font-family: 'Comic Sans MS',cursive; font-size: 120%;">In the event of a microburst, it is important to remain in your home, or if you cannot get to significant shelter, move to an open area clear of trees and other debris. Since aircrafts are at the greatest risk to microbursts, airports are crucial to avoid. In 1976, a piece of technology called the Low Level Windshear Alert System (LLWAS) was developed in reaction to a tragic aviation accident at John F. Kennedy Airport - New York in 1975 3. The LLWAS used anemometer-based technology to analyze pressure differences and compare them with other areas, reporting data to a computer 5. From there, air traffic controllers could warn pilots of extremely powerful wind and pressure shifts, which could potentially cause the plane to crash 3. <span style="color: #404040; font-family: 'Comic Sans MS',cursive; font-size: 24px;">Sources
 * 1) <span style="color: #008080; font-family: 'Comic Sans MS',cursive; font-size: 120%;">Linacre, E. and Geerts, B. "Microbursts." //University of Wyoming//. 9 Jan. 2012. <[]>.
 * 2) <span style="color: #008080; font-family: 'Comic Sans MS',cursive; font-size: 120%;">Burroughs, William J., Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot, and Richard Whitaker. "Storms." //The Nature Company Guides Weather//. Ausralia: Time Life, 1996. 2488-48. Print.
 * 3) <span style="color: #008080; font-family: 'Comic Sans MS',cursive; font-size: 120%;">Wolfson, Marilyn M. "Characteristics of Microbursts in the Continental United States."//Massachusetts Institute of Technology.// 10 Jan. 2012. []>
 * 4) <span style="color: #008080; font-family: 'Comic Sans MS',cursive; font-size: 120%;">Caracena, Fernando. "Forecasting Microbursts and Downbursts." __Forecast Research Branch__. NOAA/Forecast Systems Laboratory. 9 Jan. 2012. <[]>.
 * 5) <span style="color: #008080; font-family: 'Comic Sans MS',cursive; font-size: 120%;">Phillips, Don. "How a 'microburst' Downed a Jetliner and Revolutionized Flight Safety - The New York Times." //The New York Times//. 1 Aug. 2005. Web. 16 Jan. 2012. < <span style="color: #000000; font-family: 'Comic Sans MS',cursive; font-size: 120%;">[] <span style="color: #008080; font-family: 'Comic Sans MS',cursive; font-size: 120%;">>.