Microbursts-Purple

 //**MICROBURSTS**// Billy O and Emmett U ** Purple ** Block

Introduction Microbursts are high intensity downdrafts of air that form in thunderheads, and sink to earth causing high winds to spread out radially from the microburst's epicenter 1 . These winds and the nature of their formation causes microbursts to be commonly confused with tornadoes 5 . They are categorized into three types: wet, dry, and hybrid, depending on which factors within the thunderhead caused it 1 . They can be incredibly destructive and very dangerous 2 .

Dangers Microbursts can cause wind speeds in excess of 140 mph- in some cases faster than tornadoes 3. This poses a serious threat to people and property, as such high winds can severely damage structures and trees. However, microburst are a far more serious danger to aviation and air traffic, as planes can be slammed into the ground if they are caught in a microburst, due to the downward motion of the wind 1. The most dangerous time for planes during a microburst is during the approach and ascent, when it is highly susceptible to wind shear that can be caused by microbursts and have little room to recover from any accident 6.

Conditions of Formation Microbursts occur most of the time in **cumulonimbus** clouds (thunderheads) and form from four main conditions within a thunderhead- **entrainment**, **endothermic phase change**, "downward push" and hydrometer loading (also known as precipitation loading) 1&4. Entrainment creates a microburst when dry, midlevel air is caught in a current within the thunderhead, causing water droplets and ice crystals to evaporate or **sublimate** quickly, absorbing **latent heat** and cooling the entrained air very rapidly. This increases the entrained air's density, causing it to sink, which in turn causes more moisture to change to a gas, resulting in a chain reaction of cooling and sinking that accelerates the now downward moving air to very high speeds 4. Endothermic phase change includes entrainment but also includes the melting of ice crystals inside the thunderhead as they descend in the normal **convective cycle** of a cumulonimbus cloud. Melting is another endothermic reaction and so it cools the air around the melting ice, causing it to sink and be caught in the same cycle as downward moving entrained air 1. Hydrometer loading forces air down with the sheer weight of precipitation, which (although it does not cause a microburst on it's own) may allow one of the other factors to kick in. Finally, a downward push is the sinking of air due to unstable air rising and surrounding air sinking to take it's place. This, like hydrometer loading, does not on it's own cause a microburst 1&4. There are three types of microbursts: wet, dry or hybrid. Wet microbursts are happen when the air is moisture laden, and are caused either by precipitation loading or entrainment. Dry microbursts occur in dry conditions and are caused by sublimation or "downward push". Hybrid microbursts are caused by a mix of factors 1.

Historical Examples

In August of 1983, President Reagan, Air Force One and all of Andrews Air Force base in Washington D.C. was hit by a microburst causing winds up to 150 mph just 7 minutes after the president and Air force one had landed. This was the strongest microburst ever recorded at an airport and the president of the United states was only 7 minutes away to being caught in the most dangerous place to be during a microburst- the air 5. media type="youtube" key="BxxxevZ0IbQ" height="315" width="420" align="right"

Safety The largest danger microbursts pose when on the ground is high wind speeds. In order to shelter from such winds, seek shelter, secure unsecured objects, and if outside, avoid debris sources 7. Conditions at airports have improved since the invention of a device called the Low Level **Wind shear** Alert System (LLWAS) by the FAA (Federal Aviation Administration) in 1970. Their design was improved upon by the Research Applications Laboratory of the National Center for Atmospheric Research, adding a microburst detection system that previous systems did not have. This system places sensors around the airport and transmits data to pilots, allowing them to compensate for any potential microburst winds in their approach or takeoff 6. This is not a do-all end-all however, pilots still need to be on the watch for microbursts when flying in thunderstorm conditions.

Glossary Entrainment- to trap air in an air current Cumulonimbus- a rainy, tall, puffy cloud that is also known as a thunderhead. Endothermic Phase Change- a change between states of matter that consumes heat as it changes phase. -- Sublimation- an endothermic phase change in which solids change directly into a gas Latent Heat- the energy stored when substances undergo phase change Convective Cycle- the normal cycle of updrafts and downdrafts that exist in a thunderstorm. Wind shear- A condition in which the direction or speed of wind changes abruptly.

<span style="font-family: Impact,Charcoal,sans-serif; font-size: 24px; line-height: 22px;">Sources

1. Webmaster, AMA. "What is a Microburst?"//Southern Region Headquarters for the NOAA.// National Oceanic and Atmospheric Administration, n.d. Web. 10 Jan. 2012. <http://www.srh.noaa.gov/ama/?n=microbursts>.

2. Caracena, Fernando. "Microbursts: A Handbook for Visual Identification." //The Cooperative Institute for for Mesoscale Meteorological Studies.// Oklahoma University, n.d. Web. 10 Jan. 2012. <http://www.cimms.ou.edu/~doswell/microbursts/Handbook.html>

3. Gudgel, Dan. HighDesertNews. Youtube. Web. 11 Jan. 2012. <http://www.youtube.com/watch?=TkavH9aZue8>.

4. Linacre, Edward, PhD., and Bart Geerts, PhD. "Microbursts." //Department of Atmospheric Science.// University of Wyoming, Jan. 1999. Web. 11 Jan. 2012. <http://www.das.uwyo.edu/~geerts/cwx/notes/chap08/microburst.html>

5. "Downbursts- Severe Weather Awareness." //Eastern Regional Headquarters of the NOAA.//National Oceanic and Atmospheric Administration, n.d. Web. 12 Jan. 2012. < http://www.erh.noaa.gov/er/cae/svrwx/downburst.htm >.

6. "Low Level Windshear Alert System." //Research Applications Laboratory.// National Center For Atmospheric Research, 2012. Web. 17 Jan. 2012. <http://ral.ucar.edu/projects/llwas/>

7. "What to Do when High Winds are Expected." //City of Beaumont.// The State of California, n.d. Web. 17 Jan. 2012. <http://www.ci.beaumont.ca.us/DocumentView.aspx?DID=775>.

<span style="font-family: Impact,Charcoal,sans-serif; font-size: 210%;">Links

To see what can happen when a microburst occurs at sea, visit the following site: http://www.tsb.gc.ca/eng/rapports-reports/marine/2010/m10f0003/m10f0003.asp#sec1_12

To explore what advanced research is currently happening on making the skies safe from microbursts at NASA, go to: http://www.nasa.gov/centers/langley/news/factsheets/Windshear.html