|MadSci Network: Earth Sciences|
Hello, Bob - In general, air (or any fluid) which is forced to narrow its path (by going through a narrow passage, between mountains, through gorges, between buildings, or whatever) will flow faster. This is simply a result of conservation - as long as there isn't extra air being supplied from anywhere, and none of it is disappearing, then there has to be the same amount passing any given point on its path per unit time as there is passing any other point. So if the passage gets narrower, the flow speeds up (and the pressure decreases). This is called the Venturi effect; here is a discussion of it (as well as the continuity equation and the Bernouilli effect) from a course offerred at the University of Michigan: http://www-personal.umich.edu/~dierker/classes/p125f96/lecture24/lecture24X5Fnov25-1.supsubsym.html A general discussion of atmospheric winds, and how they vary with terrain and in storms of various kinds, can be found in http://www.civil.buffalo.edu/wind/windinf2.html. This page also contains a number of useful, but non-Web, references. These winds are of particular practical concern to aviators, especially those flying small unpowered craft such as hang gliders; educational information for them often contains advice about canyons and other formations which restrict air passage and therefore can be sources of high winds. However, the highest winds occur during storms. The highest officially recorded wind gust in the world is still the 231 mph gust recorded during a storm on Mt. Washington, New Hampshire in 1934; for a discussion of this gust, and a refutation of a claim for a higher gust during a 1997 typhoon on Guam, see http://www.mountwashington.org./mtw_windrecord.htm. In general, Mt. Washington seems to have the worst weather in the US, and perhaps in the world; a comprehensive Mt. Washington weather page is at http://www.mountwashington.com/weather/home.html. The high probability of strong winds there seems to be a combination of the general topography with the height of the mountain. A wind atlas of the US is available at http://rredc.nrel.gov/wind/pubs/atlas/chp3.html; this atlas is intended to show winds as an energy resource and so is relatively large-scale; however the detailed notes for each map indicate regions where winds may vary significantly from the average. A map showing the highest wind gusts possible if showers or thunderstorms were to occur, and the possibility of both wet and dry microbursts, is available from the GOES satellite data at http://orbit-net.nesdis.noaa.gov/ora/fpdt1/mb.html. These data are updated hourly. It is probable that winds higher than the Mt. Washington gust have occurred in tornadoes, but they have never been measured directly. Tornado wind speeds have been estimated by looking at the damage they cause; the most destructive tornadoes may have wind speeds up to 350 mph. Here are some links about tornadoes: http://www.sciam.com/explorations/052096explorations.html http://www.indirect.com/www/storm5/tornadoindex.html http://www.torro.org.uk/ http://www.discovery.com/area/science/tornado/tornado.html http://www.usatoday.com/weather/tornado/wtwist0.htm In summary, I would say: the most dangerous place in the US for winds is Mt. Washington. No winds as high as 400 mph have been recorded or even suggested. The highest winds come from tornadoes, which can arise relatively quickly but not really "from nowhere". Thunderstorms can also result in high and unpredictable winds. Clear-sky wind gusts also occur, but do not get as strong as storm gusts; they can still be dangerous, though!
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