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From: Jesse Ferrell (WeatherMatrix) (ferrell{at}weathermatrix.net)
Date: Sat Mar 26 2005 - 13:56:58 EST
>><http://email.trb.com/cgi-bin1/DM/y/jnI60H4Nas0G150GWT50EA>BIZARRE
>>WEATHER PHENOMENON SLOWS TRI-RAIL TRAINS
>>Commuters using the Tri-Rail trains throughout South Florida were delayed
>>this morning -- by Mother Nature. A bizarre high pressure weather
>>phenomenon called tropospheric ducting disrupted radio signals---making
>>it impossible to operate the commuter trains safely at more than 15 mph.
>
>
>This site has forecast maps of this, though I don't fully understand them:
>http://home.cogeco.ca/~vem3ont22/tr_map/archive/5car.jpg
>
>
>"Tropospheric Ducting (TrD)..is an abnormal condition. An inversion has
>formed at a much higher level above the ground...the vast majority of
>duct-producing inversions lie between 450 and 1500 m (1500 to 5000
>ft)..with a few between 1500 and 3000 m (5000 to 10,000 ft). These
>inversions are not formed due to nighttime radiation/cooling..but rather
>because of some other weather phenomenon (high pressure subsidence aloft,
>warm frontal boundary, cold frontal boundary, oceanic or lake inversion,
>Chinooks, etc.). Because of this..ducting can occur day or night (though
>it strengthens at night)..is not usually influenced by terrain (the
>exception being large mountain chains like the Rockies)..and from a DXers
>point of view is usually either uni- or bi-directional. In fact..typical
>ducts are sharply directional. Signals refract off of and also travel
>along the inversion..thus the analogy of a duct. Distances are
>theoretically unlimited. One large area can have multiple ducts going on
>simutaneously..but they are usually parallel paths. It is possible in a
>very strong high pressure system to have large areas of ducting creating
>multi-directional openings. These are the rare "blockbuster" openings that
>make DXers' mouths water. They are most common over the oceanic areas in
>the tropics and sub-tropics."
>
>
>--
--
===================================================================
Jesse Ferrell - Meteorologist / Webmaster / Founder - WeatherMatrix
10,000 Weather Enthusiasts Can't Be Wrong! www.weathermatrix.net
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