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Old 27-11-2003, 10:42   #1
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Excellent Weather Article

Sailnet has an excellent article on using Satelite immagry for weather forecasting.
www.sailnet.com/view.cfm?page=11975
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Old 12-07-2004, 07:22   #2
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Jet Contrails

JET CONTRAILS & WEATHER FORECASTING:

THICK &/or LINGERING CONTRAILS - indicate changing weather and/or precipitation
DISSIPATING or NO CONTRAILS - indicate continuing and/or clearing weather.

The plume of condensed water vapor in the exhaust trailing behind high flying jet aircraft is called a “contrail” - short for condensation trail. Contrails are long white streaks of ice crystals left behind in the exhaust of flying jet aircraft. At 8 to 12 km (5-7.5 mi.) above the ground, the jet engine pulls in very cold, dry air and spews out hot, water-filled exhaust. The hot water vapour mixes with the colder surrounding air. In doing so, it expands and freezes in (1 or 2 seconds) forming a trail of ice-crystal clouds. Contrails will only form into long-lasting visible cirrus-like cloud formations when the air temperature is at -40C or lower (-40F) and the humidity level is at 70% or greater.

Contrails can provide a clue to upcoming weather. If a jet leaves no trail or a short trail; or if the trail fades quickly - it is an indication that the air is relatively dry and sinking, which suggests that fair weather is likely to continue. On the other hand, if the exhaust trails linger for an hour or more; or they spread across the entire sky - the surrounding air is probably moist and rising, foretelling the arrival of a storm in a day or two. Reading contrails gives you a glimpse as to the weather over the next 12 to 24 hours.

Sky light blue to dark blue. Bright moon. Jet contrail disappears immediately or doesn't form. all indicate: Continuing Good Weather, Clear.

Large halo circling moon or sun. Jet contrail lingers thickly before falling apart indicate: Possible Change.

FWIW
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Old 12-07-2004, 15:24   #3
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Does this suggestion, change it's reliability in differing parts of the world. I mean by that, is it more reliable in say the tropical area's or southern/northern latitudes? The reason I ask, is down here in NZ, we get Large High pressure (anti_cyclone) area's, with low pressure fonts circling around them. If we happen to be in the path of the front, we will get a Southerly (cold/wet) pattern or it can bend across the country, due to our land shape and contours and become a warmer Westerly. So how does the Contrail depict that a front is coming. Or does this article explain and I should read it????
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Old 14-07-2004, 08:02   #4
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I'm not a weather expert, but reading the article the idea is that dry sinking air is stable, and warm moist air is unstable. This should hold true in most areas, northern or southern hemisphere.

However, on the extremes you could be in the tropics and have moist rising air as a normal occurence and this would not predict the coming of a disturbance. Likewise in the higher lattitudes the air is always drier so bad weather could be coming even with dissapating contrails. So, like most "rules of thumb" it is probably valid most of the time, you just have to be aware of it's short comings.
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