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Old 2nd December 2009, 08:39 PM
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rich rich is offline
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Join Date: Nov 2008
Location: Shebbear, N Devon
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It's when the air is approaching saturation point and on the verge of condensing (and forming mist) that the greatest effect will be noticed. For the meteorologists and physicists it's when the temperature is close to the dewpoint.

Colloquially we talk about humidity and get - in my view - an imperfect understanding. Warm air can contain much more water vapour than cold air. We talk about it being very humid on a muggy summer's day. Yes, there is a lot more moisture in the air than there would be on a cold day, but the summer air is able to hold a lot more before it gets saturated.

So it depends if you are talking about absolute humidity or relative humidity.

For example, today down here in Devon the relative humidity has been around 90 percent all day. Only another ten percent to saturation. In the summer we seldom see relative humidities as high as 90 percent, although the air contains a lot more moisture then.

The worst times for unexpected pellet drop are generally early morning when it's been a cold damp night, with a lot of dew on the ground. As the daylight starts to burn off the dew, so the relative humidity of the layers of air close to the ground increases as the air absorbs this moisture. In an hour or so the wind - and it doesn't take much - will have assisted mixing of the strata and the worst of the effect will have gone.

And the viscosity of the air is not an issue; air viscosity actually rises with temperature (at the levels we might be interested in) by about 5 percent from 0 to 20 degrees. Its density varies by about 7 percent in the opposite direction.

Sorry for the lecture.....
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