Dew and Water Vapour

10:59am Tuesday 25th March 2008

The air is never completely dry, even in the driest parts of our globe like the deserts, the air contains water vapour. During the day light hours the surface warms, the air in contact with the ground warms up to, but overnight, the ground cools by radiating its warm into the sky. If conditions are ideal temperatures will fall quickly, especially the ground temperature. If skies are cloudy, this acts as a barrier, most of the radiant heat is reflected by the clouds back again, slowing down the rate at which the ground cools. If the skies remain clear, the ground rapidly cools, quickly chilling the air next to it. If the air is cooled to its dewpoint temperature, water vapour will condense on to surfaces and dew will form. Dew in the morning means that the night has probably been cloudless. In dry climates this small amount accounts for a significant proportion of all precipitation.

Next week......Snow and Sleet.The answer to last weeks question: Refraction of Light.This week's question: Temperatures fall rapidly when skies are:

A/ Clear.

B/ Cloudy.Good Luck.Glossary: Chinook: A warm, dry wind on the eastern side of the Rocky Mountains in North America.Weather for coming week Monday 24th March-Friday 28th March.Low pressure predominating over this coming week. Cold air will be replaced by milder Atlantic Air Wednesday onwards.Rain or showers at times with some drier and brighter intervals between. Showers wintry on Monday. Windy at times with mist over the hills in any rain.Maximum temperature 10-13c 50-55f.Minimum temperature 3-6c 37-43f.Last Week's Observations (Monday-Sunday)Highest day temperature...........11.2c 52f on Thursday 20th March.Lowest air temperature...............Minus 1.5c 28f on Tuesday 18th March.Wettest day..............................1.3 mm on Thursday 20th March.

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