Friday, July 11, 2008

The importance of dewpoint

In an effort to educate on this blog, I bring you today's topic: the importance of dewpoint.

Dewpoint, as defined by the National Weather Service, is "the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant)" (ref: Or, more simply, it is a measure of the amount of atmospheric moisture. Why is it important? Well, the amount of moisture in the air helps determine things like whether clouds or precipitation will form and how "humid" it feels. Frequently in these parts, you'll hear someone say, "the humidity sure is high today." What they are talking about (maybe without knowing it) is how humid or sticky the air feels, as measured by relative humidity. Relative humidity is calculated using the temperature and dewpoint. Based on the definition above, if the temperature is 85 and the dewpoint is 65, that means the 85 degree air must be cooled to 65 for it to be saturated (or to reach 100% relative humidity). Therefore, the relative humidity can be calculated (using a formula) as 51%.

It is very important when forecasting temperatures and humidity to know what the dewpoint will be as well. In fact, it is so important, that it is one of the parameters I keep accuracy statistics on when making my forecasts. When I write a forecast, I always also forecast dewpoint, even though it does not appear on my actual forecast page. It is very helpful in knowing what a morning low temperature will be (the temperature can never fall below the dewpoint, otherwise humidity would be above 100%!), what the heat index will be (temperature and humidity are used to calculate heat index), and how much moisture is in place to feed potential thunderstorm development. It's also important to know the temperature and dewpoint spread above the surface (if temperature and dewpoint converge, the humidity nears 100% and clouds form at that level).

So, dewpoint is one of the parameters that must be taken into account when producing an accurate forecast. And next time you hear someone say "the humidity sure is high" - correct them by saying "perhaps it's the dewpoint." They'll probably look at you funny, and that's OK. ;-)

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