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Ask Todd: Dew point

11:50 PM, Jan 3, 2013   |    comments
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Mark Bivens asks: "How can we have a dew point of 39 and be raining 100 percent? I thought rain would bring higher dew points."

Thanks for your question, Mark. Let's start by explaining dew point.

Dew point is the temperature air would have to be cooled to in order for saturation to occur.

Another term we often associate with dew point is "humidity." Humidity refers to water vapor in the air. That's water in the form of an invisible gas. The warmer the air, the more water vapor it can have in it.

When the air can no longer hold all of the water vapor in it, the vapor begins condensing into ordinary, liquid water.

If the air is in the sky, the vapor condenses into cloud drops. If the air is right above the ground, the vapor condenses to make fog. If the air is touching something, like a store window or grass, it condenses to make dew.

Dew point is really a measure of how much water vapor is in the air, but it's given as the temperature at which that particular bunch of air has to cool for the water vapor to begin condensing.

Now, when it comes to rain, the dew point and temperature don't really matter.

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