Ask Todd: Windshield frost

10:02 PM, Sep 25, 2012   |    comments
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Bob Taylor asks: "Todd, if water freezes at 32 degrees, how can I have ice on my windshield and frost on the rooftops at 40 degrees? This was the case Monday at my house here in Jacksboro."

Well Bob, frost actually cannot form at temperatures above freezing. Frost is defined as a layer of ice that forms on surfaces that are at or below 32 degrees.

Sometimes frost does appear on your lawn overnight or your car, even though your thermometer may have never dropped to the freezing mark. That's because cold air on clear, calm nights sinks to ground level, and things like your windshield or windows tend to lose heat more quickly.

So for example, the thermometer might read 40 degrees on top of a fence or near a roof, which is several feet above your car. But closer to the ground, it might read 32 degrees.

It's also important to note that official weather measurements are taken in an instrumental shelter four to five feet above the ground, not at the level of your car windshield.

And one more thing, frost also has everything to do with the amount of moisture in the air. Temperatures below or near freezing with moisture will get you a frosted windshield, but below freezing temperatures with very dry air may not produce much frost because there's less moisture.

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