It's below freezing. Why isn't there frost on my car

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Why your car doesn't always have ice on it when its cold.

It is another brutally cold morning out there for us in central North Carolina. So, if it is so cold out, why isn't there always frost or ice on your car in the morning?

Well, it all has to do with the dew point.

For years Big Weather has been asked, "Why do you show the dew point (or DP) on your forecast?"

The short answer is it's the best way to tell how much moisture is in the air. People like to hear the relative humidity, but it doesn't always tell the real story.

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The warmer it is, the more moisture a parcel (a parcel is like a balloon full of air without the balloon around it) of air can hold.

If there is 100 percent humidity at 20 degrees, you can get chapped lips. Mix 100 percent humidity with 70 degrees and the sweat pours off you.

The higher the temperature, the more water it can hold. That's why we don't care about relative humidity in the weather center as much as what the DP is. So, what is it?

The dew point is the temperature at which, if we cooled a parcel of air down to, the air would become saturated (it can't hold any more moisture), and dew would form.

By the way, fog forms when we the air temperature drops to the DP. It can't keep any more moisture in and we see the moisture in the form of water droplets. In a cloud, the temperature and DP are the same numbers.

When we get a summer night with a dew point of 60, and the temperature gets down to 62, we will get dew on the grass because the grass actually cooled to 60 degrees and dew condensed on the cool surface.

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Now back to Sunday night. We had dew points of -4. That means we'd have to get your windshield down to -4 degrees for frost to form.

And it was cold yesterday (17 degrees), but not that cold. So, we didn't see the moisture condense, and we didn't see frost form.

On Tuesday, around 1 a.m. our temperature at RDU was 18 degrees, but the DP was 10. Some metal surfaces (and windshields) dropped to around 10 and we got some frost.

Now, just a couple hours later, the DP dropped and the temperature stayed the same. As the distance grew between the numbers, it was harder for frost to form and that's why it is just a light coating this morning.

If the numbers had been closer, for a longer period of time, the frost on your windshield would be thicker.

One other note, with the dew points so low, the amount of water in the air is so small that we, as humans, really feel the effects of the dry air.

We get dry skin. We get chapped lips. Because the air is so dry, it absorbs the water from our bodies, from our skin, and our lips. When the DP is higher, and there's more moisture in the air, the water stays with us.

Now, go find your ChapStick.
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