With cold temperatures comes the frost, but why on some days and - WBTV 3 News, Weather, Sports, and Traffic for Charlotte, NC

With cold temperatures comes the frost, but why on some days and not others?

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With this week's cold snap, temperatures have started off near freezing for many of us the past few mornings.  While some of us in the higher elevations have awoken to snow, many across the Piedmont saw frost instead.  So how does it form?

Unlike snow or sleet, frost is not any form of precipitation.  It doesn't start from a cloud and it doesn't fall from the sky.  Instead, frost begins as water vapor suspended in the air. 

Let's say we have a clear sky overnight, with a current temperature of about 35° and a dew point (or in this case frost point) of 30°.  If we cool the temperature to the dew point, we will have 100% relative humidity.  As temperatures continue to drop, the relative humidity increases, until we drop to 30°.  Once we've reached the dew point, water vapor can condense, or change states into a liquid.  But in this case, we're below freezing.  Instead of condensing, the water vapor is undergoing a phase change known as deposition- going from a gas to a solid.

One of the most common places we see frost form is on blades of grass.  They're close to the surface and are efficient emitters of radiation, meaning they lose their heat very quickly.  It's also frequently seen on cars and other objects that are near Earth's surface, but not the ground itself.  The ground holds heat longer than these objects.

So on the grass, or whatever object we're considering, small crystals of ice collect on its surface as it touches the air under the right conditions. 

The ideal weather situation for frost to form is a clear night with little wind.  Clouds have a blanketing effect, bouncing radiation back to the surface rather than emitting it into space.  That's why cloudy nights are more mild.  Strong winds allow drier air higher in the atmosphere to mix in with the more humid air near the surface.  Also, dry regions or areas that have not seen precipitation in a long time are less likely to see dew or frost.  Much of the required water vapor comes from evaporated soil moisture.  It's another reason dew and frost readily collects on blades of grass.

It's also important to note that sometimes frost can form even when temperatures are technically above freezing.  That's because temperature observations are taken about 4 feet above the ground, where temperatures can be a few degrees warmer.

With another clear, cold night ahead we may see another frosty morning.  Warmer air is on the way, though.  Here's a look at your 7 Day Forecast.

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