WRAL WeatherCenter Blog

Not Jack Frost, but a 'black frost'

Posted January 19

We have some very cold temperatures to start the day, and if you're out early you might notice that for the most part, you didn't have to scrape frost off the windshield or look out on a glittering lawn or garden.

The reason for this is a very dry air mass that settled in behind an arctic cold front as we headed through Monday afternoon and evening. By this morning, dew points across the area were generally in the low-to mid-single digits, after running around zero for part of the night, and as the graph here shows, they will likely stay quite low as we head through the day.

When dew points are that different from the temperature, it indicates very low relative humidity and can often mean that there isn't enough water vapor in the air to form visible frost on objects at the surface.

When we have a hard freeze of this sort without enough moisture to form those ice crystals, there are a couple of terms most frequently applied to the situation - one is a "dry freeze," and the other, especially when applied to any exposed vegetation, is a "black frost." This is a term that comes from the fact that many types of vegetation exposed to temperatures this cold, without the protective effects that a layer of frost can provide, will be badly damaged and turn black or dark green.

Even in conditions such as today, a few of you may note some visible frost in spots. This can occur close to localized sources of moisture such as ponds, creeks or rivers, and perhaps drainage ditches, steam vents and the like.

Otherwise, instead of "Jack Frost" painting the area with a sparkly coating this morning, we get the cold without the decoration.

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