Kevin Myatt is The Roanoke Times' weather columnist.

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Dry air can give us snow, and dry air can take it away.

Almost anytime there's a reasonable chance of snow in our area, a cold, dry air mass is parked over us. That dry air often eats ups the first waves of precipitation, as you may have noticed on radar a few hours before Thursday's snow. In some cases, it can eat up just about all of it and foil forecasters.

But that dry air also plays a huge role in cooling the atmosphere to allow snow to fall when it would not otherwise occur. The process responsible is called evaporational cooling.

Sometimes it might be hard to understand how it will get cold enough to snow and accumulate when, on the night before an expected winter storm, the temperature stalls in the mid- to upper 30s for hours under a cloudy sky.

When I first moved here and hadn't fully learned the local weather quirks yet, I doubted some snow forecasts when the high temperatures hit the 50s on the day before a forecast winter storm. Where I grew up in northern Arkansas, a high above 42 the day before a possible winter storm would almost always mean it would be rain instead. The temperature simply would not recover that much.

But the geography of our region often allows colder, drier air to become trapped on the eastern side of the Appalachians while warmer, moister air can ride over the mountains on top of the cold, dry air mass.

Temperatures at the surface may rise well above freezing, but as precipitation falls into the thick layers of dry air above the surface, it evaporates. The process of evaporation takes heat out of the atmosphere. As heat is taken out of the atmosphere, the temperature falls.

It is not unusual before a winter storm for the arrival of cloud cover to halt the radiational cooling we get on clear nights. Warmth that would simply radiate out of the atmosphere is blocked by the cloud cover, and our surface temperatures may stall above the freezing mark.

But when the precipitation begins, the atmosphere is cooled and moistened layer by layer downward until snow reaches the ground and temperatures fall below freezing.

While temperatures at the surface may be hovering above freezing, dew points are often in the teens or even the single digits. The dew point is the temperature it must be for the air to be saturated with moisture. Cold air holds less moisture than warmer air, so a dew point many degrees lower than the air temperature indicates a very dry atmosphere.

As the evaporational cooling occurs, the dew point creeps upward as the air moistens even as the temperature drops. The temperature and dew point will meet somewhere between the original readings if the atmosphere becomes completely saturated.

This is an entirely different situation than those cold, rainy evenings when there is a threat of ice. If it's raining and 34 degrees, the atmosphere is probably already nearly saturated, so there isn't much opportunity for evaporational cooling. The only way it will get to freezing is if there is something in the atmosphere strong enough to force in the cold air.

So I often pay even more attention to the dew point and its trends than the precise temperature in a borderline situation between rain and frozen precipitation.

We've had lots of chances to toe that borderline this winter, including on Tuesday. We'll probably have at least a few more, maybe as early as the weekend.