Big Low Temperature Ranges by Sean Sublette

This is the time of year when we can get large ranges in low temperatures from city to countryside. In addition to the well-known heat island effect, there are other factors that play a substantial role in low temperature forecasting:

Wind speed. When the air is moving, it is similar to having the atmosphere in a mixing bowl. The wind keeps the air stirred up a bit, so that isolated cold pockets of air can mix with the relatively warmer air nearby.

Cloud cover. Low clouds, in particular, give off a small amount of energy. The water in the clouds emits a small amount of infrared radiation, which slows down the cooling process at night. In the absence of clouds, cooling occurs much more quickly. In fact, temperatures will usually rise if clouds roll in during the course of a night.

Localized elevation. Cool air is denser than warm air, so in the absence of those mixing winds, it can separate like oil and water. Small creek valleys (e.g. Blackwater Creek) are areas where relatively cooler air can collect, leaving areas on top of a hill or bluff slightly warmer than the area a few dozen feet below.

When these conditions combine, we can have huge changes in low temperatures from city to countryside. So, knowing that the official temperature for Lynchburg is taken in a grassy location adjacent to Lynchburg Regional Airport, and that it is near a creek bed, and that the nearby woods tend to shelter it from strong north winds well we often get a temperature that is not representative of the metropolitan area as a whole.

So, consider a night where the sky is clear at the airport, but there are some patchy low clouds in Midtown. And the winds are about 5 miles per hour in town, but they have already dropped to zero at the airport. A difference of 12 degrees can occur between the two locations. We have seen it happen more than once.

But on a night with a consistent breeze across the entire region and a deck of low clouds in place, that difference is much smaller. Such was the case Monday night, when the airport went down to 22, and Midtown went down to 23.

So it can happen. In fact, as I write this Tuesday evening, it remains 23 in Midtown, while the airport has already dropped to 16. The sky is clearing and the winds are decreasing quickly.