Temperature Variations During Our Recent Hard Freezes
Most of the Ohio Valley had its first two hard freezes of the fall on Friday and Saturday mornings. The Canadian high pressure system responsible for the cold weather brought clear skies and calm winds, which are ideal for radiational cooling. Of course, not all of central Kentucky and southern Indiana will be affected equally by this cooling, because of variations in terrain. If you have an outside temperature readout in your car, you might notice the temperature fluctuating by several degrees just between high spots and low spots.
Other factors that influence how far the temperature will drop at night can include cloud cover, local wind effects, urban heat islands, or even just local differences in land use. This map shows some of the variations in the low temperatures observed across the Ohio Valley on Saturday morning, October 26th.
The most obvious areas of warmer temperatures are associated with the urban heat island of Louisville, and parts of southwestern Indiana, western Kentucky, and southern Illinois where clouds moved in overnight. If we zoom in on the Lexington area, we will see that even subtle differences in land use can make a difference.
At Blue Grass Airport, where the temperature sensor is surrounded by concrete runways and taxiways, the low was 30 degrees. Just across Highway 60 at the Keeneland Racetrack, where there is also a lot of pavement, the low was also 30. South of downtown Lexington at the Mesonet site for Fayette County, the temperature dropped to 27, with other nearby sites reaching a low of 28. These sites are all in largely suburban/residential areas, with more trees and less concrete/asphalt.
We at the National Weather Service in Louisville try to account for these differences when we publish forecast temperatures across the region. If you want a forecast for your exact location, enter your zip code up at the box in the upper left part of this screen: