The proverbial frost on a pumpkin refers to ice crystals forming on a surface once temperatures drop below the freezing mark, 32 degrees F. However, the air temperature does not necessarily need to “officially” drop below the freezing mark for frost to form. The National Weather Service, NWS, keeps its “official” temperatures in a sheltered area some five feet above the ground at the airport. However, because cold air is denser than warm air, the coldest air is found in the lowest couple of feet above the surface. On a clear and calm night, the temperature difference between the surface and five feet up can be 5 degrees or more. In addition to the coldest temps being found right near the earths surface, some surfaces radiate heat more effectively than others. For example metal, glass and vegetation are very efficient at emitting long-wave energy back into that atmosphere. This allows coldest temperatures to be achieved on these surfaces first. That is why frost can many times be found here first when air temperatures are marginal.
So far this season, our “official” lowest temperature was 33 degrees set on the morning of October 25th. Despite the fact that the “official” temperature never reached the freezing mark, many areas reported temps as low as the upper 20’s. The second half of this month has turned out to be very cold. In fact, morning temperatures at the airport have dipped into the 30’s seven times since October the 12th. Patchy frost was reported on most of those cold mornings.
A hard freeze refers to when the actual air temperature falls below the freezing mark for several hours at a time. For this to occur, typically the “official” low temperature will reach the freezing mark if not lower. Although Louisville has not had a hard freeze so far this season, but many locations across Kentuckiana have experienced subfreezing temperatures during the recent cold spell.