Tonight is going to be quite chilly here in Central PA. High pressure is building in today leaving us with a mainly clear sky tonight. Our winds will be light, our sky will be clear, and this is the perfect recipe for maximum radiational cooling and for temperatures to tumble.
Our forecast low temperatures tonight are in the lower to mid 40s. In the valley locations in our northern counties, temperatures could even fall into the upper 30s. This could lead to a bit of frost in the morning in Elk and Cameron Counties. Our average low temperatures this time of the year are in the lower 50s.
Radiational cooling is defined by the National Weather Service as the cooling of Earth’s surface. At night the heat from the day, escapes back into the atmosphere and temperatures drop. Clouds can keep some of the heat here on Earth, but on a clear night, a lot of that energy returns back into space, making us cooler.
We also will have a bit of fog that forms from this same concept. Radiation Fog forms on a clear night with calm winds. You need the temperature to meet or come close to the dew point. The sun heats the earth during the day, but at night the energy escapes back into the air cooling it. When the air is cooling, it helps the air to reach the dew point creating the fog.