How Can It Freeze at 35 Degrees?
If you are a snow removal expert, you know to watch for refreeze after snowstorms and to do "ice checks" in the morning if temperatures fall to freezing or below. You are also aware to keep a close eye on areas near snow piles where melt water during the day can refreeze at night. BUT, what about when your truck is telling you it's 35°F and your seeing ice up...What gives? Last I checked water still freezes at 32°F!
Well, the main reason this occurs is where we measure air temperature. As per the National Weather Service, air temperature is recorded 4 - 6 ft above the ground surface. While this seems like a good standard (since that captures the height of many people), the air temperature can vary from the surface to 4 - 6 feet. So the temperature right near ground surface can be 32°F, while your truck or temperature sensor is showing 35°F (since cold air sinks).
I know what your thinking, when is this temperature difference most likely to occur? The typical scenario involves a clear night sky and calm wind, when radiational cooling is maximized. This means heat from the surface can easily radiate into the atmosphere allowing for rapid cooling. So if your property's parking lots, sidewalks, roadways, or parking decks are wet at night when skies clear and winds turn calm after a storm, be alert because refreeze may be on the way even when the official temperature is 35 degrees.