The temperature this morning was 40f and there was frost on the porch railings and cars. However the temperature never got below 40f overnight. The temperature yesterday was 58f during the day.
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Lower local temperatures. The air was 40F on average but it would have hots spots and cold spots. Evaporating water, wind chill, and even more exposure to the sky can cause a surface to become much colder than the surrounding air.
So while the air wasn’t cold enough to freeze water, your window was.
Air temperature is measured 2 meters above the surface. Since cold air sinks, on a calm day the temperature at the ground can be a few degrees colder than at the height they measure temperature.
It was explained to me that…… The depth of the universe acts as an infinite heat sink, drawing the heat out of surfaces that are exposed to it. Frost can form on surfaces at temperatures as high as 40 degrees ambient. It’s why a car parked in the open will have frost while a car under a carport won’t on the same morning.
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