Look closerWeather
Why there is frost when the air never dropped below freezing
When: October to April
On a clear, still night a surface that faces the open sky radiates its heat upward faster than the air gives it back, so the surface itself drops below freezing while the air a few feet above it does not. That is why there is frost on the windshield on a morning the forecast called for the mid-thirties, and the gap is not an error in the forecast.
Why this time of year
The trigger here is a night, not a season: clear sky, light wind, and a low somewhere in the mid-thirties. The National Weather Service says the layer of air right at the surface can run up to about ten degrees colder than the air at four to five feet, where the official thermometer sits, and gives the worked example of 36 F in the air with the surface at 30 F or below. So the reading you see on a screen is taken at chest height, and the frost is reporting a different temperature from a few feet lower down.
How to notice it
- On a morning after a clear, calm night, look at the reported temperature on a phone or a car display BEFORE you look at the car.
- Then look at the windshield, the roof and the lawn. If the number said mid-thirties and there is frost, you have just measured the difference between the air at chest height and the air at the ground.
- Check the sky you had overnight. The NWS puts frost in the 33 to 36 F range with clear skies and calm wind, and says it is much less likely if the air was very dry, if cloud was in place, or if the wind was up.
- Compare a cloudy or breezy morning at the same temperature. Cloud radiates heat back down and wind stirs the cold layer away, so the frost does not form, and that contrast is the whole mechanism.
Try this with kids: The weather says it did not freeze last night, but the car is covered in frost. Who is wrong, the thermometer or the car?
radiational cooling is a surface losing heat straight up to the open sky, which can take it below the temperature of the air around it
Sources
- weather.govNWS, verbatim and load-bearing: these super-cooled temperatures can be up to 10 degrees cooler than 4-5 feet above the surface, where observations are typically taken; and the worked example, if conditions are favorable, air temperatures could be 36 F, but the air in contact with the surface could be 30 degrees or colder. Also that clear skies allow radiational cooling, that calm to light winds let the thin supercooled layer form, and that wind of 8 mph or more prevents it. NOTE this page and weather.gov/arx/why_frost carry the same NWS text word for word, so they are one source published at two offices; the duplicate is deliberately not cited here.
- weather.govNWS Charleston, independent wording and the cleanest statement of the paradox: frost can occur at above freezing temperatures because NWS temperature observations are taken at an elevation of five feet above ground level. Also that frost usually forms when forecast lows are in the 33 to 36 F range with clear skies and calm winds, and that it is not guaranteed if the air is very dry, if there is sufficient cloud cover, and/or winds are roughly 6 mph or more. The 6 mph figure is THIS page's; the Omaha page above gives 8 mph, and the card quotes neither as a threshold because the two disagree.
Checked against these sources on September 13, 2026.
In the guide for October, November, December, January, February, March, April