Frost is a thin layer of ice crystals that forms on surfaces when the air is very cold and water vapor freezes. It often appears on grass, windows, and cars on chilly mornings.
“Frost” comes from Old English “forst” or “frost,” related to “freeze” and old Germanic words for cold. The root idea is of something being hardened by cold.
Frost can form even when the air temperature is just above freezing, because surfaces like car roofs can cool down faster than the air. So your lawn might be icy while the thermometer still claims it’s not that cold.
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