Frost (also called Hoarfrost and White Frost), ice crystals produced by the freezing of water vapor on the ground, on objects near the ground, or on windows. Water vapor in the air condenses into a liquid or a solid at a temperature called the dew point. This temperature varies, depending on the amount of vapor in the air. When both the dew point and the temperature of the air are above 32o F. (Oo C.), dew is formed; when both are below 32o F., frost occurs.
Frost is most likely to occur on clear, windless nights when the layer of air next to the ground contains considerable moisture. A frost that forms at temperatures of 27 F. (-3 C.) or less is termed a killing frost because it kills plants by freezing their fluids. Black frost is a term for intense cold that blackens and kills vegetation; no actual frost is involved.
Frost is a hazard to agriculture, particularly in valleys, where cold air flowing down from the hills often causes heavy frost damage. Fruit growers protect their orchards by burning open fires, or by placing heaters or smudge pots at intervals to raise the temperature of the air near the trees.