Frost are ice crystals formed when moisture passes directly from the gaseous to the solid state. This formation occurs when the dew point of the air is below 32 °F. The process is called sublimation. Of the two kinds of frost, one, known as light frost or hoarfrost, forms on objects that are below freezing but that are surrounded by air whose temperature remains above freezing. The other kind of frost, called hard frost or black frost, freezes and blackens plants without forming hoarfrost. Black frost is formed on objects when both their temperature and that of the air around them are below freezing.
The first frosts in the United States occur a little before September 1. It is important for farmers and fruit growers to know when the first frost will occur. For that reason the United States Weather Bureau issues frost warnings. Forewarned of possible frost, farmers can cover their flower gardens and vegetable gardens with cloth or screens, and fruit growers can light their stoves or smudge pots.
