Ice crystals that form on surfaces when the temperature is below freezing are called frost. Frost usually forms when water vapor changes directly into solid ice on a sufficiently cold surface, without first becoming liquid water.
This gas-to-solid transition is called deposition, or desublimation. It occurs when a surface cools below the surrounding air’s frost point and the nearby air contains enough moisture. Clear, calm nights often provide ideal conditions because exposed objects lose heat through radiation and can become colder than the air a short distance above the ground.
Hoarfrost is the familiar delicate, white variety that appears on grass, leaves, branches, and wires. Crystal shapes vary with temperature, humidity, wind, surface texture, and how long the crystals grow. Frost may also appear on windows, where glass imperfections help determine the branching patterns.
Frost is not the same as snow or hail. Snow forms in clouds and falls as precipitation; hail grows inside powerful thunderstorms. Dew consists of liquid droplets, although dew can freeze afterward into a coating that is visually different from crystals deposited directly as frost. Frost can damage plants when ice forms inside their tissues.