When water vapor on a surface freezes without first becoming liquid, it forms frost. Frost is a white, delicate layer of ice crystals that forms on cold surfaces when water vapor in the air freezes directly into ice. It is a common occurrence during cold weather, especially in areas with high humidity. Frost is often seen on windows, windshields, and other exposed surfaces in the morning, when the temperature is low enough to cause the water vapor in the air to freeze directly into ice.