The process in which water vapor turns directly into ice, forming frost, is called deposition.
In deposition, a gas becomes a solid without first becoming a liquid. When water vapor encounters a sufficiently cold surface, such as a leaf, window, or car roof, molecules can lose enough thermal energy to arrange themselves directly into ice crystals. The resulting coating is frost or, under some conditions, hoar frost.
Deposition is the reverse of sublimation. Sublimation occurs when a solid changes directly into a gas, as with ice slowly disappearing in very dry air. Condensation is different: it changes water vapor into liquid water, producing dew when the surface is above freezing. If the surface is below freezing, direct ice formation is deposition rather than ordinary condensation.
The distinction explains why frost can have delicate crystal patterns. Surface temperature, humidity, wind, and microscopic irregularities all influence where ice crystals begin growing. Deposition also matters beyond weather: industries use related gas-to-solid processes to create thin coatings on materials.