How can water vapor form frost directly on a cold surface?

Frost forms on a cold surface through a process known as deposition (or desublimation), where atmospheric water vapor transitions directly into the ice crystal phase without first becoming a liquid.12

This occurs when the temperature of an exposed surface drops to or below the frost point—a threshold below 0 °C (32 °F) that is analogous to the dew point. When the pressure exerted by the water vapor in the air exceeds the vapor pressure of the solid ice phase at the surface's temperature, the vapor deposits as frost crystals.132

While condensation typically involves a gas turning into a liquid, deposition is an exothermic process that releases heat as the vapor converts directly into a solid structure. These crystals, often called hoarfrost, grow outward from the surface in various forms, such as needles or plates, depending on the specific subfreezing conditions.21