Frost formation

Frost forms on cold, clear nights with calm conditions as the temperature falls below freezing. Clear skies lead to radiational cooling which allows the greatest amount of heat to escape into the atmosphere. The calm to light winds prevent the air to be stirred, allowing a thin layer of super-cooled temperatures to develop at the surface.

If there is enough water vapor in the air and the temperatures can cool to the frost point (the temperature at which condensation occurs), frost could develop on exposed surfaces as the water vapor turns to ice crystals.

The temperature must be at 32 degrees or colder for frost to develop. But you might notice frost on the ground or on certain objects when the temperature is above freezing. This is because air temperature is taken several feet above the ground. Since cold air sinks and because the ground can cool very quickly, the temperature at ground level can be colder than a few feet higher.

Topography also has a role in determining if and where frost will develop. Cold air settles in valleys since it is heavier than warm air, therefore frost is more likely to form in these regions. Also, valleys shelter the area from strong winds, enhancing the potential for frost.