We often see it form on the ground in November after a cold, clear night. In fact, Salt Lake City only averages 167 days a year with temperatures staying above freezing. Generally, from October 15th to April 30th temperatures will drop below freezing.
There are 2 types of frost: rime frost and hoar frost. Rime frost occurs often when the atmosphere has plenty of water vapor in it. It forms when water in the atmosphere comes into contact with subfreezing surfaces and freezes on contact. This can happen during fog events or when clouds are blown over mountain areas and frost forms and accumulates on subfreezing surfaces.
Hoar frost, on the other hand, forms slowly when the water vapor is deposited on cold surfaces as ice. This is the most common type of frost seen when you awake in the morning and the lawn is glittering with white ice crystals.
Hoar frost first forms on surfaces that easily lose heat during the night such as rocks, metals, and glass. That is why your car windshield is often the first thing to have frost form on because glass loses heat more rapidly than the surrounding air.
So how exactly does it form? If the temperature is between 32°F and 0° F frost will most likely form as water vapor condenses on a surface as a liquid and then quickly freezes. After the initial frost crystals have formed, further frost accumulation is quicker as water vapor deposits directly as ice crystals instead of liquid.
For temperatures below 0° F, frost will form by water vapor directly depositing onto surfaces as ice rather than liquid.
So what is the driving force and meteorologically what is happening? When the temperature of a surface falls below the dewpoint temperature but remains above freezing, dew forms through the condensation of water vapor onto it. But if the dewpoint is below freezing we get frost rather than dew.
Neil Opperman
Meteorologist/Weather Producer
