The formal definition of frost in the Glossary of the American Meteorological Society (AMS) is “The fuzzy layer of ice crystals on a cold object, such as a window or bridge, that forms by direct deposition of water vapor to solid ice.” This definition requires some explanation itself. Most people are familiar with the process of condensation. Water vapor (the gas form of water) changes to liquid water. Deposition is the process by which water vapor goes straight to the ice phase.
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Frost forms in a similar fashion to dew often find on the grass or our cars. On clear, relatively calm nights, surface of the Earth can radiate heat. This process creates a relatively cool surface. As the air temperature cools to its dewpoint temperature, water vapor condenses to liquid droplets because as the National Geographic website notes, “Colder air is less able to hold water vapor than warm air.” It is a bit more complex than that statement, and there are other things going on. However, an extensive review of the Clausius-Clapeyron equation is beyond the scope of this discussion so we’ll go with it. The National Weather Service Glossary has a good explanation:
Frost describes the formation of thin ice crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans. Frost develops under conditions similar to dew, except the temperatures of the Earth’s surface and earthbound objects falls below 32°F. As with the term “freeze,” this condition is primarily significant during the growing season. If a frost period is sufficiently severe to end the growing season or delay its beginning, it is commonly referred to as a “killing frost.” Because frost is primarily an event that occurs as the result of radiational cooling, it frequently occurs with a thermometer level temperature in the mid-30s.
Okay Dr. Shepherd, so what is frozen dew? The answer is actually pretty simple. There are cases in which dew may form because the temperature is above the freezing point but then falls below it. The liquid dew simply freezes. This is different than frost which happens because the dewpoint temperature is below the freezing point.
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You might actually be surprised to learn that frost is characterized in different ways. I summarize them based on the AMS Glossary of Meteorology:
- Hoarfrost (white frost) – “deposit of interlocking ice crystals (hoar crystals) formed by direct deposition on objects, usually those of small diameter freely exposed to the air, such as tree branches, plant stems and leaf edges, wires, poles, etc.” This type of frost can actually form in caves, on aircraft, in buildings, and other locations.
- Dry freeze (black frost) – “The freezing of the soil and terrestrial objects caused by a reduction of temperature when the adjacent air does not contain sufficient moisture for the formation of hoarfrost on exposed surfaces.” If vegetation is involved, the term “black frost” is often used.
The term “freeze” is often used to describe a situation in which temperatures are below the freezing point for a period of time such that only the hardiest crops and vegetation can withstand the conditions. The National Weather Service actually has criteria that it uses for Frost Advisories and Freeze Warnings. However, the criteria can vary by office and region.