# How can I define frost based on a weather record?

If I want to (programatically, in a plant growth model) define a growing season as last frost in spring to first frost in fall, how might I determine these dates based on a weather record that contains hourly temperature and humidity? Perhaps temp < 0 and dew point > 0?

A canonical reference would be appreciated.

• What is it about frost that matters for the growing season? Does it have to be icy, or just low temperatures? If the formation of ice is important, use the frost point rather than the dew point. I can't find any single equation on it, but Wikipedia has some equations for the latent heat of ice, and Jeff Haby describes the difference between dew and frost point. – Jareth Holt Jul 2 '15 at 20:57

The dew point is the temperature at which water condenses from the atmosphere. When the dew point temperature is below the freezing point of water ($0$ $^oC$, $32$ $^oF$) frost forms.