<p>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.</p>

<p>Most weather data includes dew point temperatures. Search your data for when the dew point is below the freezing point of water.</p>

<p>If your data does not you will need to calculate it to find when it is below the freezing point of water. The formulas <a href="https://en.wikipedia.org/wiki/Dew_point#Frost_point" rel="nofollow">here</a> will help. </p>

CC BY-SA 4.0
2022-07-25T13:27:56.350Z
I've observed frost on the grass in the morning when the reported low for the night was only 49F.  How do you account for that @Fred?
boatcoder
CC BY-SA 3.0
2015-07-03T02:45:17.110Z
2015-07-03T02:45:17.110Z
Fred
<p>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 &lt; 0 and dew point > 0?</p>

<p>A canonical reference would be appreciated.</p>

CC BY-SA 3.0
2015-07-02T20:57:26.253Z
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](https://en.wikipedia.org/wiki/Latent_heat#Latent_heat_for_condensation_of_water) has some equations for the latent heat of ice, and [Jeff Haby](http://www.theweatherprediction.com/habyhints/347/) describes the difference between dew and frost point.
Jareth Holt
<meteorology>
How can I define frost based on a weather record?
David LeBauer
# How can I define frost based on a weather record?

(Asked by: username_0 on 2015-07-02)

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.

**Tags:** meteorology

> **Comments:**
> **username_1**: 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](https://en.wikipedia.org/wiki/Latent_heat#Latent_heat_for_condensation_of_water) has some equations for the latent heat of ice, and [Jeff Haby](http://www.theweatherprediction.com/habyhints/347/) describes the difference between dew and frost point.  

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## 1 answers

### Answer by username_2 on 2015-07-03. Score: 1 | Accepted answer

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.

Most weather data includes dew point temperatures. Search your data for when the dew point is below the freezing point of water.

If your data does not you will need to calculate it to find when it is below the freezing point of water. The formulas [here](https://en.wikipedia.org/wiki/Dew_point#Frost_point) will help.

> **Comments:**
> **username_3**: I've observed frost on the grass in the morning when the reported low for the night was only 49F.  How do you account for that @username_2?  

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