Was Monday Our First Frost?

Our first frost date, according to Farmer’s Almanac, is November 29th. I woke up on the morning of November 9th to frost on the roof of the house next door.

I went out to check the garden, and everything looks pretty much the same. There wasn’t any obvious frost on any of the plants. Even the basil is alive! There may have been a tiny bit of damage on one of the zucchini plants, but it’s hard to know if this is from the frost or just my own benign neglect. (Since they’ve stopped producing any fruit, I’ve hardly watered them. I should pull them out, but they keep flowering, and I don’t have anything else planned for that space, so I let them live.) Hmmm…false alarm?

The Farmer’s Almanac’s first frost date is the date in the fall when (on average) there is a 30% chance of the temperature dropping below 32° F before that date. (This was news to me. I had assumed that the first frost was just the average first frost date, meaning there’s a 50% chance of the first frost happening before and a 50% chance of the first frost happening after that date. Wrong!)

A “light frost” is when the air temperature drops just below 32° F for only a few hours at most. A “hard frost” is when the temperature drops below 28° F for at least four hours. Some plants can tolerate a light frost but not a hard frost. Zucchini and basil shouldn’t be able to tolerate even a light frost, but my kale should be just fine.

The weather reports say the temperature in Palo Alto got down to 37° F that night. Although it’s possible we are in a colder microclimate, given the state of my vegetables and herbs, I don’t think we had our first frost Monday night. The roof was, indeed, a false alarm.

As a side note, how does frost form on roofs if it doesn’t get below freezing?Frost forms from water vapor touching a freezing surface and forming a layer of ice crystals (this is called deposition, when a gas turns to a solid without becoming a liquid first). Although the measured air temperature was higher than 32° F, the roof temperature could have been less than 32° F. This has to do with radiative cooling and frost/dew points, which you can read more about here if you are curious.

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