What’s the Difference Between a Frost and a Freeze?

by Kathy Purdy on October 11, 2009 · 19 comments

in FAQ, Weather

Frost on lawn

Last night the National Weather Service issued a frost advisory for our area, and we did indeed get a frost. They are issuing a freeze warning for tonight. What’s the difference?

I contacted a member of the local weather station staff. He told me that

Both [the frost advisory and the freeze warning] are only issued during the growing season. A Frost Advisory is issued when the predicted temperature is expected to fall to 36 degrees or lower in the next 3 to 30 hours during the growing season. So temperatures 35 to 40 range would also dictate a frost advisory. A Freeze Warning is issued when there is an 80% or greater chance that the temperatures are expected to fall to 32 degrees (F) or lower in the next 3 to 30 hours during the growing season. If the temperature is expected to fall below 28 degrees (F) this is considered a Hard Freeze.

Radiational Cooling

Another way to look at it is that a frost is caused by radiational cooling, that is, the earth loses enough heat that temperatures drop below the freezing point at ground level. Sometimes when there is a very light frost, the freezing temperature occurs just a bit above ground level. Our first frost this year was a very light one, where the tops of the basil plants got damaged, but the plants themselves survived.

Advective Cooling

A freeze, on the other hand, is caused by advective cooling, where a mass of cold air comes into the area from somewhere else (like the Arctic). This usually signals the end of the growing season for all but the hardiest plants. When the weather service issues a freeze advisory, they are telling you the party’s over. It’s time to get serious about fall clean up and winter preparation.

Time to Get Serious with the Chores

I need to empty and wash the bird bath and bring it in. Oh, and I better make sure the rain gauge is empty. What’s on your must-do list?

About

Kathy Purdy is a colchicum evangelist, converting unsuspecting gardeners into colchicophiles. She would be delighted to speak to your group about colchicums or other gardening topics. Kathy's been writing since 4th grade, gardening since high school, and blogging since 2002.

Some might say a calendar is a simple construct that allows us to divide and conquer. But I prefer to think of each little numbered square as the reminder to bite off only what I can chew and savor.
Lorene Edwards Forkner

{ 12 comments… read them below or add one }

Donalyn October 13, 2009 at 1:32 pm

I kind of like fall cleanup – it reminds me a final time of what went right and wrong and how I might improve things next year. And – provided I actually get it all done- I enjoy the clean look for a bit before the snow flies. And with no tomatoes to freeze this year, it hardly mattered. :(
.-= Donalyn´s last blog ..Candied Ginger Pumpkin Cake =-.

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Country Gardener October 12, 2009 at 3:41 pm

We were away in New York City, and though it was on my list to-d0 list before going away for the weekend, I plain forgot to turn the water off at the house and barn. We had our laptop with us, so we got the freeze warning on our WeatherNetwork “weather eye”. The meant I had to call home and ask the dog sitter to drape all the faucets with towels and blankets. Thankfully, the water systems didn’t freeze and we avoided a plumbing repair. Before we went away, we had moved our non-hardy succlents and agapanthus into the hoophouse, so we were covered there.

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kerri October 12, 2009 at 2:38 pm

We too woke up to a hard frost and sad looking flowers this morning.
Vases full of annuals that I cut yesterday and Saturday are easing the pain a little :)
I’m headed out to empty some containers and pot up plants I want to overwinter.
I’m already dreaming of next spring’s garden and what I want to improve on from this past summer….what to start inside…..what to plant more of, etc., etc.
But first there’s fall clean-up…not a happy prospect :(
As for the forecast of snow on Friday….bah humbug, to say the very least! It’s much too soon!
.-= kerri´s last blog ..Lily Garden Annuals =-.

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Helen @ Gardening With Confidence October 12, 2009 at 6:57 am

This is most useful Kathy. It is so unpredictable for us here in Raleigh. I look to the future (forecasts) to know when to begin cutting back for clients. I let my garden get hit by the frost before cutting back…I don’t want to loose a moment of fall before a frost. H.

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Ilona October 11, 2009 at 11:56 pm

Thanks for the reminder to take care of the bird baths for winter… and for all the great information. Must-dos are everywhere and multiplying here, it seems;)
.-= Ilona´s last blog ..The Prairie Garden =-.

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Noelle October 11, 2009 at 10:45 pm

What a great post. I have always wondered at what the difference is. Now I know. Thank you for the information.
.-= Noelle´s last blog ..Beautiful Plant Combinations =-.

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ramblingwoods October 11, 2009 at 10:35 pm

This is good information. We will get a frost here tonight (near Buffalo), not the freeze yet… I am getting out my heated birdbaths and getting all the other cleaned out.. Big job….Michelle

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Craig @ Ellis Hollow October 11, 2009 at 8:47 pm

There are a lot of stars out there tonight. I’m expecting the double-whammy of the cold air moving in and the radiational cooling. Being in a frost pocket, you can almost feel the cold air flowing down the slopes and piling up here in the hollow.

I covered up a 6-foot brugmansia with the hope that the dozen unopened blossoms might be spared. But the rest of the tender stuff is ready to be put out of its misery.
.-= Craig @ Ellis Hollow´s last blog ..Sunday music: Lyle Lovett (coming to town) =-.

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Cindy, MCOK October 11, 2009 at 7:48 pm

I do have a little envy for you cold climate gardeners this time of year. You get to put the garden to bed and leave it there until spring. We, on the other hand, are covering/uncovering, hauling in/hauling out, and sweating/freezing on a semi-regular basis! Texas, it’s a whole nuther country.

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Lorraine October 11, 2009 at 6:50 pm

Hi Kathy,

I like to think of frost as “frozen dew” — sounds so much more benign, like a smoothie for plants, maybe. But, whether light frost or freeze, it’s arriving much too soon for my liking, especially as our summer here in Toronto didn’t start until the end of August!

Cheers,
Lorraine
http://citygardeningonline.com/
.-= Lorraine´s last blog ..9 of 12 Great Things I Found at Raleigh =-.

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Pam/Digging October 11, 2009 at 4:52 pm

Thanks for the explanation, Kathy. Here in Austin, where freezes are few and hard freezes rarer, the weather folks will issue freeze warnings all winter, since we’re always in a growing season. “Pets, pipes, and plants” is the mantra they chant, reminding us warm-weather folks of what to take care of during freezing weather.
.-= Pam/Digging´s last blog ..Mule skinning in the Grand Canyon =-.

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Mr. McGregor's Daughter October 11, 2009 at 4:14 pm

Thanks for the explanation. We had our “first frost” yesterday, which was actually a freeze, followed by a hard freeze last night, and I wish I had brought in my terracotta pot. But we emptied the fountain today, and I had brought in Coleus cuttings and the tender plants I’m going to overwinter.
.-= Mr. McGregor’s Daughter´s last blog ..It’s A Bittersweet Symphony =-.

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