Frost vs. Freeze: What Each Means, the NWS Alerts and Your Growing Season

Frost is ice that forms on cold surfaces such as grass, leaves and car roofs. A freeze is the air itself at 32°F or colder over a wide area, whether or not ice crystals form. Frost often forms while the official thermometer reads in the mid-30s, so a frosty night isn’t always a freeze. A hard freeze, 28°F or colder for an extended time, kills most vegetation.^2,3

The word you pick also changes your growing season. In NOAA’s 1991–2020 normals for 37,743 US ZIP codes, the median season is 166 days between 36°F “frost” nights, 188 days between 32°F freezes and 212 days between 28°F hard freezes.¹

Frost and freeze, defined side by side

Sources: the National Weather Service (NWS) glossary entries for frost and freeze,^2,3 and the American Meteorological Society (AMS) glossary.^14,15

Why frost forms when the thermometer reads above 32°F

Official temperatures are measured about 5 to 6 feet above the ground.^8,17 On a clear, calm night the ground loses heat to the sky, so the air near the ground gets colder than the air a few feet up.¹⁶ As the NWS glossary puts it, frost

frequently occurs with a thermometer level temperature in the mid-30s

NWS La Crosse, Wisconsin, says air at the surface can be up to 10 degrees colder than at thermometer height: 36°F on the thermometer, 30°F or colder at the ground.¹²

Frost also needs little wind and some moisture. NWS Tampa Bay notes that a 3–5 mph breeze can keep the air mixed and prevent widespread frost, and that cars and rooftops can frost at thermometer readings as high as 38°F.⁷

That still-night kind is a radiation frost.¹⁸ The NWS uses “freeze” mostly when a cold air mass moves in or wind prevents frost:³ frosts happen close to the ground, freezes come with deeper cold air.¹⁶

How the definition changes your growing season

Which “frost” a garden calendar counts from moves the answer by weeks. We measured the season three ways for every US ZIP code with a NOAA station within 50 miles, using NOAA’s season-length figures for the 36°F, 32°F and 28°F thresholds.¹

Nationally, counting from 36°F instead of 32°F trims a median 23 days from the season (21 to 25 days for the middle half of ZIP codes). The last 36°F night of spring comes a median 12 days after the last freeze, and the first 36°F night of fall 10 days before the first freeze. Counting to the 28°F hard freeze instead adds another 24 days.¹

NWS Memphis’s own “frost” (36°F) and freeze (32°F) averages put the two seasons 21 days apart;¹³ our normals for the station nearest downtown give 23.

Look up your ZIP code’s frost and hard-freeze dates; your planting zone won’t give you these dates.

Where frost comes without a freeze

Of 40,911 ZIP codes with a station within 50 miles, 930 (2.3%) get a 36°F night in most years but a 32°F freeze in fewer than half. That is about 1 in 5 ZIP codes in California and Florida, and 1 in 9 in Arizona.¹

At Tampa’s airport station, a 36°F night comes in 74% of years, a freeze in 35% and a hard freeze in 9%. At Phoenix Sky Harbor the figures are 78%, 18% and 1%. In these places, frost-possible nights, not freezes, are the usual cold threat.

Which is worse for plants?

A freeze usually does more harm. Frost damage stays mostly on the leaf surface; in a longer freeze, ice forms inside the plant and breaks the cell walls. A brief dip to 32°F may do little, while hours below freezing can kill tender plants.^17,4

In dry air, plants can even freeze inside with no frost on the outside. The AMS calls this a dry freeze, or black frost, after the blackened leaves:¹⁴

A black frost is always a killing frost

Frost advisory, freeze watch and freeze warning

The NWS issues three alerts for plant-killing cold during the growing season:^4,5,8

The freeze warning is the more serious alert (“Take Action!” against the advisory’s “Be Aware”), and it is issued whether or not frost forms.^4,3

What happened to the hard freeze warning

Many offices, from Raleigh to Tampa, used to issue a separate Hard Freeze Warning at about 28°F.¹⁹ On October 1, 2024, the NWS merged it into the Freeze Warning nationwide.⁶ NWS Tampa Bay says

specific Hard Freeze Warning/Watch products are no longer issued

Instead, its freeze warnings mention a hard freeze when lows of 27°F or colder are likely for two hours or more.⁷ Some office pages still describe the old product, so read the warning text for the expected low and duration.^9,10

Why your local office’s criteria may differ

Each NWS office sets its own criteria:

• Tampa Bay, FL: freeze warning for 32°F or lower for at least 2 hours.⁷
• Charleston, WV: freeze warning for lows of 29–32°F.⁸
• Grand Forks, ND: the low must cover the whole county or zone.⁹
• South Florida: no frost advisory; frost is mentioned in the forecast.¹⁰

Alerts also stop for the year: NWS Charleston after a killing freeze,⁸ NWS Binghamton on fixed dates in a test it was running in fall 2023 (October 11 for most of its area).¹¹ After that, watch the forecast low yourself.

What to do with each forecast

Cover sensitive plants before sunset at 36°F or below. At 28°F or below, NWS Charleston warns there is little you can do for plants left outside.⁸ Our guide to covering plants for frost explains how.

The 28°F hard freeze also sets the last day to overseed, and our garlic guide decodes “killing frost” advice. See all our frost and freeze guides and the methodology page.