“Frost” and “freeze” are often used interchangeably, but the National Weather Service (NWS) treats them as different things. The difference matters for gardeners, because a frost can damage tender plants while a hard freeze kills most of them.
Frost
Frost is ice crystals that form on surfaces (leaves, grass, windshields) when they cool below the dew point and below freezing. Surfaces can get colder than the air measured a few feet up, so frost can form even when the air temperature stays above 32°F, especially on clear, calm nights.
NWS offices issue a Frost Advisory when overnight lows are forecast in the 33°F to 36°F range with good conditions for radiational cooling (clear skies, light wind). Frost is less likely when the air is dry or winds are around 8 mph or more.
Freeze
A freeze is when the air temperature falls to 32°F or below over a widespread area for a climatologically significant period. NWS issues Freeze Watches and Freeze Warnings during the growing season whether or not frost actually forms.
Hard freeze
A hard freeze is 28°F or below for an extended period, which the NWS describes as cold enough to kill most types of vegetation. How long “extended” is varies by NWS office (from about an hour to several hours).
Summary
| Term | Temperature | NWS product |
|---|---|---|
| Frost | 33–36°F, clear and calm | Frost Advisory |
| Freeze | 32°F or below | Freeze Watch / Freeze Warning |
| Hard freeze | 28°F or below, extended | Hard Freeze Warning (some offices) |
Which temperature do frost dates use?
Most published “frost dates”, including the headline dates on this site, are 32°F dates: the typical last freeze in spring and first freeze in fall. Because frost can form on plants at 33–36°F, tender seedlings can still be damaged after the 32°F date. That’s why each frost dates page also shows the typical dates for 36°F (frost possible) and 28°F (hard freeze) from NOAA’s 1991–2020 climate normals, plus active NWS frost and freeze alerts.
Sources: NWS glossary: freeze; NWS Raleigh frost/freeze information. Criteria vary slightly between NWS offices.