Frost Advisory vs. Freeze Warning vs. Hard Freeze: What Each Means
The three cold-weather alerts measure different risks, their cutoffs change from one forecast office to the next, and all of them disappear once your growing season is over.
Three alerts, three thresholds, and one rule almost nobody knows: the National Weather Service only issues frost and freeze headlines during the local growing season. Once your area has had its first killing freeze of the fall, the alerts stop, even if the nights keep getting colder. That single detail explains why a gardener can get two frost advisories in a row in early October and then nothing at all for a 20-degree night in December.
The three products are a frost advisory, a freeze warning and a hard freeze warning. Each tells you something different about what will survive the night in your yard.
What is the difference between a frost advisory and a freeze warning?
A frost advisory means ice crystals are likely to form on surfaces even though the air itself may stay above freezing. A freeze warning means the air temperature itself is expected to fall to 32 degrees Fahrenheit or lower. The first damages tender plants. The second can kill them outright and, in many places, effectively ends the growing season.
Where it gets confusing is that the exact cutoffs are set by each local forecast office, not nationally. Compare two offices a few hundred miles apart in the eastern U.S. and one in Colorado:
| Alert | NWS Tallahassee, Fla. | NWS Charleston, W.Va. | Southern Colorado (as reported by KOAA) |
|---|---|---|---|
| Frost advisory | 32° to 36°, calm winds, humidity near 100% | 33° to 36° | Frost expected for 2+ hours, possible up to 37° |
| Freeze warning | 32° or lower for at least 2 hours | 29° to 32° | 32° or lower for 2 hours or longer |
| Hard freeze | 25° or below | 28° or less for more than one hour | Below 28° for 2 hours or longer |
The definitions above come from the Tallahassee office's frost and freeze guide, the Charleston office's program page and KOAA's summary for southern Colorado. The practical lesson: a "hard freeze" in north Florida is a colder night than a hard freeze in West Virginia. Read your own office's thresholds rather than a national rule of thumb.
Can it frost when the temperature is above 32 degrees?
Yes, and this is the part that trips people up most. The official temperature is measured about 2 meters (roughly 6.5 feet) above the ground. On a clear, calm night, heat radiates away from the ground, leaving the air right at the surface colder than the air at thermometer height. The Tallahassee office notes frost can form at ground level while the measured air temperature reads anywhere from 32 to 36 degrees.
That is why your car windshield or a low-growing lettuce bed can be coated in ice on a morning when the forecast low was 35. It is the same physics behind why windshields frost above freezing, and it is why the frost advisory exists as a separate product at all. Wind changes the picture: a breeze mixes the air and usually prevents frost, which is why the Tallahassee criteria specify calm nights.
What temperature is a hard freeze, and what does it kill?
Depending on the office, a hard freeze starts at 28 degrees or 25 degrees, usually sustained for an hour or two. The Charleston office describes it as a "killing freeze" that destroys most vegetation and ends the growing season. KOAA frames it as the last call to winterize: outdoor pipes and sprinkler systems are now at risk, not only the tomatoes.
A plain freeze is more selective. Hardy plants may ride out temperatures in the low 30s, KOAA notes, while frost-tender crops are far less forgiving. Gardeners who cover beds for a frost advisory are buying a few more weeks of harvest. Gardeners who cover them for a hard freeze warning are mostly buying time to pick what is left.
Why did the frost advisories stop?
Because the season they protect is over. The Charleston office defines the growing season as the stretch between the median date of the last spring freeze and the median date of the first fall freeze, and says that outside that window "the NWS will not issue any Frost or Freeze headlines." Once a severe killing freeze happens, the products cease until spring. Tallahassee runs a similar rule, generally suspending freeze products after the first light freeze, with an exception for hard freeze events of 25 degrees or below.
The logic is about signal and noise. An alert is meant to prompt action, and after a killing freeze there is little left in an unprotected garden to act on. For people, the cold-weather advisories and extreme cold warnings take over, based on wind chill rather than plant damage.
That growing-season window is also moving. A Climate Central analysis of 200 U.S. cities, published in October 2024, found the first fall freeze now arrives 11 days later on average than in the early 1970s, with 85% of locations showing a later date. Some western cities have shifted far more than the average.
A later first freeze sounds like good news for anyone growing late squash. Climate Central's caveat is that it also stretches pest and pollen seasons, which helps explain why fall allergies seem to drag on, and it can unsettle the timing a nearly $27 billion fruit and nut industry depends on. For the person deciding whether to drag a bedsheet over the tomatoes tonight, the change shows up more simply: the first advisory of the year tends to arrive a little later than it did for their parents, and the local office's thresholds, not the calendar, are the thing to watch.
If the weather alert alphabet still feels arbitrary, our guides to heat advisories versus excessive heat warnings and why drought makes leaves change color early cover the other end of the seasonal calendar.
