Frost, freeze, and hard freeze

Three words, three jobs

A garden forecast can use frost, freeze, and hard freeze in one night, and a climate table can use 36°F, 32°F, and 28°F in one row. They are related. They are not the same product. The National Weather Service writes the forecast words. NOAA’s National Centers for Environmental Information writes the climate dates this site displays. Mixing them is how a 36°F advisory gets treated as a 32°F planting date, or a 28°F climate row gets treated as a warning that was never issued.

This guide stays with the published definitions and with the way local Weather Service offices actually set their thresholds. Those thresholds are not identical from office to office. That disagreement is part of the subject.

What the national glossary says

The National Weather Service glossary defines a freeze as the surface air temperature expected to be 32°F or below over a widespread area for a climatologically significant period of time. It says use of the term is usually restricted to advective situations, or to occasions when wind or other conditions prevent frost. “Killing” may be used during the growing season when the temperature is expected to be low enough, for a sufficient duration, to kill all but the hardiest herbaceous crops. A freeze warning, in that same glossary, is issued during the growing season when surface temperatures are expected to drop below freezing over a large area for an extended period of time, whether or not frost develops.

The glossary’s hard-freeze entry is a temperature and a duration, not a plant list. A hard freeze occurs when the surface air temperature reaches 28°F or lower for an extended period of time, killing most types of vegetation. “Extended” is not given a number of minutes in that sentence. Local offices fill the gap, and they do not all fill it the same way.

Frost, in forecast practice, is the ice that forms on surfaces. It is not the same sentence as “the air at the thermometer reached 32°F.” Several local offices say frost can form while the reported air temperature is still above 32°F, because the leaf, the car roof, or the ground can cool below the air a few feet up. The Raleigh office’s frost-freeze page says frost can occur when the temperature falls below 36°F, especially in rural areas, and that it is local and variable across a small area. The Tampa Bay office, describing radiational cooling, says frost can be observed on cars and rooftops when the temperature about 2 meters (about 6 feet) above the ground is still as high as 38°F, because materials such as metal and glass radiate heat efficiently. That 38°F figure is Tampa Bay’s description of surfaces, not a national rule that every garden frosts at 38°F.

What a warning is, office by office

The Raleigh, North Carolina, office publishes criteria that are easy to line up with the three words.

A frost advisory there is issued when minimum temperatures are forecast to be 33 to 36 degrees and conditions are favorable for at least areas of frost, during the locally defined growing season, which that page describes as typically March 15 through November 15. The same page’s frost-freeze maps say a frost advisory is for the 33°F to 36°F range with good radiation conditions.

A freeze warning there is issued when minimum temperatures are forecast to be 32 degrees or less during that growing season, with or without frost. The growing season officially ends in autumn when minimum temperatures drop to 32 degrees or less, across half or more of a forecast zone, for three or more hours. A note on the criteria page says 32°F to 28°F is a freeze, whereas temperatures less than 28°F are considered a hard freeze. “Less than 28” is a slightly different line from the glossary’s “28°F or lower.” A night that bottoms at exactly 28°F sits on that crack. If you are reading a Raleigh product, use Raleigh’s note. If you are reading the glossary, use the glossary.

The Charleston, West Virginia, office writes the hard-freeze duration in hours. A frost advisory there is for the locally defined growing season on nights with radiational cooling, light winds and clear skies, usually when minimum temperatures are forecast in the 33°F to 36°F range, with at least minor damage possible to sensitive plants. A freeze warning is for minimum temperatures of 29 to 32 degrees. A hard freeze warning is for minimum temperatures of 28 degrees or less for more than one hour. That office says a hard freeze usually means most sensitive plants or seasonal vegetation will be destroyed, and that unprotected outdoor plumbing can be at risk. It also ties the phrase “killing freeze” to a severe frost that ends the growing season, associated with temperatures of that kind.

The Tampa Bay office is a useful contrast because of the Hazard Simplification changes. That office says specific hard-freeze warning and watch products are no longer issued. A mention of hard freeze is placed in the “what” bullet of a freeze warning or watch when temperatures of 27°F or lower are likely for 2 or more hours. A freeze warning there is for 32°F or lower for 2 or more hours. A frost advisory is for 33°F to 36°F with good radiational cooling, light winds and clear skies. If you learned “hard freeze means 28” from the glossary and then read a Tampa product that uses 27°F and will not issue a separate hard-freeze headline, both statements can be true of the documents that published them. They are not a single national switch.

The practical reading is short. Look at the office that forecasts your county. Read the threshold in that product. Do not import another office’s hour count. Do not treat the glossary’s 28°F sentence as a promise that a hard-freeze warning will be issued, under that name, everywhere.

Where the 36, 32, and 28 rows on this site come from

The dates on a city page are not warnings. They are 1991–2020 climate normals from NCEI. The calculation methodology for those normals computes probability dates at several minimum-temperature thresholds, including 36°F, 32°F, and 28°F. This site shows those three because they sit next to the forecast words people already use: a frost advisory is often aimed at the mid-30s, a freeze warning is aimed at 32°F, and a hard freeze is aimed at 28°F or a neighboring threshold. The match is approximate. NCEI is counting how often a daily minimum crossed a line in a 30-year record. The Weather Service is forecasting one night, with a duration, inside a locally defined growing season.

At Austin-Bergstrom, station USW00013904, the 50% dates in the access file are not the same day across those three lines. The median last 36°F date is March 27. The median last 32°F date is March 12. The median last 28°F date is February 27. The median first dates in autumn are November 7 at 36°F, November 16 at 32°F, and December 1 at 28°F. A plant that is only bothered by a true 32°F air minimum and a plant that is damaged by frost at 36°F do not share a calendar, even at one airport. The probability guide explains why the 10% date is the late spring date on a last-freeze row and the early autumn date on a first-freeze row. That direction applies at 28°F and 36°F the same way it applies at 32°F.

A qualified station on this site must have a complete 32°F set. The 28°F and 36°F rows are shown when they pass the same flag rules. If a warm station has no 28°F date because that temperature almost never occurs, the page leaves the cell empty. It does not borrow a date from 32°F.

What to do with a forecast that names one of these

Iowa State University Extension’s home-garden page on protecting plants says to start watching overnight lows in the context of the mid-30s, and that covering is typically no longer effective once temperatures drop below about 28°F. That is a protection statement, not a Weather Service criterion, and it is developed in the late-frost guide. The point here is the handoff. A frost advisory in the mid-30s is the night Iowa State tells you to notice. A hard freeze near 28°F is the night that same page says a cover often stops being enough. Between them, a 32°F freeze warning is the line NCEI uses for the frost dates on this site, and the line Texas A&M’s planting table calls the frost-free date and the first freeze date without naming a probability. This site applies those week offsets to the median 32°F date and says so on the crop page. It does not apply them to 36°F or 28°F unless you choose that row yourself in the fall calculator.

None of these products is a verdict on a single plant. Duration matters, which is why Charleston writes “more than one hour,” Tampa writes “2 or more hours,” and Raleigh ends the growing season with a three-hour test across half a zone. A brief touch of 32°F at the airport thermometer is not the same night as five hours at 29°F in a low spot. The station distance printed on each city page is there because the airport and the bed are not the same instrument. Microclimates is the rest of that argument.

Sources

• National Weather Service glossary, “freeze”: https://forecast.weather.gov/glossary.php?word=freeze
• National Weather Service glossary, “hard freeze”: https://forecast.weather.gov/glossary.php?word=hard+freeze
• NWS Raleigh weather criteria, frost and freeze: https://www.weather.gov/rah/weathercriteria
• NWS Raleigh frost and freeze maps: https://www.weather.gov/rah/FrostFreezemaps
• NWS Charleston frost-freeze information: https://www.weather.gov/rlx/frost-freeze-information
• NWS Tampa Bay cold-weather criteria, including the Hazard Simplification note: https://www.weather.gov/tbw/colddss
• NOAA NCEI normals calculation methodology, frost-freeze thresholds: https://www.ncei.noaa.gov/data/normals-daily/1991-2020/doc/Normals_Calculation_Methodology_2020.pdf
• Station USW00013904 access file, 1991–2020 annual/seasonal normals.