Frost
Frost is a thin layer of ice crystals that forms on surfaces when they cool to 32°F (0°C) or below and water vapor in the air freezes onto them. It is the surface that has to reach freezing, not the air: frost can form while the official air temperature is still in the mid 30s.
Why It Matters
Frost damages and kills tender plants, which makes the first frost of autumn and the last frost of spring the dates gardeners and growers plan around.
Official air temperatures are measured about 4 to 5 feet above the ground. On a clear, calm night the ground and plants lose heat faster than the air above them, and the layer right at the surface can be up to 10 degrees colder. An official reading of 36°F can mean 30°F or colder where frost forms.
National Weather Service Frost and Freeze Alerts
| Alert | Typical criteria |
|---|---|
| Frost Advisory | Forecast low of 33°F to 36°F on a clear, calm night during the growing season |
| Freeze Watch | Significant, widespread freezing temperatures are possible within 24 to 36 hours |
| Freeze Warning | Significant, widespread freezing temperatures are expected during the growing season |
Criteria and growing-season dates are set by each local NWS office. Since October 1, 2024, the former Hard Freeze Warning has been part of the Freeze Warning. Temperatures below 28°F for an extended period can kill most commercial crops and residential plants.
Source: NWS Baltimore/Washington: Watch, Warning, and Advisory Definitions
When Frost Is Most Likely
- Clear skies, which let heat escape from the ground overnight.
- Calm or light winds, which stop warmer air from mixing down to the surface.
- Cool, moist air that can reach its dew point at the surface.
- Low ground. Cold air settles in valleys and hollows, which are also sheltered from the wind.
Frequently Asked Questions
At what temperature does frost form?
Frost forms when a surface cools to 32°F or below. Because the ground and plants can be several degrees colder than the air a few feet up, frost is possible when the official air temperature is in the mid 30s, especially on clear, calm nights.
What is the difference between a Frost Advisory and a Freeze Warning?
A Frost Advisory is issued when frost is expected and threatens sensitive plants, typically with forecast lows of 33°F to 36°F. A Freeze Warning is issued when significant, widespread freezing temperatures are expected, cold enough to damage or kill unprotected plants whether or not frost forms.
Can frost form when it is above 32°F?
Yes, by the official reading. The air temperature is measured about 4 to 5 feet above the ground, and the surface can be up to 10 degrees colder on a clear, calm night. The surface itself still has to reach 32°F or below.
Related Terms
Freezing Fog
Fog that forms when air temperature is below 32°F (0°C), depositing ice crystals on exposed surfaces. Freezing fog creates treacherous driving conditions and can coat everything with a layer of rime ice.
Dew Point
The dew point is the temperature to which air must be cooled to become saturated, so that water vapor begins to condense into dew or fog. The higher the dew point, the more moisture is in the air. In summer, a dew point of 55°F or lower feels dry and comfortable, and 65°F or higher feels muggy.
Black Ice
A thin, transparent layer of ice on road surfaces that is nearly invisible, making it extremely dangerous for drivers. Black ice forms when temperatures drop rapidly after rain or from refreezing meltwater, often at night or in shaded areas.
Temperature Inversion
A layer of the atmosphere where temperature increases with altitude instead of decreasing. Inversions trap pollutants and moisture near the surface, causing fog, smog, and haze. They also cap thunderstorm development.
Wind Chill
Wind chill is how cold the air feels on exposed skin once wind is taken into account. Wind strips heat from the body faster than still air does, so the wind chill temperature is lower than the air temperature. At 0°F with a 15 mph wind, the wind chill is -19°F, and exposed skin can freeze in 30 minutes.
Frost: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
In-depth guide
Frost Advisory vs Freeze Warning vs Hard Freeze: What Each One Means
A Frost Advisory usually means lows of 33 to 36°F, a Freeze Warning 32°F or colder, and a hard freeze about 28°F or colder. Learn why frost forms above freezing, why the alerts stop in late fall, and what to protect at each level.
Weather Education
Lake Effect Snow: Why One Town Gets Three Feet and the Next Gets Flurries
What lake effect snow is, how it forms, why Buffalo gets so much of it, why totals change within a few miles, and what a Lake Effect Snow Warning means today.
Weather Education
Winter Storm Watch vs Warning vs Winter Weather Advisory (Plus Blizzard, Ice Storm, and Snow Squall Warnings)
What a Winter Storm Watch, Winter Storm Warning, and Winter Weather Advisory mean, how many inches of snow trigger a warning where you live, and how Blizzard, Ice Storm, Snow Squall, and Extreme Cold Warnings differ.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Guide
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The key differences between winter precipitation types, how each forms, and how to stay safe.
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Winter Weather Guide
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Temperature Converter
Convert between Fahrenheit, Celsius, and Kelvin instantly.
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Wind Chill Calculator
Estimate cold-weather exposure risk from temperature and wind speed.
Sources
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In the WeatherAI App
WeatherAI displays official NWS alerts, which include Frost Advisories and Freeze Warnings, and Standard and Pro subscribers can set a custom notification rule on temperature, checked at a time of day they choose.
Availability, coverage, update frequency, and alert delivery vary by data source, region, subscription plan, device permissions, and service conditions. WeatherAI supplements official sources and is not an emergency notification service. For life-safety decisions, rely on your national weather service, local emergency management, and official warning channels.