Frost, decoded. Ice that grows out of clear air.
Frost does not fall from the sky. It grows where it sits: on grass, on car roofs, on a pumpkin left out overnight. This page is the short version of how that happens, why a night can frost over even when the forecast never quite hits freezing, and the two things worth protecting before the first cold night of the season.
WHAT IT ACTUALLY IS.
A surface colder than the air around it.
Frost forms when a surface cools to 32 degrees Fahrenheit or below and water vapor in the air turns directly into ice crystals on that surface, without ever becoming liquid water first. It happens most often on clear, calm nights, when the ground radiates its heat straight up to the sky and grass, leaves, and car roofs cool faster than the air above them. That is why the first frost of the season so often arrives on a quiet, starry night rather than in the middle of a storm.
FROST VS FREEZE.
Not the same cold, and the warnings say so.
A freeze means the air temperature itself has dropped to freezing or below; frost means ice crystals have formed on a surface, and the two do not always arrive together. Frost can appear while the official air temperature still reads a degree or two above freezing, because an exposed surface can cool below the air around it, and a dry freeze can pass with no visible frost at all. The warnings keep the distinction: a Frost Advisory means frost is expected and tender plants are at risk, while a Freeze Warning means air temperatures at or below freezing are expected, a longer and harder cold that threatens pipes and plants alike.
WHAT TO DO ABOUT IT.
Cover the plants, scrape the glass, take it slow.
Before a frost night, bring potted tender plants inside and cover garden plants with cloth or an old sheet, which insulates better than plastic laid directly on leaves. On the car, scrape the windshield fully clear instead of peering through a peephole, and never pour hot water on frosted glass, which can crack it. Bridges and overpasses frost first, so treat the first cold morning drive like the first icy one: slow down, leave space, and assume a shady patch of road may be slick. Brzzy's forecast, live radar, and severe weather alerts turn the same kind of ground truth into a plain-language answer before the cold night: cover, scrape, or go as usual.
What is frost?
Frost is ice crystals that form on a surface when that surface cools to 32 degrees Fahrenheit or below and water vapor in the air turns directly into ice, without ever becoming liquid water first. It does not fall from the sky; it grows where it sits.
Why does frost form on a clear night?
On a clear, calm night the ground radiates its heat straight up to the sky, so grass, leaves, and car roofs cool faster than the air above them. Once a surface drops to freezing, water vapor deposits on it as ice crystals, which is why the first frost often arrives on a quiet, starry night rather than in a storm.
What is the difference between frost and a freeze?
A freeze means the air temperature itself has dropped to freezing or below. Frost means ice crystals have formed on a surface. Frost can appear while the official air temperature still reads slightly above freezing, because an exposed surface can cool below the air, and a dry freeze can pass with no visible frost at all.
What is the difference between a Frost Advisory and a Freeze Warning?
A Frost Advisory means frost is expected and tender plants are at risk. A Freeze Warning means air temperatures at or below freezing are expected, a longer and harder cold that threatens pipes and plants alike. Exact thresholds vary by local forecast office.
Does Brzzy warn me about frost?
Brzzy shows your local forecast with hourly updates, live radar, severe weather alerts, GIF forecasts, and activity ratings that weigh the sky for your plans. Brzzy is free to download on iPhone.
More from Brzzy: Full weather glossary, Freezing, Cold
