Freezing level
Precipitation & snow zero-degree level zero-isotherm level
The freezing level is the height where air temperature reaches 0 °C. It helps describe the atmosphere, but does not determine snowfall or ground frost on its own.
Where the thermometer reaches zero aloft
A weather balloon measures temperature as it rises, allowing the height of 0 °C to be identified. Models calculate these heights too. The freezing level is usually given in metres above sea level; check the legend for the particular product.
There can be more than one
Air does not always cool steadily with height. A warm layer above cold surface air can take the temperature above zero before it drops below freezing again higher up. Several freezing levels then exist. A single displayed number simplifies that structure.
Why snow can fall below it
A snowflake does not melt the instant it reaches warmer air. It can fall some distance farther. The snowfall level is therefore often below the freezing level, with the gap depending partly on humidity and precipitation intensity.
What about the road?
A road surface can cool below the air temperature by losing heat through radiation. Ground frost is therefore possible with a higher freezing level. Conversely, a warm layer can melt snow into rain that freezes on cold ground. Predicting precipitation type requires the full temperature profile.