The freezing level is the lowest altitude in the atmosphere at which the environmental temperature reaches 0 degrees Celsius when moving upward from the surface. On a sounding it is the height at which the temperature curve first crosses the 0 C isotherm, and it is reported either as an altitude above ground or above mean sea level or as the corresponding pressure level.
What the freezing level marks
Above the freezing level the environment is subfreezing. Cloud droplets can still exist as supercooled liquid down to about -40 C, but any ice particle that reaches this level and continues descending begins to melt. That is why cloud physics texts often prefer the label melting level for the same height, because the ice-to-liquid transition is the physically meaningful process.
In warm-season thunderstorms the freezing level sits somewhere between roughly 3 and 5 kilometers above the ground across most of the continental United States, and drops sharply during cool-season storms. In tropical air masses it can be higher, and in cold-air aloft regimes behind a strong upper trough it can drop close enough to the ground that even the low levels of a storm are subfreezing.
How forecasters use it
The most direct use is in hail forecasting. Hailstones grow above the freezing level, where they can accrete supercooled liquid water, and melt below it. A lower freezing level gives a hailstone more time to grow in the growth zone and less depth of warm air to fall through on the way to the ground. A higher freezing level shortens the growth window and stretches the melting layer, which favors smaller stones or complete melting to rain.
The freezing level and the height of the wet-bulb zero are read together for the same reason. The wet-bulb zero accounts for the additional cooling a falling hailstone gets from evaporation into subsaturated air, and studies of severe hail environments point to sub-cloud wet-bulb-zero heights in a specific window as being most favorable for large stones reaching the surface.
In cool-season and mountain forecasting the freezing level determines snow level in mountainous terrain and separates rain from snow at the surface. It also anchors the location of a bright band on radar reflectivity, where melting snowflakes briefly produce enhanced returns as they transition from ice to rain.
Where a single value can mislead
The atmosphere does not always cross 0 C only once on the way up. Warm noses aloft in cool-season storms produce environments where the temperature climbs above freezing in a mid-level layer and then falls back below freezing higher up, creating multiple 0 C crossings. In those cases the surface freezing level does not describe the ice-processing structure of the storm, and the full temperature profile has to be read.
The freezing level also moves. Daytime heating raises it, evaporative cooling in precipitation lowers it, and cold-air advection can drop it sharply within hours. Any operational use has to consider not just the initial value but the trend and the depth of the layer above and below it, because those together determine the hail growth and melting picture the storm will actually experience.