The elevation at which the air temperature transitions from below to above freezing.

In typical weather patterns, the air temperature cools with altitude. The freezing level is the imaginary line marking where the air temperature is at 32°F or 0°C. The freezing level provides a good snapshot of the relative warmth of incoming storms. The freezing level is also an important consideration for melt-freeze processes and wet avalanche conditions. 

The freezing level is the elevation where the air temperature is at freezing. The freezing level varies spatially and temporally depending on influences from air masses, diurnal heating, and other factors. Credit: National Oceanic and Atmospheric Administration

Precipitation falls as snow a few hundred feet below the freezing level before transitioning to rain. Credit: National Oceanic and Atmospheric Administration