The temperature at which snow falls is often misunderstood. Many assume the air temperature must be at or below the freezing point for snowflakes to reach the ground. The reality is more complex, involving the temperature high in the atmosphere where snow forms, the temperature near the surface, and the humidity of the air mass. Understanding the life cycle of a snowflake requires examining the physics of water across the entire atmospheric column, not just the ground-level reading.
The Freezing Point Baseline
Snowflakes originate in clouds where the temperature is at or below the freezing point of water, 0°C (32°F). Water vapor must deposit directly onto microscopic particles, such as dust or pollen, known as ice nuclei. This process of deposition allows ice crystals to form, bypassing the liquid water phase entirely.
The region most favorable for crystal growth, called the dendritic growth zone, is typically much colder, often between 10°F and 0°F (-12°C and -18°C). The tiny ice crystals collect additional water vapor and supercooled water droplets, growing into complex, hexagonal snowflakes. The physical requirement for snow to form is that the temperature in the cloud must be sub-freezing.
Why Snow Falls When Air Temperatures Are Above Freezing
It is common to see snow falling when the surface air temperature is measured at 35°F or even 40°F. This occurs because the melting of a snowflake is not instantaneous but requires a significant transfer of heat energy. Snowflakes can survive a brief passage through a shallow layer of warm air near the ground, especially if they are large or the snowfall rate is heavy.
The primary mechanism allowing snow to survive in above-freezing temperatures is evaporative cooling. As the snowflake falls through dry air, some ice sublimates directly into water vapor, while other parts may melt into liquid water. Both sublimation and melting absorb heat energy from the surrounding air. This absorption causes the air temperature immediately surrounding the falling snow to cool rapidly.
Evaporative cooling can lower the air temperature near the ground by several degrees, sometimes enough to reach the freezing point. The true measure of whether snow will survive is the wet-bulb temperature, which is the temperature the air reaches when cooled by evaporation. If the wet-bulb temperature is at or below 0°C (32°F), snow has a high probability of reaching the ground, even if the actual air temperature is 35°F. Low humidity accelerates this cooling effect, making snow more likely on a dry 35°F day than a humid one.
Temperature Profiles and Precipitation Types
The fate of a snowflake is determined by the vertical temperature profile, which measures the air temperature at different altitudes from the cloud base to the surface. For precipitation to reach the ground as pure snow, the entire column of air must remain at or below the freezing point. Any deviation in this profile can change the precipitation type, even if the storm starts as snow high above.
A slightly warmer layer aloft, where temperatures rise above 32°F, creates conditions for other winter precipitation forms. Sleet, also known as ice pellets, occurs when snow falls into a relatively shallow warm layer, causing the flakes to partially melt into raindrops. The precipitation then re-enters a deep freezing layer near the surface, where the partially melted drops refreeze into solid ice pellets before hitting the ground. This warm layer is typically thin, often less than 2,000 feet thick.
Freezing rain is produced by a more significant temperature inversion. Snow falls into a deep layer of air that is well above freezing, causing the snowflakes to melt completely into liquid rain. The raindrops then fall through a very shallow layer of sub-freezing air right at the surface. This layer is not deep enough to allow them to freeze before impact, so the supercooled liquid water freezes instantly upon contact with surfaces at or below 32°F, creating a glaze of ice.