Snow requires the formation of ice crystals high in the atmosphere where temperatures are well below freezing. While water freezes at 32°F (0°C), snowflakes are often observed reaching the ground when the ambient temperature is several degrees warmer. This occurs because atmospheric processes allow snowflakes to survive their journey through a layer of air that is above the freezing point. The maximum temperature at which snow can reach the ground is determined by the balance of heat exchange between the falling flakes and the surrounding air.
The Physics of Near-Freezing Snowfall
Snowflakes persist in air slightly above freezing due to an ongoing cooling process as they descend. When a snowflake enters air warmer than 32°F, it begins to melt, changing phase from solid ice to liquid water. This phase change requires energy, which the melting snow absorbs from the surrounding air, a process known as the latent heat of fusion. This heat absorption cools the air mass immediately surrounding the snowflake.
The survival of the flake is aided by a second cooling mechanism: evaporative cooling. If the air is not saturated with moisture, the liquid water on the surface of the partially melted snowflake begins to evaporate. Evaporation requires a substantial energy input, which is absorbed from the air and the snowflake as latent heat of vaporization. This dual cooling effect—melting and evaporation—creates a localized micro-climate around the falling snow that remains near the freezing point.
This constant removal of heat allows the snowflake to maintain its structure and avoid complete melting. The drier the air, the more vigorous the evaporative cooling, enabling snow to survive higher ambient temperatures. This mechanism effectively chills the entire column of air, enhancing the chance for the snow to reach the surface before turning entirely into rain.
Defining the Critical Temperature Threshold
The maximum temperature at which snow is typically observed reaching the ground is approximately 37°F to 40°F (3°C to 4°C). This range represents the threshold where the cooling mechanisms are overwhelmed by the ambient heat of the air. Above this temperature, the air has too much heat, and the latent heat processes cannot cool the air quickly enough to prevent the snowflake from melting completely.
The true meteorological determinant for persistent snowfall is not the standard air temperature, known as the dry-bulb temperature, but the wet-bulb temperature. The wet-bulb temperature is the lowest temperature to which air can be cooled by the evaporation of water. It is measured by a thermometer wrapped in a water-soaked cloth.
This measurement accounts for both the air temperature and the relative humidity. When the air is dry, water evaporates quickly, leading to a much lower wet-bulb temperature than the dry-bulb temperature. For snow to continue falling, the wet-bulb temperature must remain at or below 32°F (0°C), even if the dry-bulb temperature is warmer. Low humidity permits snow to fall at the higher end of the dry-bulb temperature range.
Atmospheric Layer Considerations
For snow to fall when the surface temperature is above freezing, the entire vertical temperature profile of the atmosphere must be considered. Snowflakes must originate in clouds where the temperature is well below freezing, typically around 10°F (-12°C) or colder. The precipitation then begins its descent through the atmospheric column.
The air layer above 32°F is called the “warm layer” or melting zone, and the snow must pass through it. For the snow to survive and reach the ground, this warm layer must be shallow. Alternatively, the rate of snowfall must be heavy enough to facilitate significant evaporative cooling within the layer. If the warm layer is only a few hundred feet deep, the cooling effect of the snow can effectively drop the temperature within that layer closer to the freezing point.
This condition is distinct from other winter precipitation types, which are caused by a deeper warm layer. If the warm layer melts the snowflake completely, but there is a deep layer of freezing air near the ground, the rain will refreeze into ice pellets known as sleet. If the warm layer is deep, but the sub-freezing layer near the ground is very shallow, the water droplets will become supercooled and freeze instantly upon contact, resulting in freezing rain.