It may seem counterintuitive, but snow can fall when the air temperature is 40°F. While snow is frozen precipitation, the temperature measured near the ground is only one part of the equation determining the type of precipitation. A snowflake’s survival depends on temperatures at different atmospheric levels, a physical cooling mechanism, and the dryness of the air.
Temperature Aloft
Snowflakes form high in the atmosphere, typically between 5,000 and 20,000 feet, where the air temperature is significantly colder than the 40°F at the surface. For snow to form, the air mass in the cloud layer must be at or below the freezing point of 32°F (0°C). This cold air allows water vapor to deposit directly onto microscopic particles, creating the ice crystals that aggregate into snowflakes.
The atmosphere is structured in layers, and the temperature profile, or how temperature changes with altitude, dictates the precipitation type. Even if the air at ground level is above freezing, the snow begins its descent as a fully formed crystal from the sub-freezing layer above. For the snow to reach the ground, the layer of above-freezing air near the surface must be relatively shallow.
If this warm layer is too deep or too warm, the snowflakes will completely melt into rain before they hit the ground. A warm layer with a maximum temperature above 37.4°F typically results in rain, as the snowflakes melt fully. The survival of the snowflake depends on how quickly it can pass through this warmer air before melting is complete.
How Evaporative Cooling Saves the Snow
When a snowflake falls into air warmer than 32°F, it begins to melt, but this process helps the snowflake survive. The change of state from solid ice to liquid water requires energy, known as the latent heat of fusion. This heat energy must be drawn from the surrounding air.
As the snowflake partially melts, it absorbs heat from the air immediately surrounding it, causing that air to cool down. This cooling effect, known as evaporative cooling, creates a small pocket of cooler air around the falling snow. The surrounding air temperature can drop enough that the snowflake’s melting slows significantly, allowing it to remain mostly intact.
Some of the meltwater on the snowflake’s surface can also evaporate into water vapor, which is a second cooling process involving the latent heat of vaporization. This phase change requires substantial heat energy, which is pulled from the air. This dual cooling action—melting and evaporation—can effectively chill the entire column of air from the cloud base to the ground.
Low Dew Point
The efficiency of evaporative cooling is governed by the humidity of the air near the ground, measured by the dew point. The dew point is the temperature to which the air must be cooled to become saturated, making it a direct measure of the air’s dryness. A low dew point, indicating very dry air, is required for snow to fall when the air temperature is 40°F.
When the air is dry, the meltwater from the falling snow evaporates much more effectively. This enhanced evaporation maximizes the cooling effect, allowing the air temperature to drop several degrees, potentially from 40°F down to freezing by the time the snow reaches the surface. This is why snow at warmer temperatures often occurs when the air is dry.
Meteorologists use the wet-bulb temperature, which is the lowest temperature the air can reach through evaporative cooling, as a better indicator for predicting snow than the air temperature. If the wet-bulb temperature is 32°F or less, snow is possible, even if the air temperature is 40°F. This condition requires a dew point significantly lower than the air temperature, often in the 20s or lower, for the cooling to be sufficient.