The common understanding is that snow requires temperatures at or below 32°F (0°C) to fall, but this is a scientific oversimplification. Snow is possible with an air temperature of 45°F, though the conditions are specific. Snowflakes form high in the atmosphere where temperatures are well below freezing, and the challenge is their survival through the warmer air layers below. The air temperature alone does not determine the precipitation type.
The Initial Answer: Falling Versus Accumulating
Snow can fall through air that measures 45°F, but the critical distinction is whether it will accumulate on the ground. Air temperature is typically measured several feet above the surface; surface temperature determines sticking. Surfaces like roads and sidewalks often retain heat, meaning their temperature is frequently higher than the surrounding air.
When a snowflake lands on a surface warmer than 32°F, it melts instantly, preventing accumulation. Even if the air temperature is 45°F, the snow may fall briefly as wet snow or mixed rain-and-snow. This brief snowfall is a sign of specific atmospheric conditions allowing the snowflake to survive its descent.
The Science of Evaporative Cooling (The Wet-Bulb Effect)
The primary reason snow can survive in warm air is a process called evaporative cooling, which meteorologists analyze using the wet-bulb temperature. The wet-bulb temperature is the lowest temperature air can reach by evaporating water into it, and it factors in both the air temperature and the amount of moisture in the air.
When snow falls into a layer of dry air that is above freezing, the edges of the snowflakes begin to melt and evaporate. This change from solid to gas requires energy, which the snowflake absorbs as latent heat from the surrounding air. By drawing heat away, the air immediately surrounding the snowflake cools itself down, sometimes by several degrees.
If this cooling process can maintain the air near the snowflake close to the freezing point, the flake is insulated and can survive the fall. The wet-bulb temperature must be at or very close to 32°F for snow to reach the ground, even if the ambient air temperature (dry-bulb temperature) is much higher, like 45°F. This explains why the precipitation itself is actively cooling the air column to the freezing point. The drier the air, the more effectively evaporation can take place, allowing the wet-bulb temperature to drop significantly below the ambient temperature.
How Snowflakes Survive Warmer Air
For snow to fall at a temperature as high as 45°F, several atmospheric variables must align to optimize the wet-bulb effect. One important factor is the amount of moisture in the air, or the humidity. Low humidity is essential because dry air maximizes the evaporative cooling process, allowing the falling snow to cool the air more substantially and quickly.
Another factor is the rate of precipitation, as heavy snowfalls are much more likely to survive a warm layer. A large volume of snowflakes falling rapidly cools the entire air column more effectively, a process known as dynamic cooling. The sheer number of flakes collectively draws enough latent heat from the air to drop the temperature in the warm layer closer to freezing.
The vertical temperature profile of the atmosphere is also a determining factor for survival. Snow forms high up where it is cold, and the depth of the warm layer it must fall through is important. If the warm layer is very shallow, the snowflake simply does not have enough time to completely melt before reaching the surface. The combination of low humidity, high precipitation intensity, and a shallow warm layer creates a microclimate that allows the snowflake to survive the warm air temperature reading at the surface.