Can snow fall when the thermometer reads 40 degrees Fahrenheit? The answer is definitively yes. While the formation of ice crystals requires temperatures at or below the freezing point of 32°F, it is a common meteorological observation for snow to reach the ground when the air temperature is significantly warmer. This phenomenon appears paradoxical because the snow must survive a relatively warm air column during its descent to the surface.
The Vertical Temperature Profile
The temperature reading taken by a surface thermometer often misrepresents the conditions throughout the column of air where precipitation originates. Snow crystals first develop high in the atmosphere, typically within clouds where the air temperature is well below freezing, sometimes 20°F or colder. These ice particles begin their descent and encounter a layer of air closer to the ground that may be above 32°F, which meteorologists call the melting layer. The altitude of the freezing level, where the air temperature hits 32°F, is a primary determinant of the precipitation type.
The depth of this melting layer determines the ultimate fate of the snowflake. If the warm layer is deep, the snow fully melts into rain before reaching the surface. For snow to survive at 40°F, the layer of air above freezing must be quite shallow, minimizing the duration the precipitation spends in the melting environment. The speed of the falling snow is therefore important, as a quick descent increases the chance of the ice crystal surviving the passage through the warmer air mass.
Evaporative Cooling and the Wet-Bulb Effect
The primary mechanism that allows snow to persist through temperatures like 40°F is evaporative cooling, which relates directly to the wet-bulb temperature (WBT). The WBT measures the lowest temperature to which air can be cooled by the evaporation of water, and it is almost always lower than the standard air temperature, known as the dry-bulb temperature. This difference is particularly pronounced when the air is dry, indicating a greater potential for atmospheric cooling.
As dry snowflakes fall through a warmer, drier layer of air, some ice crystals begin to sublimate, turning directly from a solid state into water vapor. This phase change requires a significant amount of energy, demanding 677 calories of latent heat for every gram of ice that sublimates. This heat is drawn directly from the surrounding air mass.
The removal of this heat effectively lowers the temperature of the entire column of air as the snow falls. This process is highly efficient because the snowflakes present a large surface area for heat exchange. The resulting temperature drop can be substantial enough to bring the air temperature closer to the freezing point, creating a self-sustaining cold environment near the surface.
If the wet-bulb temperature is at or near 32°F, the surrounding air has enough cooling potential to preserve the snow, even if a standard thermometer initially registers 40°F. The snow essentially creates its own micro-climate by rapidly cooling the atmosphere around it as it falls, allowing it to reach the ground intact.
Conditions Required for Warm Snow
Several specific meteorological ingredients must align for snow to reach the ground when air temperatures are above freezing. A high precipitation rate, meaning heavy snowfall, is important for maximizing the cooling effect. When many snowflakes fall together, collective sublimation and evaporation cool the air mass more rapidly than light flurries. This high rate of precipitation helps saturate the air with moisture, which limits further melting once cooling is established.
The thickness of the warm layer near the surface also needs to be minimal, ensuring the snow’s transit time is short. The air’s humidity plays a complex role: while high humidity helps preserve the snowflake by reducing evaporation, a slightly drier layer near the surface is necessary to initiate the evaporative cooling that drops the temperature toward the wet-bulb limit.
The resulting snowfall typically consists of large, wet, and heavy flakes, often described as slushy snow. These flakes aggregate, making them more resistant to full melting than smaller crystals. This heavy, wet nature results from clumping as the flakes partially melt and refreeze during their journey, increasing their mass and fall speed. This type of snow often struggles to accumulate on surfaces warmer than 35°F but can stick to grass and elevated objects.