The common belief is that snow can only fall when the air temperature is at or below the freezing point of 32°F (0°C). While snow formation requires sub-freezing temperatures high in the clouds, the snow’s journey to the ground is a complex process that can occur even when the surface air is slightly warmer. The question of whether it can snow at 50°F introduces a threshold that is almost always too warm for a snowflake to survive its entire descent. Understanding this phenomenon requires looking beyond the ground-level thermometer to the entire vertical temperature profile of the atmosphere.
Surface Temperature is Misleading
The temperature reported by a weather station, often taken a few feet above the ground, reflects only the conditions in the lowest layer of the atmosphere. This surface temperature is not a reliable indicator of the conditions a snowflake encounters during its entire journey from the cloud base to the earth. A surface reading of 50°F (10°C) is a near-certain indicator that the air column is simply too warm for snow to survive.
For snow to reach the ground intact, the air from the cloud to the surface must be cold enough to prevent complete melting. At 50°F, the warmth of the lower atmosphere is typically too extensive and deep. Any snow that forms high above would melt into rain long before it reached the ground, as it would be exposed to above-freezing air for an extended period.
The Critical Air Temperature Zone
Snow can reliably reach the ground when the surface air temperature is between 32°F (0°C) and about 39°F (4°C), occasionally reaching 41°F (5°C). This narrow range allows snowflakes to persist through partial melting and cooling. The determining factor for the type of precipitation is the vertical temperature profile, which maps the air temperature at various altitudes.
Snow forms in the upper atmosphere, where temperatures are well below freezing. As the ice crystals fall, they must pass through air cold enough to sustain them. If the temperature in the lowest atmospheric layer is only slightly above freezing, the snow will be heavy and wet because of partial melting. If the maximum temperature in this melting layer exceeds about 37.4°F (3°C), the snowflake is likely to melt completely into a raindrop.
The Role of Evaporative Cooling
The mechanism that allows snow to fall in air temperatures above freezing is a process known as evaporative cooling, which is linked to the wet-bulb temperature. As a snowflake descends into air slightly warmer than freezing, it begins to melt, changing phase from solid ice to liquid water. This phase change requires energy, which the melting snowflake draws as latent heat directly from the surrounding air.
The removal of heat cools the air immediately around the snowflake, effectively lowering the local air temperature closer to the freezing point. This cooling effect slows the melting process, giving the snowflake a better chance of reaching the ground intact. The effectiveness of evaporative cooling is significantly enhanced in drier air, meaning that snowfall at warmer temperatures, such as 45°F, requires extremely low relative humidity.
This phenomenon is measured by the wet-bulb temperature, which is the temperature an air parcel would have if it were cooled to saturation by the evaporation of water into it. If the wet-bulb temperature of the lower atmosphere remains at or below 32°F, snow can fall even if the actual air temperature (dry-bulb temperature) is higher. For instance, a 40°F air temperature with high humidity might still result in rain, but a 40°F air temperature with very dry air could have a wet-bulb temperature low enough to produce snow. The theoretical maximum air temperature for snow is around 48°F, but this requires an extremely dry environment with single-digit relative humidity.