When people wonder how cold it must be for snow to stick, they often look at the nearest thermometer. This is misleading because the air temperature reported by a weather station is typically measured five feet above the ground. This single measurement does not tell the full story of whether snow will accumulate on the surface. The snow’s fate—whether it melts on contact or piles up—is determined by a more complex interaction of temperatures and conditions.
Air Temperature Is Not the Only Factor
The air temperature is important because it governs the condition of the falling precipitation. Snow forms when the atmospheric temperature is at or below 32°F (0°C). If the air temperature is slightly above freezing, the falling snow begins to melt, which creates evaporative cooling. This process uses latent heat to locally chill the air immediately surrounding the snowflake, slowing the melting.
The temperature of the air is distinct from the temperature of the ground surface. While air temperature can fluctuate rapidly, the ground temperature changes much more slowly due to thermal inertia. The ground retains heat much longer than the air does. Therefore, an air temperature of 30°F might still result in no accumulation if the ground holds residual heat from previous days.
How Ground Surface Temperature Determines Sticking
For snow to accumulate and last, the surface temperature must be at or slightly below 32°F (0°C). The ground’s ability to reach this threshold depends on specific factors. Previous weather conditions, such as a warm spell or rainfall, cause the ground to store a significant amount of heat, known as thermal mass. This stored heat must be pulled out of the surface before snow can begin to stick.
The composition of the surface material itself is a major influence on heat retention and transfer. Surfaces like asphalt and concrete have low albedo, meaning they absorb more solar radiation and retain heat longer than lighter surfaces like grass or soil. Snow will often stick first to grass, mailboxes, and car roofs because these surfaces cool down faster and do not hold deep residual warmth. Conversely, dark pavement can remain several degrees above freezing even when the air is cold, causing snow to melt immediately upon impact.
Even on a cloudy day, the ground temperature is affected by solar radiation and the surrounding environment. If the ground was warm the day before, it can take hours of sub-freezing air temperatures for the surface to cool enough to permit accumulation. This is why the snow may initially stick only in shaded areas or on elevated objects that are not connected to the warm thermal mass of the earth.
The Impact of Snowfall Rate and Duration
A high rate of snowfall can overcome a ground temperature that is slightly above freezing, such as 34°F. This heavy, continuous precipitation works to cool the air layer immediately above the ground. The large volume of falling snow effectively draws latent heat away from the surface faster than light snow would.
The duration of the snowfall also plays a significant role in lowering the surface temperature to the sticking point. As the cold precipitation continues to fall, it eventually exhausts the residual heat stored in the surface layer. Once the surface temperature cools to 32°F or below, the snow begins to accumulate quickly. This accumulation forms an insulating layer that helps preserve the fresh snowpack, allowing a storm that starts as melting flakes to transition into significant accumulation.