Snow accumulation is more complicated than simply checking if the air temperature is below the freezing point of 32°F (0°C). Accumulation results from a delicate balance between air temperature, the temperature of the surface it lands on, and several other environmental factors. This process involves the physics of heat transfer and the variable nature of the snowflakes. Understanding this complex interplay provides a clearer picture of why one location may see a blanket of white while a nearby area only experiences wet slush.
The Critical Air Temperature Range
Accumulation is most likely when the air temperature near the surface is between 30°F and 34°F (-1°C and 1°C). While snow forms high in the clouds well below freezing, it must survive the journey through the lower atmosphere. If the air temperature is significantly warmer than 34°F (1°C), the snowflake is likely to melt into rain before it touches the surface.
Slightly warmer air temperatures can sometimes lead to better sticking due to the creation of “wet snow.” Wet snow forms when the temperature is just below freezing, allowing liquid water to form on the outside of the ice crystals. This moisture acts like an adhesive, causing the individual snowflakes to clump together into larger, heavier aggregates that stick easily to surfaces. In contrast, “dry snow” falls in much colder air, remaining light and powdery because it has minimal liquid water content, which makes it less adhesive and easily blown around by the wind.
The Decisive Role of Ground Surface Temperature
The temperature of the ground surface is often more important than the air temperature in determining whether snow will accumulate. Snowflakes melt on contact if the surface temperature is above 32°F (0°C), regardless of the air temperature. Surfaces like asphalt, concrete, and soil store heat absorbed from previous days.
This stored energy must be removed before accumulation can begin. The initial flakes that hit a warm surface melt, and this melting process consumes a large amount of energy, known as the latent heat of fusion. This heat consumption effectively cools the immediate environment, helping subsequent snowflakes survive.
Different materials cool at different rates based on their thermal mass. Grass and soil cool much more quickly, meaning snow often sticks to lawns and fields first, even when roads remain clear. Pavement, such as roads and sidewalks, has a higher thermal mass, allowing it to hold heat longer. Consequently, a road surface may remain above freezing for hours into a snow event, delaying accumulation until the persistent cold overcomes the pavement’s retained heat.
Other Environmental Factors Driving Accumulation
The rate at which the snow is falling, or the precipitation rate, is a significant non-temperature factor influencing whether snow sticks. Heavy, rapid snowfall can overcome a warmer ground more easily than light flurries. When snow falls intensely, it blankets the surface faster than the surface heat can melt it, creating an insulating layer that slows the heat transfer from the ground.
Evaporative cooling, which is related to humidity and dew point, also plays a part. When a snowflake falls through air that is not saturated (low humidity), a small amount of the ice can sublimate or melt and then evaporate, drawing heat from the surrounding air. This cooling effect lowers the air temperature right near the surface, helping the remaining snow stay frozen and accumulate. Conversely, high humidity reduces this cooling effect, making it more difficult for the snow to survive if the air temperature is borderline.
Wind conditions modify accumulation by affecting both temperature and distribution. Strong winds can prevent light, dry snow from settling evenly by blowing it into drifts or off exposed surfaces. Furthermore, wind can mix warmer air near the surface with the colder air above, delaying the necessary cooling of the ground.