Whether snow accumulates on the ground involves more than just the air temperature. Snow’s ability to “stick” depends on a delicate interplay of atmospheric and surface conditions. Understanding these factors helps explain why some snowfalls create a picturesque winter landscape, while others seem to vanish upon contact.
The Critical Air Temperature Range
Air temperature plays a significant role in snow accumulation. Snow typically sticks best when the air temperature is at or slightly above freezing, around 32°F (0°C). This temperature range is optimal because snowflakes are near their melting point. At these temperatures, the ice crystals develop a thin layer of liquid water on their surface, making them somewhat “sticky.” This moist outer layer allows individual snowflakes to easily bond upon contact, forming larger, heavier, “wet snow.” This characteristic stickiness contributes to efficient accumulation. Even if the air temperature is a few degrees above freezing, snow can still fall and stick, particularly if the warmer air layer near the ground is shallow. As snowflakes descend, some melting occurs, which cools the surrounding air through evaporative cooling. This cooling effect helps snow survive its journey to the surface and maintain its ability to bond.
Ground Temperature
While air temperature is important, the temperature of the surfaces snow lands on holds considerable sway over accumulation. Surfaces like roads, trees, and grass must be at or below freezing for snow to bond and pile up. If the ground is warmer than 32°F (0°C), falling snowflakes melt instantly upon impact, preventing lasting accumulation. This is a common reason why early season snowfalls may not stick, as the ground retains heat from previous warmer conditions. Ground temperatures often lag behind air temperatures, so a cold snap might not immediately cool the ground enough. However, prolonged freezing air temperatures or heavy snowfall can eventually cool the ground, making accumulation possible. When snow falls at a high rate, it can cool the surface faster than the ground’s warmth can melt it, allowing accumulation even on initially above-freezing surfaces.
The Role of Moisture in the Air
Atmospheric moisture, or humidity, significantly influences the type of snow that falls and its ability to stick. For snow crystals to form and grow into sizable flakes, sufficient moisture is necessary within the clouds. High humidity allows snowflakes to develop intricate structures that interlock, contributing to “stickier” snow when temperatures are near freezing. This type of snow is often heavier due to its higher water content, leading to more substantial accumulation. In contrast, very dry air produces small, lightweight, powdery snow that does not readily bond. Such snow is less likely to accumulate effectively, even if air and ground temperatures are well below freezing. While high humidity benefits “sticky” snow formation, it can also accelerate the melting of existing snowpack, as water vapor condensation releases heat.
Why Snow Doesn’t Always Stick
Several conditions can prevent snow from accumulating, even when it appears to be snowing. If the air temperature is significantly below freezing, snowflakes tend to be very dry and powdery. These dry flakes lack the thin liquid layer needed for strong bonding, making them less likely to stick to surfaces or each other, often resulting in snow that blows around easily. Conversely, if air temperatures are too warm, snowflakes may melt into rain or sleet before reaching the ground, or turn to slush upon impact. This is especially true if the layer of warm air near the surface is deep. Additionally, a ground temperature that remains above freezing will consistently melt falling snow, preventing lasting accumulation. Even if the air temperature seems right, a warm surface will act as a natural obstacle to snow accumulation. Low atmospheric humidity can also lead to non-sticking snow by preventing the formation of large, cohesive flakes.