Snow accumulation determines whether fresh snowfall remains on the ground or melts immediately. The difference between a light dusting that vanishes and a snowfall that coats the landscape is not simply the amount of precipitation. Accumulation relies on specific thermal conditions, the microscopic structure of the ice crystals, and the characteristics of the surface the snow falls upon. Understanding what makes snow stick involves looking beyond the air temperature to the conditions occurring at the ground level.
The Essential Temperature Conditions
The air temperature alone does not tell the whole story of snow accumulation. While snow forms when the air temperature is at or below the freezing point of water, \(32^\circ\text{F}\) (\(0^\circ\text{C}\)), it can still fall when air temperatures are slightly warmer. This is due to evaporative cooling, where melting snow creates a small bubble of colder air around the flake, slowing the melting process as it descends.
The temperature of the ground is more significant than the air temperature in determining if snow will stick. Ground surfaces retain heat from the sun and previous days, leading to a thermal lag. If the ground temperature is above \(32^\circ\text{F}\), snow will melt upon contact, but accumulation begins only when the ground cools sufficiently. This cooling happens faster on surfaces like grass and soil than on heat-retaining materials like asphalt and concrete.
How Snow Crystals Physically Bond
Once cold temperatures are met, the physical structure of the snowflakes dictates how they adhere to each other. Snowflakes are intricate, six-sided structures, often with elaborate branches called dendrites. These branches allow the individual crystals to interlock and tangle together upon landing, forming a cohesive snowpack.
The primary microscopic mechanism that causes snow grains to bond and strengthen the snowpack is called sintering. Sintering involves the growth of ice necks between adjacent snow grains, essentially welding them together over time, facilitated by a thin layer of water molecules on the crystal surface.
Surface Impact and Snow Type
The moisture content of the snow plays a role in its stickiness and accumulation. Wet snow occurs when the air temperature is just around the freezing point, causing the flakes to partially melt and acquire a layer of liquid water. This moisture makes the flakes heavy and sticky, allowing them to easily adhere to surfaces like trees and power lines, and to pack into dense snowballs.
In contrast, dry snow falls when temperatures are well below freezing and contains significantly less liquid water. Dry snow is light, powdery, and lacks the moisture needed for the ice crystals to bond strongly upon impact, making it less likely to stick to vertical surfaces. The surface material also affects initial sticking; surfaces with low thermal mass, such as vegetation and wooden decks, cool down faster and offer a better initial anchor than pavement or concrete.