The question of the temperature required for snowfall is often met with the simple answer of 32 degrees Fahrenheit, but the reality is more nuanced than the freezing point of water. Snow is frozen precipitation that must form in cold air high in the atmosphere. The temperature at ground level can be surprisingly variable, requiring consideration of the entire atmospheric column and the physics of cooling.
The Freezing Point Misconception
The widely accepted temperature for water to solidify is 32°F (0°C), which is frequently cited as the threshold for snow. Snow crystals initially form high in the clouds where the air temperature is always at or below freezing, often much colder. As the snowflake descends, it begins to melt if it passes through air above 32°F. The melting process requires energy drawn from the surrounding air. If the warmer air layer is shallow or not significantly above freezing, the snowflake may not melt completely before reaching the ground. This explains why snow can sometimes be observed even when the surface temperature is a few degrees above freezing.
The Science of Evaporative Cooling
Snow can fall when the surface air temperature is slightly above freezing, sometimes up to 40°F, due to a process called evaporative cooling. This phenomenon requires the surrounding air to have a low dew point, which is a measure of humidity. When falling snowflakes pass through a dry column of air, the air absorbs moisture from the edges of the flake through evaporation or sublimation.
Evaporation requires a significant amount of heat energy, which is pulled directly from the air. This transfer of energy causes the air temperature between the cloud and the ground to drop. If the air is dry enough, this cooling effect can lower the temperature of the entire column to 32°F or below, allowing the snow to survive the trip without melting.
Forecasters rely on the wet-bulb temperature as the metric for predicting whether precipitation will fall as snow or rain. The wet-bulb temperature is the lowest temperature that air can be cooled to by the evaporation of water into it. If the wet-bulb temperature of the air column is below 32°F, the evaporative cooling process keeps the falling precipitation frozen, regardless of the standard temperature reading.
The Difference Between Wet and Dry Snow
The temperature and humidity of the air column determine the physical characteristics of the snow that reaches the ground, creating a distinction between “wet” and “dry” snow. The quality of the snow is a direct consequence of the thermal and moisture balance in the atmosphere during its descent.
Dry Snow
Dry snow, often referred to as powder, forms when temperatures are well below freezing and humidity is low. These conditions result in small, light, and fluffy ice crystals with a low water content. Dry snow is easily blown by the wind and is preferred for winter sports like skiing because it provides less resistance.
Wet Snow
Wet snow falls when the air temperature is near or just above freezing, and the humidity is high. The warmer, moister air causes the edges of the snowflakes to partially melt, making them sticky and allowing them to clump together into larger, heavier flakes. This higher water content makes wet snow dense and heavy, making it ideal for packing into snowballs or building snowmen. The increased weight of wet snow can be problematic, potentially leading to power outages by accumulating on trees and power lines.