Snow is defined as frozen precipitation resulting from thermodynamic and microphysical processes occurring high above the ground. The temperature needed for snow formation is complex, as the survival of a snowflake depends on the temperature profile throughout the entire column of air, not just a single reading. Understanding the process requires looking first at the cloud where the snow begins, then at its journey down to the surface.
The Process of Ice Nucleation
The initial stage of snow formation, known as ice nucleation, happens in clouds where the air temperature is well below the standard freezing point of water. Clouds often contain supercooled water droplets, which are liquid water existing at temperatures between 0°C (32°F) and approximately -40°C (-40°F). These droplets do not freeze spontaneously because they lack a structure to organize the water molecules into an ice lattice.
For ice crystals to form at temperatures warmer than -40°C, they require microscopic particles called ice nucleating particles (INPs). These particles, such as mineral dust or organic matter, act as templates upon which water vapor deposits directly as ice, a process called heterogeneous nucleation. Efficient snow production typically occurs between -12°C and -18°C (10°F and 0°F), known as the dendritic growth zone. In this narrow range, ice crystals grow rapidly, allowing them to reach a size large enough to fall from the cloud.
Defining the Necessary Atmospheric Layer
After the ice crystal forms inside the cloud, its survival depends on the temperature of the air column beneath it. For snow to reach the ground, the ideal condition is a vertical temperature profile that remains continuously at or below 0°C (32°F). This means the air layer from the cloud base to the surface must be cold enough to prevent melting.
If a layer of air warmer than 0°C exists between the cloud and the ground, it is termed a melting layer. The thickness and maximum temperature of this warm layer determine the fate of the precipitation. A deep or significantly warm melting layer will cause the snowflakes to melt completely, resulting in rain at the surface.
Why Snow Can Fall Above Freezing
Snow frequently reaches the ground when the surface thermometer reads slightly above freezing, often between 0.5°C and 3°C (33°F and 37°F). This is possible because of two cooling processes that occur as the snowflake descends through the shallow melting layer near the surface. The first is the latent heat of fusion, where the initial melting of the snowflake absorbs heat from the surrounding air, cooling it down.
The second, and often more significant, process is evaporative cooling, which relates directly to air humidity. As snowflakes fall through unsaturated air, some ice sublimates or melted water evaporates. This process draws substantial heat from the immediate environment, lowering the temperature surrounding the flake enough to preserve the ice crystal’s structure until it hits the ground.
This dynamic is best predicted not by the standard air temperature, known as the dry-bulb temperature, but by the wet-bulb temperature. The wet-bulb temperature represents the lowest temperature to which air can be cooled by water evaporation. If the wet-bulb temperature is at or slightly above 0°C (32°F), snow is highly likely to reach the surface, even if the dry-bulb temperature is warmer. Consequently, surface temperatures up to 5°C (41°F) have been observed during snowfall, though this requires very dry air to maximize evaporative cooling.