Snow is a form of frozen precipitation, consisting of individual ice crystals that grow in the atmosphere and fall to Earth. While many assume the temperature must be at or below the freezing point for snow to fall, the actual ground temperature often tells a more complex story. The technical requirement for snow formation is straightforward, but conditions between the cloud and the surface allow snow to persist even when the thermometer reads a few degrees above freezing.
The Standard Answer: Freezing Point
The journey of a snowflake begins high up in the clouds where the air temperature is at or below \(32^\circ\text{F}\) (\(0^\circ\text{C}\)). Within these cold clouds, water vapor must convert directly into ice crystals without first becoming a liquid droplet. This process, known as deposition, requires microscopic particles suspended in the air to act as seeds, which are called ice nuclei. Particles like dust or pollen provide the necessary surface for water molecules to arrange themselves into an ice structure.
Many water droplets in the cloud remain liquid even at temperatures well below freezing, a phenomenon known as supercooling. When a supercooled water droplet collides with an ice nucleus, it freezes instantly. This allows the newly formed ice crystal to grow rapidly by attracting surrounding water vapor. This initial formation process requires the air aloft to be at or below the freezing mark.
Why Snow Often Falls Above Freezing
Despite the requirement for sub-freezing temperatures in the cloud, snow can fall and accumulate when the temperature at ground level is as high as \(38^\circ\text{F}\). This seemingly counterintuitive event is possible because of the cooling effect that precipitation has on the air below the cloud. When snowflakes descend into an air mass that is slightly above freezing, they begin to melt and, simultaneously, some of the water on the snowflake’s surface begins to evaporate.
Both melting and evaporation are processes that require energy, which they draw from the surrounding air. This energy transfer cools the air mass immediately around the falling snow, a process known as evaporative cooling. If the air is dry, the evaporative cooling effect is maximized, sometimes dropping the near-surface temperature to \(32^\circ\text{F}\) or below very quickly. This cooling creates a dome of colder air, allowing the snow to survive the journey to the ground without fully melting.
Meteorologists often use a measurement that combines air temperature and humidity to predict the precipitation type, which is related to the wet-bulb temperature. This value represents the lowest temperature a parcel of air can reach through the evaporation of water. When the air is cold and dry, the wet-bulb temperature is low enough to support snowfall even if the actual air temperature is a few degrees above freezing. Furthermore, the temperature measured on the ground is not the only factor; the temperature of the air several hundred feet above the surface is more important for determining whether the snowflakes melt before reaching the surface.
How Atmospheric Layers Influence Snow Type
The type of winter precipitation that ultimately reaches the ground is determined by the vertical temperature profile, which is the temperature of the atmosphere at various heights. If the entire column of air from the cloud to the surface remains at or below the freezing point, the precipitation will fall as snow. However, the presence of alternating layers of warm and cold air can transform the precipitation into other forms.
Sleet occurs when snow falls from the cloud and encounters a layer of air that is above freezing, causing the snowflakes to melt completely into raindrops. These liquid drops then fall into a deep layer of cold air near the ground, where they have enough time to refreeze into tiny pellets of ice before hitting the surface. For this to happen, the cold layer at the bottom must be sufficiently deep to allow for the full refreezing process.
Freezing rain is produced by a similar, but distinct, atmospheric structure. The snow melts entirely in a warm layer, but the cold air layer near the surface is very shallow. The raindrops do not have enough time to fully freeze into ice pellets before reaching the ground; instead, they become supercooled liquid. These supercooled drops freeze instantly upon contact with any surface that is at or below \(32^\circ\text{F}\), creating a coating of clear ice.