Many people believe snow only falls when the air temperature is precisely at or below 32°F (0°C). This simple rule is often contradicted by real-world experience, where people frequently observe flakes falling even when the thermometer registers a few degrees above freezing. The true answer to “at what temperature will it snow?” is not a single number, but a complex interplay of atmospheric conditions extending from the clouds down to the ground. Understanding the survival of a snowflake requires looking beyond the surface reading to the entire column of air through which the precipitation falls.
Where Snow Crystals Form
Snowflakes always begin their life in a frigid environment high in the atmosphere, regardless of the temperature at the ground. Snow crystals form in clouds where the temperature is well below freezing, typically between 10°F and 0°F (-12°C and -18°C). The process starts when supercooled water droplets—liquid water below 32°F—freeze onto tiny particles known as ice nuclei.
As these ice crystals grow by collecting more water vapor, their shape is determined by the specific temperature and humidity of the surrounding air. For instance, large, ornate stellar dendrites, the classic star-shaped snowflakes, primarily grow near 5°F (-15°C). Once the crystal is heavy enough, it begins its long descent toward the surface, determining whether it arrives as snow or rain.
Air Temperature Near the Ground
As the ice crystal falls through the atmosphere, it can encounter layers of air that are above the freezing point. The survival of the snowflake depends on the depth and maximum temperature of this “melting layer.” If the air temperature remains only slightly above freezing, say between 32°F and 37°F (0°C and 3°C), the snowflake may not have enough time to melt completely before reaching the ground.
A deep layer of air with temperatures significantly above freezing will fully melt the snow into rain. However, a shallow, above-freezing layer (especially if the maximum temperature is less than about 34°F or 1°C) often only results in partial melting. The resulting precipitation may reach the surface as wet snow or slush.
The Role of Humidity
The humidity of the air below the cloud is the most important factor allowing snow to reach the ground when the surface temperature is above freezing. This factor is accounted for by the concept of the wet-bulb temperature. The wet-bulb temperature is the lowest temperature to which air can be cooled by the evaporation of water into it.
When snow falls through relatively dry air, a portion of the ice begins to melt or sublimate, a process that requires energy. This energy is drawn directly from the surrounding air, causing rapid cooling known as evaporative cooling, which can drop the air temperature down to the wet-bulb temperature. If the wet-bulb temperature is 32°F (0°C) or lower, the air surrounding the snowflake cools to the freezing point. This protects the crystal from melting even if the standard thermometer reads 35°F or 37°F. Low humidity is necessary for snow to fall at above-freezing surface temperatures.
Distinguishing Winter Precipitation Types
The specific type of winter precipitation that reaches the ground is determined by the vertical temperature profile—the temperature at every altitude from the cloud to the surface. Snow requires a temperature profile that is largely at or below freezing throughout the entire column. Tiny variations in this profile produce the other forms of frozen precipitation.
Sleet (Ice Pellets)
Sleet occurs when the snow falls through a warm layer deep enough to cause complete or partial melting, but is then followed by a deeper layer of air that is refreezing near the ground. The rain or partially melted flakes freeze into small, hard ice pellets before they hit the surface.
Freezing Rain
Freezing rain requires a deep warm layer aloft that fully melts the snow into rain, followed by only a very shallow freezing layer at the surface. The liquid raindrops become supercooled, meaning they are below freezing but remain liquid. They freeze instantly upon contact with cold objects on the ground, creating a dangerous glaze of ice.