The temperature required for snowfall is often misunderstood, as many assume the air must be at or below 0°C (32°F) at ground level. While freezing is necessary for water to solidify, this simple answer ignores the complexities of the atmosphere. Snow often falls when the ground temperature is slightly above freezing. The actual precipitation type reaching the surface is determined by the temperature profile of the entire column of air from the cloud base down to the ground. Understanding this vertical temperature gradient explains why a seemingly warm day can still deliver winter precipitation.
The Basic Requirement for Snowfall
The fundamental requirement for snow is that it must begin as ice crystals high in the atmosphere. Snow forms when the temperature within the cloud is at or below 0°C (32°F). At this temperature, water vapor converts directly into ice, a process known as deposition, without first becoming liquid water. These microscopic ice crystals attract and freeze additional water vapor, allowing them to grow into complex snowflakes.
This formation must occur in the cold upper layers of the storm system. If the cloud temperature is above freezing, precipitation forms as liquid raindrops instead of ice crystals. Therefore, the temperature far above the surface determines whether the process starts as snow. The temperature near the ground only affects whether the snowflake survives its journey.
Why Snow Can Fall Above Freezing
The key to understanding above-freezing snowfall is that air temperature does not remain constant from the clouds to the surface. A snowflake’s journey may take it through a shallow layer of air warmer than 0°C. Snowflakes can survive brief exposure to temperatures slightly above freezing, typically up to about 4°C (40°F), before melting completely.
The survival of the snowflake is influenced by evaporative cooling. As the snowflake begins to melt, liquid water on its surface evaporates into the surrounding air. This change of state requires energy, drawing heat from the snowflake and the immediate air. This heat transfer cools the snowflake and the air layer, significantly slowing the melting process.
This cooling effect means the actual temperature experienced by the falling snowflake is often lower than the air temperature measured by a standard thermometer. Meteorologists use the wet-bulb temperature, which is the lowest temperature air can reach through evaporation. For snow to reach the ground, the wet-bulb temperature of the air column must remain at or below 0°C (32°F). Drier air allows for more evaporation and cooling, making snowfall possible at a higher recorded air temperature.
How Temperature Determines Precipitation Type
The type of precipitation that ultimately reaches the ground—snow, sleet, or freezing rain—depends entirely on the vertical temperature profile of the atmosphere. When the temperature remains below 0°C (32°F) from the cloud base to the surface, precipitation remains frozen and falls as snow. If falling snow encounters an above-freezing layer, it begins to melt.
Sleet (Ice Pellets)
For sleet, this warm layer is shallow, and the snowflakes only partially melt. These partially melted drops then fall through a deep second layer of air that is below freezing, causing them to refreeze into small, hard ice pellets before hitting the ground. Sleet is often characterized by a bouncing sound upon impact.
Freezing Rain
Freezing rain occurs when the warm layer high up in the atmosphere is much deeper, causing the snowflakes to melt completely into liquid raindrops. These raindrops then fall through a very shallow layer of sub-freezing air just above the surface. Because the cold layer is too thin, the drops do not have time to refreeze before landing. Instead, the supercooled liquid water freezes instantly upon contact with any surface at or below 0°C, creating a clear glaze of ice.