It is a common belief that snow, defined as frozen precipitation, can only fall when the air temperature is at or below the freezing point of \(32^\circ\text{F}\) (\(0^\circ\text{C}\)). While this assumption makes logical sense, atmospheric physics reveals that snow often reaches the ground when the surface temperature is a few degrees above freezing. This phenomenon is possible due to the interplay between the atmosphere’s temperature structure and the cooling effect of the falling precipitation.
The Temperature Profile Aloft
Snowflakes begin high up in the atmosphere where temperatures are well below freezing, regardless of surface conditions. Ice crystals typically form within clouds at altitudes where the air temperature is often between \(10.4^\circ\text{F}\) and \(0.4^\circ\text{F}\) (\(-12^\circ\text{C}\) and \(-18^\circ\text{C}\)). This layer is known as the dendritic growth zone, where ice crystals grow efficiently into complex flakes.
The air temperature near the ground, measured by a standard thermometer, is often very different from the temperature several thousand feet above. Precipitation starts as snow in these frigid upper layers and begins its descent. For snow to reach the ground, it must survive the journey through the lowest layer of the atmosphere, which may contain air warmer than \(32^\circ\text{F}\).
The Mechanism of Evaporative Cooling
The mechanism allowing snow to survive a fall through above-freezing temperatures is evaporative cooling, which actively lowers the air temperature near the ground. As a snowflake falls into warmer air, it begins to melt, changing from solid ice to liquid water. This phase change requires latent heat, which the snowflake absorbs from the surrounding air.
This absorption of heat chills the pocket of air immediately around the falling flake. The resulting liquid water on the snowflake’s surface also begins to evaporate, especially if the air is relatively dry. This evaporation draws heat from the air, further cooling the surrounding environment. The combined effect of melting and evaporation creates a localized “cold bubble” that allows the snowflake to persist.
Meteorologists use the “wet-bulb temperature” to determine the true temperature falling precipitation will encounter. The wet-bulb temperature considers both the air temperature and the relative humidity. This provides a more accurate measure of the potential for evaporative cooling. If the wet-bulb temperature remains at or below \(32^\circ\text{F}\) (\(0^\circ\text{C}\)), the falling snowflakes can cool the air sufficiently to prevent complete melting before reaching the surface.
The Practical Surface Temperature Limits
Applying these principles reveals the typical maximum surface temperature range where snow can still occur. Snow rarely reaches the ground intact if the surface air temperature is warmer than about \(38^\circ\text{F}\) (\(3.3^\circ\text{C}\)). Most snow events with above-freezing surface temperatures happen when the thermometer is between \(33^\circ\text{F}\) and \(38^\circ\text{F}\).
For snow to survive at these marginal temperatures, the air must have relatively low humidity. Dry air allows for more rapid evaporation of melting flakes, which enhances the evaporative cooling effect and drops the air temperature quickly. The rate of snowfall also plays a role. A heavy, intense snowfall introduces a large volume of cold flakes, which can cool the entire layer of air down to \(32^\circ\text{F}\) (\(0^\circ\text{C}\)) in a short period.
The combination of high snowfall rates and effective evaporative cooling means that snow can sometimes accumulate even when the temperature is slightly above freezing, such as \(36^\circ\text{F}\) or \(38^\circ\text{F}\). Accumulation is possible because the rate of falling snow is greater than the rate at which it melts on warmer surfaces. Once the snow stops, the melting process takes over as the air warms back up.