Weather forecasts often predict snow even when the surface temperature is a few degrees above freezing. This can be confusing, as basic science suggests water freezes at or below 32°F (0°C). The contradiction reveals a complexity in atmospheric science that goes beyond a simple temperature threshold. Understanding the temperature required for snow involves looking high into the clouds where precipitation forms and analyzing the physical processes that occur as snowflakes fall toward the ground.
The Theoretical Freezing Point
The scientific baseline for the phase change of water is 32°F (0°C). This is the maximum air temperature at which snow can fall and remain completely intact without melting. Above this point, the ice crystals begin melting into liquid water. If the surface air temperature is 33°F or higher, the falling snow will encounter air capable of causing the ice to melt. However, snowflakes do not melt instantly because they possess a latent heat of fusion. This means a certain amount of energy must be absorbed for the phase change from solid to liquid to occur, giving the snowflakes a brief window of time to survive in above-freezing temperatures.
The Crucial Role of Evaporative Cooling
The true determining factor for snow reaching the ground above freezing is the wet-bulb temperature. This measurement represents the lowest temperature air can reach through the evaporation of water into it. The wet-bulb temperature is a more accurate indicator of the precipitation’s phase than the ambient air temperature (dry-bulb temperature) because it accounts for cooling.
As a snowflake falls through unsaturated air, some ice or liquid water on its surface evaporates. This evaporation is a cooling process because water requires heat energy to change into vapor. This necessary heat is drawn from the surrounding air and the snowflake, lowering the temperature of the air column near the ground.
Evaporative cooling can effectively drop the air temperature down to the wet-bulb temperature. If the air is dry enough, the wet-bulb temperature can be below 32°F (0°C), even if the ambient temperature reads as high as 38°F or 40°F. The falling snow essentially creates its own sub-freezing environment, allowing the snowflakes to survive the descent without fully melting. Snowfall often occurs when the wet-bulb temperature is below 34°F (1.1°C), regardless of the dry-bulb temperature.
How Snow Crystals Form in the Atmosphere
The journey of a snowflake begins high in the clouds, where temperatures must be below freezing for ice crystals to form, often far colder than the surface. Precipitation in cold clouds usually starts as ice, even if it later falls as rain. The mechanism responsible for forming these initial ice crystals is the Wegener–Bergeron–Findeisen process, often simplified to the Bergeron Process.
This process occurs in clouds cold enough to contain both supercooled water droplets—liquid water unfrozen below 32°F—and a small number of ice crystals. The saturation vapor pressure over ice is lower than over liquid water at the same sub-freezing temperature. This difference causes water vapor molecules to preferentially deposit onto existing ice crystals, making them grow rapidly at the expense of the surrounding supercooled water droplets, which evaporate. The ice crystals quickly become large and heavy enough to fall from the cloud. As they descend, they may collide with other supercooled droplets, which freeze onto the crystal on contact, increasing the snowflake’s mass.
Other Factors Influencing Snowfall Type
Beyond temperature, two other factors significantly influence the type of precipitation that reaches the ground, whether it is light, fluffy snow, or heavy, wet snow: air humidity and precipitation rate.
The humidity of the air below the cloud base is important. High humidity reduces the potential for evaporative cooling, meaning the air temperature must be closer to or below 32°F for the snow to survive. Conversely, dry air allows for greater evaporation, increasing the cooling effect and making snow possible even with surface temperatures several degrees above freezing. Higher humidity in the cloud layer also provides more water vapor for growing ice crystals, leading to heavier, wetter snowflakes.
The rate of precipitation also influences the outcome. A heavy precipitation rate means a greater volume of ice is falling through the air. This increases the total surface area for evaporation, which cools the air column more quickly and significantly. A high precipitation rate can rapidly cool the column, increasing the likelihood of snow even at borderline temperatures.