Many believe snow only falls when temperatures are at or below 32 degrees Fahrenheit (0 degrees Celsius). This perception doesn’t capture the full complexity of how snow forms and reaches the ground. Various atmospheric factors beyond just surface temperature are involved, explaining why snow can surprise us even when the thermometer reads above freezing.
The Nuance of Snow Temperature
Snow can indeed fall when surface temperatures are slightly above freezing, sometimes even between 33 and 37 degrees Fahrenheit (1 to 3 degrees Celsius). This occurs because the air layer immediately above the ground can still be cold enough for snowflakes to remain frozen during their descent. As snowflakes fall through this marginally warmer air, they begin to melt, a process that draws heat from the surrounding environment. This evaporative cooling effect chills the air directly around the snowflakes, creating a small pocket of colder air that helps preserve their frozen state. If the layer of above-freezing air near the surface is shallow, the snow may not have enough time to melt completely before reaching the ground.
Atmospheric Conditions for Snowfall
The temperature throughout the entire atmospheric column, from the clouds to the ground, dictates whether precipitation falls as snow. Snowflakes initially form in clouds where the temperature is at or below freezing. Ice crystals begin to form on tiny particles in the atmosphere, known as ice nuclei.
These initial ice crystals then grow through accretion, collecting supercooled water droplets, and aggregation, clumping together with other crystals to form larger snowflakes. A sufficient amount of moisture, indicated by high relative humidity, is also necessary for robust ice crystal growth within the cloud. For snow to reach the ground intact, a consistently cold air mass must extend from the cloud base down to the surface, ensuring the snowflakes do not melt into rain.
Distinguishing Winter Precipitation
Different temperature profiles throughout the atmosphere lead to distinct types of winter precipitation.
Sleet
Sleet forms when snowflakes fall through a shallow layer of warm air, causing them to partially melt into raindrops. These partially melted drops then fall through a deeper layer of freezing air closer to the surface, refreezing into small ice pellets that bounce upon impact.
Freezing Rain
Freezing rain occurs when snowflakes melt completely into rain as they pass through a substantial warm layer aloft. This rain then encounters a very shallow layer of sub-freezing air just above the surface, cooling the raindrops but not allowing them enough time to refreeze before hitting the ground. These supercooled raindrops freeze instantly upon contact with cold surfaces, creating a glaze of ice.
Graupel
Graupel, sometimes called soft hail or snow pellets, forms when supercooled water droplets freeze onto snowflakes, coating them with ice. Unlike sleet, graupel is soft and opaque, resembling tiny polystyrene balls, and can be easily crushed.