Snow is a form of precipitation consisting of frozen water crystals, but the conditions required for it to reach the ground are more intricate than simply a freezing temperature at the surface. The formation of snow begins high in the atmosphere, requiring a specific combination of atmospheric moisture and a sustained temperature profile from the cloud base to the ground. True snowfall depends on the successful creation of ice crystals in the cloud and their subsequent survival through all layers of air during their descent to the earth.
The Essential Ingredients for Snow Formation
The journey of a snowflake begins hundreds or thousands of feet above the ground within a cloud that contains both water vapor and liquid water droplets. For snow to form, a moist air mass must be lifted and cooled through adiabatic cooling, which causes the water vapor to become saturated. This saturation alone is not enough, as pure water droplets can remain liquid in a supercooled state at temperatures down to approximately -40°F (-40°C).
The process of forming an ice crystal requires a solid particle to serve as an anchor, known as an ice nucleus. These nuclei are microscopic airborne particles like dust, clay, or even certain bacteria that possess a structure similar enough to ice to encourage water molecules to freeze onto them. Once an initial ice crystal forms around a nucleus, it grows rapidly at the expense of the surrounding supercooled water droplets, driven by the difference in saturation vapor pressure between ice and liquid water.
The ice crystal then grows into a recognizable snowflake through two primary mechanisms: riming and aggregation. Riming occurs when the crystal collides with supercooled water droplets, which instantly freeze upon contact and adhere to the surface. Aggregation is the process where multiple ice crystals collide and stick together, often forming the large, complex snowflakes seen during heavy snowfall events. This dual growth process allows the initial ice crystal to quickly accumulate enough mass to overcome updrafts and begin its fall toward the surface.
The Critical Temperature Profile for Descent
The most common misconception about snow is that the surface air temperature must be 32°F (0°C) or below for it to fall. In reality, the temperature profile of the entire atmosphere, from the cloud to the ground, is the determining factor. Snowflakes can survive a fall even when the surface temperature is slightly above freezing, typically up to 40°F (4.5°C).
The factor that allows snow to survive the warmer air near the ground is evaporative cooling, which is highly dependent on the air’s humidity. As the snowflake falls through an above-freezing layer, it begins to melt. This phase change requires energy, which the snowflake draws from the surrounding air, thus cooling the air immediately around it.
This cooling process creates a small pocket of colder air that slows the melting, effectively insulating the snowflake as it descends. The true measure of whether snow will survive the fall is the wet-bulb temperature, which takes into account both the ambient temperature and the relative humidity. This temperature represents the lowest point to which air can be cooled by the evaporation of water.
If the wet-bulb temperature remains at or below 32°F (0°C) throughout the lower atmosphere, the air is cold and dry enough for the snowflakes to keep their integrity or only partially melt. This explains why snow can fall with a thermometer reading of 37°F (3°C) but not at 32°F (0°C) if the air is extremely humid. When the air is drier, more evaporation occurs, leading to greater cooling and a lower wet-bulb temperature, allowing snow to reach the ground intact through a shallow warm layer.
Distinguishing Snow from Other Frozen Precipitation
The type of precipitation that reaches the ground is determined by how the vertical temperature profile of the atmosphere is structured. True snow requires a temperature profile that is entirely below freezing, or one with only a very shallow, above-freezing layer near the ground. A slight variation in this profile can lead to entirely different winter precipitation.
Sleet occurs when snow falls into a deep layer of air that is above freezing, causing the snowflakes to melt completely into rain droplets. These droplets then fall through a much deeper layer of sub-freezing air near the surface, giving them enough time to refreeze into small, transparent ice pellets before hitting the ground. This requires a substantial warm layer aloft followed by a thick freezing layer near the surface.
Freezing rain is caused by a deep warm layer that completely melts the snow, followed by a very shallow layer of sub-freezing air directly at the surface. The rain droplet cools below freezing as it passes through this thin, cold layer, but it does not have enough time to freeze into a solid pellet. Instead, the supercooled liquid freezes instantly upon contact with any surface at or below 32°F (0°C), coating objects in a layer of glaze ice.