Snow is a form of precipitation consisting of ice crystals that fall from the atmosphere. The creation of snow requires a precise sequence of events involving specific atmospheric components and environmental conditions. It is a product of an intricate microphysical process that begins high in the clouds. For snow to form and reach the ground, the air must contain three fundamental ingredients: moisture, cold temperatures, and a microscopic solid particle to act as a seed.
The Foundation: Water Vapor and Sub-Freezing Temperatures
Sufficient moisture, existing as water vapor, is the fundamental requirement for any precipitation. This gaseous water must be present in the upper atmosphere to supply the material for ice crystal growth. Warmer air can hold substantially more water vapor than colder air, which is why the heaviest snowfalls often occur when air temperatures are only slightly below freezing, typically around \(-9^\circ\text{C}\) (\(15^\circ\text{F}\)) in the cloud layer.
The second part of the foundation is a temperature profile at or below \(0^\circ\text{C}\) (\(32^\circ\text{F}\)) within the cloud where the snow is forming. Water droplets can remain liquid at temperatures far below the standard freezing point, a state known as supercooled water. These droplets can persist in liquid form down to approximately \(-38^\circ\text{C}\) (\(-36.4^\circ\text{F}\)) without a suitable surface to initiate freezing.
The Critical Ingredient: Ice Nuclei
The atmosphere must contain microscopic particles, called ice nuclei (INPs), to initiate the formation of ice crystals. Water vapor cannot spontaneously freeze into an ordered crystal structure without a solid surface to act as a template, a process known as heterogeneous nucleation. These particles provide the necessary structure for supercooled water molecules to align and freeze.
Ice nuclei are composed of various airborne materials, including mineral dust from deserts, soot, volcanic ash, pollen, and even certain types of bacteria. The most effective INPs, such as the bacterium Pseudomonas syringae, can trigger freezing at temperatures near \(-2^\circ\text{C}\) (\(28.4^\circ\text{F}\)). Without these seeds, the supercooled water droplets would struggle to transform into ice, limiting the cloud’s ability to produce snow or even rain.
The presence and type of ice nuclei determine the temperature at which ice formation begins in a cloud. Mineral dust particles often require temperatures below \(-10^\circ\text{C}\) (\(14^\circ\text{F}\)) to become active. Once an INP is introduced into a supercooled water droplet, it acts as a scaffold, overcoming the energy barrier required for the phase change to solid ice.
The Formation Process: From Nucleus to Snowflake
Once an initial ice crystal forms around an ice nucleus, its growth cycle is primarily driven by the Bergeron Process. This mechanism explains how ice crystals grow at the expense of surrounding supercooled liquid water droplets. At sub-freezing temperatures, the saturation vapor pressure over ice is lower than the saturation vapor pressure over liquid water.
This difference causes water vapor molecules to move away from the liquid droplets and deposit directly onto the ice crystal’s surface, a process called deposition or sublimation. The ice crystal grows rapidly, drawing moisture from the surrounding air and causing the supercooled droplets to evaporate. As the ice crystal grows, its final shape, known as its crystal habit, is determined by the precise temperature and humidity of the air it passes through.
At temperatures between \(-10^\circ\text{C}\) and \(-15^\circ\text{C}\) (\(14^\circ\text{F}\) and \(5^\circ\text{F}\)), for example, the ice crystals tend to form delicate, branched structures called dendrites, the classic snowflake shape. Colder, drier air typically produces simpler forms like columns or plates. The snowflake we observe on the ground is rarely a single crystal, but rather an aggregate of many crystals that collide and stick together as they fall, a process called aggregation.
Delivery Conditions: Ensuring Snow Reaches the Ground
For snow to be observed at the surface, the atmospheric conditions must persist from the cloud base to the ground. This requires the entire vertical temperature profile to remain at or below freezing to prevent complete melting. If the snowflake encounters a layer of air with temperatures significantly above \(0^\circ\text{C}\) (\(32^\circ\text{F}\)) on its descent, it will begin to melt.
A shallow warm layer will cause the snowflakes to partially melt and then refreeze into ice pellets, or sleet, before reaching the ground. If the warm layer is deeper or warmer, the snowflake will melt completely into a raindrop. The temperature closest to the ground is crucial, and the concept of the “wet-bulb temperature” often determines the outcome.
The wet-bulb temperature is the temperature air would have if cooled to saturation by the evaporation of water into it. As snow melts, it draws heat from the surrounding air, cooling the layer immediately around it through evaporative cooling. This cooling effect allows snow to sometimes persist and reach the surface even when the air temperature measured by a standard thermometer is slightly above freezing, often up to \(2^\circ\text{C}\) (\(35.6^\circ\text{F}\)).