How Does Snow Grow? (Photo Credit: NESDIS)

Winter weather can have a major impact on all aspects of life. From school closings to travel delays and cancellations, snow and ice cause a wide range of difficulties during the winter months. Additionally, snowflakes are present in almost all clouds (as most clouds exist at altitudes that experience below freezing temperatures) and as such, it is important to learn about the processes by which they grow. Whether falling to the ground as a melted summer rain or a frozen mid-winter snowstorm, ice crystals play a major role in our weather and the hazards it can sometimes produce. 

Once an initial snowflake has formed, there are three primary mechanisms by which it may grow: deposition, accretion and aggregation. In the deposition process, ice crystals grow as a result of the difference in saturation vapor pressure between liquid water and ice at a given temperature. Saturation vapor pressure is the pressure exerted outward by a vapor (in this case water vapor) when the surrounding air is saturated. Since liquid water has a much higher saturation vapor pressure than ice, and molecules in the atmosphere move from high pressure toward low pressure, water vapor travels from liquid droplets toward the lower-pressure crystals. This flow of water vapor aids the growth of ice crystals and development of snowflakes. It is important to note that deposition relies heavily on the presence of both ice and liquid water and is thus temperature dependent. Maximum rates of growth by deposition occur around -15℃. 

When ice crystals collide with supercooled water droplets, the crystals grow by a process known as accretion. Supercooled water simply refers to liquid water molecules that exist in liquid form at a temperature below freezing (0℃). As an ice crystal falls through a cloud, any supercooled water it comes in contact with will freeze to the crystal surface, quickly increasing the crystals size. This mechanism is most efficient between 0℃ and -10℃, where supercooled water droplets are commonly present. 

Lastly, snowflakes can grow in size by aggregation, a process by which ice crystals collide with one another to form larger ice crystals. The probability that two random snowflakes collide and combine depends strongly on the shape of each crystal and the presence of liquid water on each molecule. This liquid water helps ice crystals bond together molecularly, creating larger and larger ice crystals over time. In fact, snowflakes formed by aggregation can reach 3 to 4 inches in diameter. It is important to keep in mind that all of these mechanisms for snowflake growth refer to the ice crystals within a cloud, not near the surface. However, if the temperature is consistently below freezing between the cloud and surface, these crystals can fall toward the ground and create dangerous winter weather conditions! 


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©2019 Weather Forecaster Dennis Weaver