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How is snow made? – Tenley, age 7, Rockford, Michigan
The thought of snow can conjure up images of powdery slopes, days out of school or hours of shoveling. For millions of people, it’s an inevitable part of life – but you may rarely stop to think about what made the snow.
As a professor of atmospheric and planetary sciences, I’ve studied how ice crystals floating in the sky become the snow that coats the ground.
It all starts in the clouds.
Clouds form when air near the Earth’s surface rises. This happens when sunlight warms the ground and the air closest to it, just like the Sun can warm your face on a cold winter day.
As the slightly warmer air rises, it cools – and the water vapor in that rising air condenses to form liquid water or water ice. From that, a cloud is born.
You need just two things for snow to form.
Endless pathways
When temperatures are well below freezing on the ground, the clouds are primarily made of water in the form of ice. Under 32 degrees Fahrenheit – that’s zero degrees Celsius – the frozen water molecules arrange themselves into a hexagonal, or six-sided, crystalline shape. As ice crystals grow and clump together, they become too heavy to stay aloft. With the help of gravity, they begin to fall back down through and eventually out of the cloud.
What these ice crystals look like once they reach land depends on the temperature and humidity of the atmosphere. As the humidity – or the amount of water vapor in the cloud – increases, some of the ice crystals will grow intricate arms at their six corners. That branching process creates what we think of as the characteristic shapes of snowflakes.
No two ice crystals take the same path through a cloud. Instead, every ice crystal experiences different temperatures and humidities as it travels through the cloud, whether going up or down. The ever-changing conditions, combined with the infinite number of paths the crystals could take, result in a unique growth history and crystalline shape for each and every snowflake. This is why you’ve likely heard the saying, “No two snowflakes are exactly alike.”
Many times, these differences are visible to the naked eye; sometimes a microscope is required to tell them apart. Either way, scientists who study clouds and snow can examine a snowflake and ultimately understand the path it took through the cloud to land on your hand.
It takes approximately one hour for a snowflake to reach the ground.
LiLi/iStock via Getty Images Plus
Liquid water as glue
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