What comes to mind when you hear the word snow? Does it make you anxious? Do you rush to the market for milk and bread? Or do you get excited, looking forward to a nice, relaxing snow day? In the Atlantic Corridor we see our fair share of snow, with higher concentrations in the Northeast.
But do we ever stop and think, “What makes snow, snow?” Of course we know snow is a form of precipitation – a major component of the water cycle that keeps our natural world going. But what exactly is the science of snow? This article contains everything you didn’t think you needed to know about snow.
What is snow?
By definition, snow is an accumulation of packed ice crystals. So no, snow is not frozen rain (that’s called sleet). Snow forms when water vapor converts directly to ice without becoming water through a process called deposition or, desublimation.
How does snow form? Surprisingly, it is not the temperature that we feel at ground level that influences snow. The formation of snow requires freezing atmospheric temperatures and very little air moisture. Snow can occur at extremely low temperatures as long as there is some source of moisture and some way to lift or cool the air. Most heavy snowfalls occur when there is relatively warm air near the ground—typically 15 degrees Fahrenheit or warmer— due to the fact that warmer air can hold more water vapor.
What is a snowflake?
Snowflake is the term used to describe snow crystals. They are formed from water vapor that condenses directly into ice inside of clouds (remember deposition?). They take their familiar, hexagonal shape as water vapor molecules from cloud droplets condense and freeze on the surface of a seed crystal. Seed crystals form on tiny specks of dust in the air, and patterns emerge as these crystals grow (below).
The size and composition of a snowflake depends on the amount of ice crystals grouped together. This is always dependent on air temperature. Snowflakes that fall through dry, cool air will be small and powdery. They will not stick together. This ‘dry’ snow is ideal for snow sports such as skiing.
When the temperature is slightly above freezing, the snowflakes will melt around the edges and stick together to become big, heavy flakes. This creates ‘wet’ snow which sticks together easily. This snow is perfect for building snowmen and snowball fights!
As far as the saying “there are no two snowflakes alike” – its true. Physics professor Kenneth G. Libbrecht stated, “It’s like shuffling a deck and getting the exact same shuffle for 52 cards. You could shuffle every second for the entire life of the universe, and you wouldn’t come close to getting two of the same.”