As winter approaches, the northern hemisphere begins to prepare for snow. This week, Ask Science explains the science behind snowfall, including snowflakes' shape and their implication towards the Earth's climate, and even shows us how to make our own snow.
In the northern hemisphere, winter is upon us and for those living far enough from the equator, that means snow, and lots of it. According to the long-range weather forecast from the Farmer’s Almanac, the 2019 winter will be “colder-than-normal” with plenty of snow. According to the National Snow & Ice Data Center, snow can cover up to 46 million square kilometers (that’s almost 18 million square miles) of the globe each year, and nearly every location in the U.S. has seen snowfall at some point (even you, Florida!).
So let’s take a look at the science of snowfall. Where does it snow and how? Is it really true that no two snowflakes are ever alike? What does snowfall tell us about the Earth’s overall climate? And for those, like me, who live somewhere without snow, I’ll tell you how to make your own.
How Does Snow Form?
For snow to form, two conditions have to be met. First, there has to be some amount of moisture in the air. This is why some deserts, like the Dry Valleys of Antarctica, can have no snow despite being very, very cold. This dependence on moisture also leads to what is often called “lake effect snow” or heavy snow fall near a large lake. Cold, dry air passing over a lake will gain moisture from the warmer water below. Colder air also has a harder time holding onto water vapor, which means that snowfall is rarer at very cold temperatures but more common at temperatures that are just below freezing. Which brings us to the next requirement…
Second, the temperature has to be cold enough, or below 0 degrees Celsius or 32 degrees Fahrenheit. That temperature refers to atmospheric temperature, or the temperature in the layer of atmosphere above us where that snow will form. If the temperature on the ground is also cold enough, snow will reach the ground and accumulate. Otherwise, it can melt when it reaches the warmer layers of air, which usually happens if the ground temperature is more than 5 degrees Celsius (or 41 degrees Fahrenheit).
Are Two Snowflakes Ever Alike?
Snow can fall as either single snowflakes, which are themselves clusters of ice crystals, or clusters of snowflakes clumped together. Snowflakes are always symmetric and six-sided.
When a droplet of water meets a pollen or dust particle in the atmosphere, they merge to create an ice crystal. Water vapor in the air will then freeze onto this original ice crystal building outward into the arms of a snowflake. The symmetric and six-sided nature of the snowflake arises from the fact that water molecules have their own internal order. In other words, the oxygen and hydrogen atoms that make up a water molecule fit together most efficiently in a hexagonal pattern.
So, it is true that no two snowflakes are alike. Snowflakes get their shapes based on the atmospheric conditions they pass through on their way to the ground, conditions like temperature and humidity. Since no two of those original ice crystals, the ones that will build into snowflakes, take the same path to the ground, each crystal has to make its own journey through its own unique set of atmospheric conditions. For example, snowflakes formed at 23 degrees Fahrenheit tend to appear elongated while snowflakes formed at a colder 5 degrees Fahrenheit are flatter. However, the six arms of a single snowflake all share the same journey and so they remain identical, resulting in a symmetric snowflake.