Let It Snow!
It is a question practically everyone has either asked or been asked. Is it true that no two snowflakes are the same? First a little background on how snowflakes form.
It all begins with water vapor that condenses into a droplet. The droplet cools and, depending on the temperature around it, freezes. Temperature is critical for snowflake formation.
There are six basic snowflake types. Which one occurs depends on the temperature at the time. As the crystal grows (provided there is enough water vapor around to let it grow), the crystal gradually grows six arms. They grow into six-sided forms because water molecules take that configuration as they freeze.
As more and more water vapor condenses and freezes onto the crystal, it grows larger and heavier. Eventually it begins to fall. As it falls, it continues to acquire more water vapor in a process called riming. Snowflakes continue to grow until they hit the ground, some times just by riming action but sometimes by collision with other snowflakes.
Large snowflakes are typically conglomerates of several that stuck together after collision. They are more likely to stick together while they are falling through warmer air rather than colder air because they melt a little and the water forms a bond between flakes. The table below gives you an idea of what type of flake develops at what temperature.
|Hexagonal Plates||32 to 25F|
|Needles||25 to 21F|
|Hollow Columns||21 to 14F|
(flat plates in a >6-sided star shape)
|14 to 10F and again at 3 to -8F|
(6-sided flakes with many sub-branches on each arm)
|10 to 3F|
|(Dendrites are what most people think of when they picture a snowflake.)|
Water content matters in how much snow comes from the available water vapor. Although the accepted average ratio of snow to water content is 10 inches of snow for every one inch of water content. Water content is what you get when you melt down the snow.
Wet snow has a higher water content than average and can be as low as 4-6 inches of snow for one inch of water content. Dry snow, generally formed under very cold conditions, is often 20-30 inches of snow (called champagne powder) to one inch of water and there have been cases of snow so dry, it worked out to be 50 inches of snow for an inch of water. Snow that dry and light can be moved with a puff of air that would not even blow out a match.
How do we know so much about snowflakes? Although scientists have wondered about them for centuries, the questions only began to be answered when a curious Vermont farm boy developed an avid interest in snow. Wilson Bentley began to examine snow very closely in 1880 when he was just 15 years old. During his first three winters, Bentley put snowflakes under a microscope and drew what he saw.
Can you imagine what it must have been like for this teenager who stood out in the cold catching snowflakes and then looking at them under the microscope. His family probably thought he was nuts! In his fourth winter, he began to photograph the snowflakes and over the next 50 years, he photographed thousands of them. He published a book “Snow Crystals” in 1931 that contained 2,300 snowflake photos.
Ukichiro Nakaya, of the Hokkaido University in Japan, began to study snowflakes in 1930 and he photographed snowflakes until his death in 1962. A scientist, he took his interest further than Bentley did. Bentley was only interested in the snowflakes, not their origin and he made few observations about them beyond the photographs.
Nakaya, on the other hand, was curious about how snowflakes developed and researched which conditions led to which type of snowflake. He not only spent several winters out in the snow, he constructed a test booth and created snowflakes in the laboratory. The precise control of the lab environment allowed him to determine what shape a crystal would take under a range of temperature and humidity conditions. It was his research that gave us new understanding of snow.
Modern cloud physicists continue to study snowflakes but the answer to the question at the start of this article is already known. Is it true that no two snowflakes are the same? The answer is, it depends on the snowflake.
The simpler the form is, like hexagonal plates, needles, columns and sector plates, the more likely it is they will look alike. That is not to say theyíll be precisely the same since their molecules wonít ever be in the same exact place twice, but theyíll certainly look identical and thatís really all we care about, isnít it?
The more complex the form a snowflake takes, the less likely it is two will look the same. Dendrites (snowflakes shaped the way most of us think of them) form in infinite patterns that change as the snowflake rises and falls through varying layers of the atmosphere, melts a little, collects more rime, or collides with other snowflakes. So yes, there are some snowflakes that occur only once, as individual as fingerprints, but there are other, simpler ones that are the same, and, pardon the pun, thatís no snow job!
Written by Melody L. Higdon, 14 WS/DOPA