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Overview

Have you ever seen somebody experiencing snow for the first time? Or the spark in a child’s eye when they wake up and see streets covered in snow? Snow is one of those natural phenomena which does not cease to leave us bewildered and amazed. The same stands for the scientific community. They are still researching how snowflakes are formed and discovering new shapes they can take. Learn more about snow and you will appreciate it even more.

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How Are Snowflakes formed?

A snowflakes forms when water vapor during its travel through the air condenses on a particle. As water vapor changes from gas to solid state and freezes on the particle, it slowly grows into a crystal. Snow crystals are different from sleet, which are frozen raindrops, because water vapor converts directly into ice without first becoming liquid water.

Once snow crystals form in the atmosphere, they grow one molecule at a time, by absorbing surrounding water droplets. There are two basic ways in which snowflakes develop:

  • The first way leads to forming what are called facets crystals. A facet is a flat face on a 3D shape, like a prism.
  • The second way is branching. These are the snow crystals we are used to seeing in photographs and being represented in commercials, movies, and cartoons. Branches on a flake form because the water vapor condenses on the first thing it touches, and as this happens over and over again the intricate structures of snowflakes are formed.

A snowflake starts as a prism with six facets, but it can switch back and forth between creating facets and forming branches multiple times during its path to the ground. The final shape of a snowflake will depend on the atmospheric conditions in the cloud in which it was formed, as well the conditions in the surrounding environment as it was falling. Because even the slightest changes in temperature and the amount of moisture in the air change how the molecules act and how they condense, no two snowflakes will be alike. Professor Librecht from Caltech observed that the most intricate snowflake patterns are formed when there is a lot of moisture in the air and temperatures are warmer.

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Types of Snow

Scientists and meteorologists have made classifications of snow based on the shape of flakes, description of how it is falling, and by how it collects on the ground.

Even though no two snowflakes are alike, they can still be classified into some most common patterns. However, different classification systems have been created, and there is no consensus on which one is the proper one to use. The most popular are the International Classification System (defines seven types of snow crystals), Nakayo’s Classification (describes 41 types of snow crystals), and Magono and Lee Classification (distinguishes between 80 types of snow crystals).  The most common patterns included in these classifications are prisms, plates, dendrites, needles, columns, and stars.

There are several ways in which snow can fall to the earth:

  • Snow flurry (a light shower of snow that does not last long)
  • Snow squall (brief, but very intense snow storm)
  • Blizzard (lasts three or more hours with wind speeds of at least 35mph and visibility less than a quarter of a mile)
  • Snowstorm (like a blizzard, but without strong winds)
  • Thunder snow (snow storm which includes thunder)

The information on types of snow after it reaches the ground is especially important for all of those enjoying winter sports since some are more pleasant to land on.

The types of snow after it collects on the ground are:

  • Snow powder (untouched snow)
  • Crud (when snow powder has been tracked out)
  • Crust (snow powder with a layer of frozen crust on top)
  • Slush (melting snow)
  • Ice (snow that has melted and refrozen several times)

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Why are snowflakes always six sided?

The atmospheric conditions experienced by an entire crystal as it falls is what causes the snowflake to take a specific shape. The conditions of the atmosphere can change quickly, and even the slightest change in surrounding temperature and humidity causes the crystal to grow differently. However, the six-sided shape is always maintained, as all of the branches of a snowflake experience the same atmospheric conditions.

The six-fold symmetry comes from the arrangement of water molecules in the ice crystal. A water molecule has a small V shape, with the oxygen atom at the center, and hydrogen atoms on both sides. The oxygen atom has a strong attraction to the electron clouds of the two hydrogen atoms and pulls them closer. This means that two hydrogen ends are more positively charged, while the center of the molecule is more negatively charged. When water molecules “bump” to each other strong forces between the negatively charged and positively charged parts of the molecules cause them to join together in a three-dimensional pattern with a six-sided symmetry. Each new water molecule which joins the snowflake reflects this pattern until eventually a macroscopic six-sided shape is formed.

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Why isn’t all snow in the form of snowflakes?

Snow does not necessarily form as a flake, but also a ball, which is known as graupel. Under certain atmospheric conditions, descending snow crystals during their passing through the atmosphere may encounter super-cooled water droplets. These droplets can remain in a liquid state even at temperatures of -40° C.

When snow crystals and the super-cooled droplets come in contact, it results in freezing of the liquid droplets onto the surface of crystals. This process is known as accretion. Graupel forms when this process continues, and shape of the original snowflake is no longer identifiable, but it falls to the ground in the shape of a ball.

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Facts about Snow

Is snow really white?

In order to know if snow is white, we must first understand how we experience colors. The reason why we see objects as colors is because some wavelengths of light are absorbed, while others are reflected. We see an object in a color which it reflects. Snow is made up of many tiny surfaces, and when light hits it, it is scattered in many directions and will bounce from one surface to the next leaving no wavelength absorbed, thus making snow appear white to us.

However, snow does not always appear white; it can also appear blue or pink. Deep snow tends to act as a filter, absorbing most of the red light, which makes it look blue. On the other hand, snow in high alpine areas and the coastal polar regions sometimes contains cryophilic fresh-water algae which have a red pigment coloring snow and making it appear pink.

Where was the highest snowfall ever recorded?

The highest snowfall ever recorded in one year period was in Mount Rainier, Washington State, between February of 1971 and February of 1972, when 1224 inches of snow fell.

No two snowflakes are alike. Myth of fact?

In 2007 there were news stories which claimed that the adage “no two snowflakes are alike” might not be true. The answer from the scientific community is that it is very unlikely for two larger complex snowflakes to be identical in molecular structure and appearance. On the other hand, snow crystals which have a small number of molecules (e.g. 10) can be alike. However, this does not seem to mean much when a typical snowflake contains 1018 water molecules.

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Additional Resources

Wilson “Snowflake” Bentley (1865-1931) from Jericho, Vermont, was the first person to take photographs of snowflakes. He was a pioneer of “photomicrography,” the photographing of very small objects, including snowflakes.

Because snowflakes can easily melt and are very delicate to work with, Bentley connected his camera to a microscope and would stand for hours outside in the snow creating photographs that would show all the intricate details of snow crystals. Once a snowflake landed to the ground, he would pick it up with a feather, place it under the lens, and after a minute and a half of exposure, he captured the image.

From his first photograph in 1885, he went on to photograph 5000 snow crystals until his death. Throughout his life, he also published 60 articles in scientific and popular journals and worked with the physicist William J. Humphreys on a book, Snow Crystals, which contains 2300 of his photographs.

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A Complete Review For Kids: How Snow is Formed
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