Snow: How its made

Atmospheric Process

The marvel wonder we all enjoy – snow. It starts with a snowstorm, and in order for that to happen we need atmospheric lift. Atmospheric lift in simple terms is anything that causes warm air to rise which forms clouds. There are several processes that can make this occur, but essentially it occurs when two air masses collide.

Nucleate

Whatever the process, when water vapor rises it cools, and eventually transforms back in to a liquid. Liquid needs something to condense on to, or a nucleus. Microscopic debris in the atmosphere in the form of dust, dirt, salt, pollen and various other particles. This is the ingredient necessary for droplet formation.^[1]Christner, Brent Q.; Morris, Cindy E.; Foreman, Christine M.; Cai, Rongman; Sands, David C. (2007). Ubiquity of Biological Ice Nucleators in Snowfall. Science 319 (5867): 1214

Crystallization

Water vapor in the clouds eventually cools as it reaches high in the atmosphere, and droplets typically freeze once they drop below 14° F in the supersaturated environment. These droplets solidify in to tiny ice particles which then begin to collect surrounding water vapor and water droplets to their surface. The droplet then grows by diffusion of water molecules in the air, or vapor, onto the ice crystal surface where they are collected. Water droplets are more numerous than the ice crystals due to their sheer abundance, so the crystals are able to grow by a process known as the Wegner-Bergeron-Findeison process (better known as the Wegener–Bergeron–Findeisen process (after Alfred Wegener, Tor Bergeron and W. Findeisen), (or "cold-rain process") is a process of ice crystal growth that occurs in mixed phase clouds (containing a mixture of supercooled water and ice) in regions where the ambient vapor pressure falls between the saturation vapor pressure over water and the lower saturation vapor pressure over ice." class="glossaryLink " target="_blank">Bergeron Process).^[2]Kenneth G. Libbrecht (2006-09-11). Guide to Snowflakes. California Institute of Technology. Retrieved 2009-06-28

Snow crystals begin to form as more ice crystals are attracted to the surface, eventually amassing enough The force created by gravity acting on a mass." class="glossaryLink " target="_blank">weight to fall through the cloud. By the time the crystal formation drop through the cloud base they have developed in to intricate lattice formations that we have come to know as snowflakes.

If the air temperature remains at freezing from the cloud base to the ground, the snowflakes keep their form and accumulate as ground snow. Usually though, many transformations happen as crystal form falls from the clouds. Often melting and refreezing and changing form. Each ice crystal has a unique shape that is dictated by air temperature and moisture content.^[3]Bohannon, John. “ScienceShot: The True Shape of Snowflakes“. ScienceNOW. American Association for the Advancement of Science. Retrieved 12 April 2013.

Crystal Shape

These ice crystals that make up snowflakes are symmetrically patterned, because they reflect the internal order of the crystal’s water molecules as they arrange themselves in predetermined spaces, known as crystallization, to form a six-sided snowflake.

Ultimately, it is the temperature at which a crystal forms, and to a lesser extent the humidity of the air that determines the basic shape of the ice crystal. We see long needle-like crystals at 23° F and very flat plate-like crystals at 5° F.

The intricate shape of a single arm of the snowflake is determined by the atmospheric conditions experienced by entire ice crystal as it falls. A crystal might begin to grow arms in one manner, and then minutes or even seconds later, slight changes in the surrounding temperature or humidity causes the crystal to grow in another way. Although the six-sided shape is always maintained, the ice crystal, and its six arms, may branch off in new directions. Because each arm experiences the same atmospheric conditions, the arms look identical.

Morphology

The shape of the snowflake is determined by the temperature and humidity at which it is formed.^[4]M. Klesius (2007). The Mystery of Snowflakes. National Geographic 211 (1): 20. ISSN 0027-9358.  Planar crystals (thin and flat) grow in air between 0 °C (32 °F) and −3 °C (27 °F). Between −3 °C (27 °F) and −8 °C (18 °F), the crystals will form needles or hollow columns or prisms (long thin pencil-like shapes). From −8 °C (18 °F) to −22 °C (−8 °F) the shape reverts to plate-like, often with branched or dendritic features. At temperatures below −22 °C (−8 °F), the crystal development becomes column-like, although many more complex growth patterns also form such as side-planes, bullet-rosettes and also planar types depending on the conditions and ice nuclei.^[5]Matthew Bailey; John Hallett (2004). Growth rates and habits of ice crystals between −20 and −70C. Journal of the Atmospheric Sciences 61 (5): 514^[6]Kenneth G. Libbrecht (2006-10-23). A Snowflake Primer. California Institute of Technology. Retrieved 2009-06-28.^[7]Kenneth G. Libbrecht (January–February 2007). The Formation of Snow Crystals. American Scientist 95 (1): 52–59.

A snowflake consists of roughly 10¹⁹ water molecules (that’s billions of billions for those of us that struggle with exponents), which are added to its core at different rates and in different patterns, depending on the changing temperature and humidity within the atmosphere that the snowflake falls through on its way to the ground. As a result, it is extremely difficult to encounter two identical snowflakes.^[8]John Roach (2007-02-13). No Two Snowflakes the Same Likely True, Research Reveals”. National Geographic News. Retrieved 2009-07-14.^[9]Jon Nelson (2008-09-26). Origin of diversity in falling snow. Atmospheric Chemistry and Physics. Retrieved2011-08-30.^[10]Kenneth Libbrecht (Winter 2004–2005). Snowflake Science. American Educator. Retrieved 2009-07-14.

Did you know?

• If a falling crystal collides with a water vapor as it falls, the water vapor instantly freezes to the surface in a super-cooling effect process known as riming. These appear as tiny dots on the snowflake surface. These droplets may accumulate by the thousands to the point that the crystal surface is completely erased hiding it’s shape. This is better known as a snow pellet or graupel.

• The crystal changes, although not completely understood, are linked to the H₂0 clusters on the surface. Part is chemistry, and to a greater extent it is the changing environment of temperature and humidity. Snow crystals are encoded within their very structure everywhere they have been.

• 

  Snowflakes always have six points or sides due to the hexagon shape. The hexagon shape is formed because the H₂0 molecules chemically bond into a hexagonal network of lattice. They hydrogen and oxygen arrangement is illustrated.

• The old adage “No two snowflakes are alike” can be true for crystals that complete the entire process shown above. However, this can be false if the crystals fall through the clouds in the early stages of formation such as needles, columns, hollow columns, and plates.

• The world’s largest ever snowflake to date found was 38 centimeters wide and 20 centimeters thick. This snowflake occurred at Fort Keogh, Montana, USA on 28 January 1887.

• The average snowflake falls at 3.1 miles per hour.

• Scientists have tried classifying snowflakes for centuries. In 1951, the International Association of Cryospheric Sciences classified snowflakes into 10 basic shapes. The IACS^[11]http://www.cryosphericsciences.org/snowClassification.html categorize snowflakes as plates, stellar crystals, columns, needles, spatial dendrite is a crystal that develops with a typical multi-branching tree-like form.
  
   " class="glossaryLink " target="_blank">dendrites, capped columns, irregular particles, graupel (soft hail), sleet, and hail. Among the 10 basic categories are subgroups of snowflakes.

• Snow actually isn’t white. Snow is actually clear and colorless.“The complex structure of snow crystals results in countless tiny surfaces from which visible light is efficiently reflected. What little sunlight is absorbed by snow is absorbed uniformly over the wavelengths of visible light thus giving snow its white appearance.^[12]National Snow and Ice Data Center

• The microscopic particles necessary for crystal formation can come from mineral dust, volcanic ash, pollen, smoke particles from wild and grassland fires, fungal material, bacterial cells and even pollution particles from your car exhaust. The particles aren’t localized, since they are in the atmosphere they can come any number of sources; the Gobi Dessert, Mumbai, Mongolia, or the Middle East. For instance, in Ulaanbaatar, Mongolia annual seasonal average particulate matter concentrations have been recorded as high as 279 µg/m³ (micrograms per cubic meter). To put this in perspective, the World Health Organization’s recommended PM₁₀ level is 20 µg/m^{3 .} A good majority of particulate comes from this region.^[13](2010)Mongolia: Air Pollution in Ulaanbaatar – Initial Assessment of Current Situations and Effects of Abatement Measures. The World Bank.

• The most snow ever recorded in a 24 hour period in the USA occurred at Silver Lake, Colorado in 1921 and was 76 inches of snow in a 24 hour period.^[14]J. L. H. Paulhus (1953) Record Snowfall A snowfall of 87 inches in of April 14-15,1921, at Silver Lake, Colorado, http://docs.lib.noaa.gov/rescue/mwr/081/mwr-081-02-0038.pdf, accessed Dec 1, 2014 The second highest was recorded in Georgetown, Colorado on December 4, 1913 with 63 inches in 24 hours.

• If you have a fear of snow, it is called chionophobia.  If you are crazy about snow, it is chionomainia.

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