At first sight, a snowflake is like another one. And even if we want to study them a day of snowfall, it’s quite hard to do it holding one in our hands, seeing at which speed they melt. Even though, an American discovered a technique to observe them and take a photo of them. Wilson Bentley is the first man that success in studying them using black velvet to take pictures before melting of the snowflake. Don’t hesitate to consult its work, it’s wonderful!
Through its photographs, he successes to catch the beauty of snowflakes and their distinctive features. Its work also highlighted the existence of different shapes of snowflakes and the wide variety that exist.
Three main shapes of snowflakes can be observed: plates, stellar crystals and needles or columns. According to the International Association of Cryospheric Sciences (IACS), there are more categories than that: needle, column, plate, column and plate combination, side plane, rime, irregular and germ. Even though these major categories have been established, it mustn’t be forgot that a multitude of shapes and aspects exists.
Globally, each snowflake is classified according to one of these categories. And each snowflake has a unique structure. However, they all share a common point, they all have a centre with a hexagonal structure. It was also highlighted that the shape of snowflakes depends on climate conditions like temperature and humidity.
How these snowflakes are formed?
To obtain snow and snowflakes, altitude clouds are needed, and they require some conditions: the temperature in the cloud must be negative and the atmosphere must contain water vapor and micro dust.
The formation of snowflakes begins when water steam condenses until it formed some hexagonal ice crystals. Globally, the birth of a snowflake can be resumed as water molecules that get stick on a seed of dust.
As a function of cloud’s conditions, like temperature and humidity, branches of a snowflake grow thinner or larger and more or less importantly. The last step is the one that makes the snowflake passing from the cloud to make it falls. This step depends on the weight of the snowflake. Indeed, at the beginning, a snowflake is around few micrometres and during its evolution it gets bigger and is going to measure some millimetres. This will lead to its fall and it snows.
Despite the differences between each snowflake and the categories of families that exists, they always have a hexagonal base. This structure can be explained by the way water molecules grip together to form a crystal. The water molecule is made of one atom of oxygen and two of hydrogens, so the molecule has a bent geometry. Between them, molecules are going to interact and form hydrogen bonds. Molecules are going to arrange between them to form a network, leading to a hexagonal structure.
What are the different shapes of snowflakes according to climate conditions?
According to different conditions, and to the various families of shapes that have been established, standard conditions can be established to find some snowflakes families.
- From 0 to -4°C, snowflakes quite flat like plates, can be observed
- From -4 to -6°C, needles are observed
- From -6 to -10°C, the corresponding shape is column
- From -10 to -10°C, a shape of star is observed
- Dendrites are obtained at temperatures between -12 and -16°C.
To simplify things, the Japanese physicist Ukishiro Nakaya developed a crystal morphology diagram for snowflakes. As a function of the temperature and humidity, this graph shows which shape of snowflake is obtained. It is also him that is credited with the creation of the first artificial snowflake.
Even if two snowflakes have a chance very unlikely to be similar, some ground rules have been established according observations:
- The more it gets colder; branches of snowflakes will be longer.
- During “warmer” temperatures, branches will get thicker, without being longer.
- At temperatures closer to 0°C, snowflakes are quite humid and get stick together easily, making groups of snowflakes quite large.
- Crystals that pass through cold air flow are harder and don’t stick together.
Once they have fallen, the structure of a snowflake can vary, in function of ground’s temperature. In this case, it is quite difficult to know how this structure behave and if it has an influence on the quality of the snow.
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