
Snowy 1.0 by vectoroom is available for your downloading pleasure today. Put up on brusheezy just a few short days ago — Dec. 15, 2009 … it already has 7028 downloads. If you are a mind to create some snowy looking stuff these days … then by all means give it a download.
How is snow made? I mean real snow!
from this article.
High up in the sky clouds begin to form. Clouds are made of water vapour. The temperature is cooler up in the clouds than it is on the ground as a general rule. However, the temperature is not constant throughout the clouds. There are different layers or zones of temperature. These different temperature zones play an important role in the forming of a snow flake or crystal.
The water vapour in the clouds is being blown around by winds through the different temperature zones. Typically liquid water will freeze at approximately 0°C. However, for water to become solid (freeze) the water molecule (H2O) needs to slow down and to touch something solid. By touching something solid and slowing down the water molecule can, under normal circumstances transition from a vapour/gas to a liquid to a solid, or from a vapour to a solid directly. But up in the cloud there is mostly water vapour (very few solids) and lots of movement. So the water molecules continue to cool while still in a vapour form. This is called super-cooling.
When a water molecule is in a super-cooled state it can go from a vapour/gas to a solid quickly and directly. So what it mostly needs now is something solid. The “something solid” is called a nucleator. Most commonly the nucleators found in clouds are dust. The nucleators are very, very small. Small enough that the winds can lift them high into the sky. Small enough that you can’t see one with the naked eye. The job of the nucleator is to start the snow crystal. Once the crystal is started then other water molecules can join onto this crystal.
Eventually after blowing around for a period of time a water molecule will join with a nucleator. The process of building a crystal has started. Other water vapour molecules in their super-cooled state will find this crystal. Some of these molecules will join the crystal and some will not. The process of building a crystal is slow.
As the crystal builds the base shape is a flat six-sided (hexagon) crystal. This is a three dimensional (3-D) hexagon. The flat parts of the six sides/edges are called prism faces. The other two sides are called basal faces. In total there are 8 faces (top basal, bottom basal, and prism faces 1 through 6) The base hexagonal shape is a result of how the water molecules connect together. The faces of the crystal are the smoothest parts and thus the most difficult parts for additional molecules to attach to. It’s much easier to attach to a rough surface or at an edge. As it happens the crystal has 18 edges: 6 bordering the top basal face, 6 bordering the bottom basal face and 6 where the each prism face meets another prism face. So crystal growth will occur the fastest at the 18 edges and slowest on the 8 faces. The growth faces are often refered to a facets.
The crystal faces will grow evenly so that they always remain smooth (it’s a property). The faces will not really increase in surface area because on the faces the new molecules will be laid down something similar to a sheet. This arrangement is called a crystalline lattice.
The growth at the edges happens differently than with the faces. These growths are refered to as “dendrites”. Commonly these are often refered to as “snowflake arms”. But we’ll always refer to these grows as dendrites.
How a crystal grows is determined mostly by the amount of water available (the super-saturation level of ice relative to water in air; we’ll call this the “SSL(I)” for personal typing sanity) and the temperature with an influence by the atmospheric pressure and (it seems) possibly the electrical charge. Once the crystal starts it is still subjected to the same forces that the water vapour molecules are subjected to. The crystal is blown around throughout the cloud through a variety of different temperature zones. The crystal can continue to be blown about for tens of minutes to hours before falling to the ground. At different temperatures fresh water molecules will attach to the crystal at different locations. In other words the crystal will “grow” differently at different temperatures. Since the crystal is being subjected to a variety of temperatures and humidities as it is blown about, the crystal will grow differently at different times.
SSL(I) in a cloud is strongly tied to the saturation level of water, “SL(W)”. Usually when the threshold of SL(W) is met this marks the upper level for SSL(I). This threshold is low at 0°C and rises as the temperature drops. The thresehold is not a perfectly linear releationship. So at -10°C the SSL(I) is about 8% while at -25°C the SSL(I) is about 25%. At 0°C the SSL(I) is 0% and at -35°Cthe SSL(I) is about 40%. But the threshold can be exceeded because of the nature of what is happening in the clouds. Remember all of the blowing is moving the crystals around. This effectively raises the limit of the SSL(I). Exceeding the threshold opens the opportunity for a greater variety of crystal growth types. Above the threshold growth types have the opportunity to vary from growth types below the threshold. This allows for a greater variety of crystal end shapes.
Some of the crystal growths go by the names plate dendrites, needles, hollow, plates, thick plates and sector plates. There are more types we won’t bother listing. There are textbooks that you can reference if you want to delve more into the growth types. But for our end goal of understanding for skiing it’s not critical. These various growth occur as the snow crystal is blown through different temperatures and different SSL(I)’s. This growth can happen over minutes or hours before it begins to fall to the ground. While falling crystals can bump into each other damaging each other and/or joining to make a larger snow flake.
The snow flakes are now on the ground piling on top of each other. The weight of the upper snow flakes will press down on those underneath packing the snow. With all of the crystal shapes initially the snow pack contains a lot of air. Over time the snow pack will compress making it denser. There is a myth that 10cm of snow equals 1cm of water when it is freshly fallen. The truth is that the density of freshly fallen snow can vary widely depending on the type of snow crystals that have fallen and the air temperature at ground level. The density can range from as high as 5:1 to a as low as 20:1 with the same snowfall due to wind, ground-air temperature and ground heating.
Source: skiwax.ca/tp/snow.php — you can catch the rest of this article on how they make man made snow as well.