How Snow is Made

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This page covers how snow is created. By understanding how (and why) snow is created this should help with understanding how skis and wax interact with snow to grip and glide. This page will not discuss how glide or grip wax works.

First we will dicuss how natural snow is created. Then we will discuss how man-made or artificial is made.

The Preparation For Natural Snow

This section is about what nature does that prepares all of the elements for the making of snow.

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.

Machine or Man-made Snow Making

Machine or man-made snow is done with the clouds taken out of the equation. But what is needed to make snow is pretty much the same. There is still the need for water, cold temperatures, a nucleator and time to form the crystal. So, how does snow making machine work?

There are essentially two types of machines: the pressurised gun and the blower. Before the water reaches these machines the same things need to happen. At the machine things vary a little bit.

Water is the, of course, the key component to making snow. And since the water will need to freeze to make the snow flakes the colder it is the better. Most locations have a pond. Being outside the water does get colder. But frozen water isn't much good. So the water is circulated to slow down or prevent the formation of ice. This also helps make it a little bit colder. And colder means better for making snow.

But cold water won't be enough to make really good snow. You still need that nucleator. Way up in the clouds tiny dust particles serve as the nucleator. With ground water a nucleator gets mixed in. Often the nucleator is a product called SnoMax. Some locations don't mix a nucleator into the water. They rely on water droplets freezing to become the nucleators. With a nucleator already in the water snow can be made at warmer temperatures than without.

With a snow gun the water comes out of an attachment that looks like an oversized garden hose sprayer. This sprays the water to break it into small droplets and atomize it as much as possible. The small droplets are needed so that the water cools fast and starts to form crystals instead of ice pellets. The guns are aimed up toward the sky to maximize the amount of time the water has to cool and form snow crystals. Remember in a cloud the crystals are swirled around for minutes to hours. With a snow gun the crystal must form within tens of seconds. With less time to form the snow crystals don't grow as large as cloud make snow crystals.

With a snow gun pressurized air is also used to help in the spraying and atomizing of the water. This gives the water more time to freeze and form crystals because it it shot higher into the air. A side-effect of using pressurized is that the atomized water gets a cooling boost. As pressurized air expands at the mouth of the gun it cools the water and air around it.

The other type of snow making machine is a blower model. It has a very large fan. Often the fan is 1m or larger in diameter. The fan is responsible for blowing the water high into the air. Again this is to maximize the time for cooling and crystal forming. While the snow gun has a single point of spray the blower has many around and across the fan. Once the snow is airborne with this model there are tens of seconds to form crystals.

The machine made snow doesn't have the time to form large snow crystals or flakes. This makes the snow pack of a higher density. So it isn't as soft as cloud-made snow. But over time cloud-made snow will age and take on a form similar to the machine-made snow. A difference between the aged cloud- and fresh machine-made snow is the machine made snow crystals will have sharper more defined structure. This will affect your glide and how grip wax interacts with it.

At many ski locations the machine made snow is mixed with the cloud made snow to bulk up the snow base. This allows for the ski season to start earlier and last longer. It also helps the trails survive warm spells and chinooks. It may not be the prettiest snow. But it's helpful and better than no snow at all!

Why is Snow White?

Snow being white is really more of a question about the properties of light than it is snow.

Light coming from the sun as humans see it appears white. This white light is really a composition of many different colours of light (red, yellow, etc.). If this white light is passed through a prism the colours will break apart and can then be seen by the human eye. It will look like a little rainbow.

Snow crystals, particularly in a snow pack, have a lot of flat reflective surfaces (the faces or facets). These surfaces are oriented in a multitude of directions. Regardless of the position you observe a snow pack a large number of facets are reflecting the white light at your eyes. So since white light hits the snow, white light is reflected. Hence, snow appears white.

Well, some of you no doubt are thinking "snow is a little bit of ice and when I look at a big bit of ice it isn't white". The quick answer is the big bit of ice isn't reflective enough. The longer answer involves why objects appear other colours. When white light strikes a tree leaf it appears green to your eye. What is happening is that the leaf is absorbing all of the colours except green. The leaf reflects the green light. So your eye sees a green leaf. The process is the same for a red rose, but for the colour red. No light absorbed means everything is reflected which means you see white. The snow crystals are small enough and the facets are smooth enough for the snow pack to be highly reflective.

There is some small absorbtion of red light by snow. But it is so small, and your eyes not sensitive enough to detect it, that the snow appears 100% white. As a snow pack reaches higher densities the absorbsion of red light is greater. A blue tinge will be more visible because not enough red is being reflected to maintain the light as white. You will see blue because the colour composition of light is now made up of more blue than red and blue stands out the most. However, the density will be high enough that technically it's no longer a snow pack. It would be a glacier or ice pack.



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