THE TECHNIQUE OF SNOWMAKING
The basic objective of snowmaking is to atomize water into droplets, blow the droplets up into the air, allowing them to substantially freeze before they contact the ground (hopefully on the ski trail). The smaller the droplets, the faster they will freeze. Therefor on a warm day you will be running the gun leaner, producing finer droplets in order to make snow.
The atmospheric factors affecting snowmaking are (in order of importance):
Wind Direction (relative to the gun)
Temperature, relative humidity, and altitude can be combined into a single factor called “wet-bulb temperature”. This is the temperature a wet thermometer will cool to if air is blown over it – a condition similar to what the water droplets are being exposed to during snowmaking. When the air temperature is near freezing, the wet-bulb temperature becomes very important. In fact with the right equipment, snowmaking can take place at 40 degrees Fahrenheit (+4.4 degrees celsius) if the relative humidity is low enough to yield a wet bulb temperature below 32 degrees Fahrenheit (0 degrees celsius). This may seem a bit bizarre, but the weather in the Western US frequently allows this. On one day I personally made snow at 50 degrees Fahrenheit in Utah. The atmosphere was extremely dry and so was the snow.
The key to efficiency is the air to water ratio at a given wet-bulb temperature. The best way to predict snowmaking weather is to watch the dew point upwind if the surrounding terrain is frozen. The dew point is that temperature at which water will start to condense out of the air. When the terrain upwind of you is frozen (including any body of water), or unless it gets warm enough to melt the snow, the dew point of a large air mass tends to be fairly constant throughout. This means that if you are making snow at night, when the sun comes up in the morning, the temperature rises and the relative humidity drops. The wet bulb temperature goes up slowly and the dew point is constant throughout the day and night. You can often continue to make snow without change unless the temperature rise is too great. The real change in snowmaking weather is when a new air mass moves through and the dew point suddenly changes. Of course if the temperature goes up enough to melt the surrounding terrain, the dew point will also go up and snowmaking conditions will deteriorate.
Don’t try to fight the wind – you’ll lose. Aim the gun with the wind. You’ll be able to add more water to the gun and make more, dryer snow. If necessary, add an extra length of hose to get the snow where you want it. Start making snow with the hose fully extended, periodically pulling the gun backwards in a zigzag fashion during the night/day until you are finished. this will cause the snow to be layered and evenly distributed. If you do make wet snow it will be covered with dry snow each time you pull the gun back, leaving a perfect surface when you’re done. If there is no wind, or if the wind is blowing in line with the trail, place the gun in the center of the trail and aim the gun up or down the trail with whatever wind there is. This will allow you to build broad smooth mounds of snow in the middle of the trail where it belongs. The skiers will spread the snow out to the edges for you.
Anticipate the wind characteristics of your mountain! Does the wind direction tend to change as the sun sets? If you can predict where the wind will go you will save yourself a lot of work. Watch the wind when testing snow quality as it falls on your arm. A sudden gust can change the apparent snow quality from one second to the next. In very warm weather or windy conditions the only valid measure of snow quality is to test what has actually fallen on the ground. Grab a handful of snow and squeeze it to see how much water there is in it.
Provide rain gear to snowmakers – it pays! To maximize a gun it should be adjusted wet and then carefully throttled back until the desired snow is produced. It is not fun to be soaked to the bone on a cold night. If snowmakers are not wearing rain gear, they aren’t going to want to test if the snow is wet, therefor they are likely to err on the dry side – that is a lot more expensive than rain gear.
Aim the gun up at about a 35 degree angle. This allows you to put more water to the gun and more than compensates for any extra loss off-trail. It often appears that more snow is lost off-trail than actually is. During atomization of the water, as each main droplet is formed there are also two very small droplets formed due to a harmonic when the main droplet breaks off. These tiny droplets tend to be reflective and noticeable although they are only a very small portion of the snow mass produced. Frequently they are carried upwards by convection and disappear altogether. Fortunately they are not significant, the vast majority of the snow is falling near the gun.
Try to let the snow set up before grooming if possible. Man-made snow is dense enough, it doesn’t need to be compacted. The first day is the most important. Ice inside the snow mass is continually sublimating (evaporating directly from a solid) and recrystallizing into a bonded mass. Grooming too soon will compress the snow, squeezing out air pockets if the snow is not allowed to set up first. This is where layering the snow with a dry layer on top can help reduce the need for grooming.
When trying to cover ice, start by making wet snow then cover it with dryer snow. This bonds the snow to the ice and builds a durable base that won’t be worn off as quickly. You should also make damp snow when the wind is blowing hard enough to drift dry snow. If there is natural snow drifting, you can trap it on the trail by making damp snow, producing large piles.
Small output snowguns tend to be more efficient than large snowguns of similar design. This is most obvious in warm weather when the water to a large gun has to be throttled way back due to the limited cooling effect of the air within its plume. This is one of the reasons the Whispergun™ has a wide pattern – to maximize cooling.
Large systems should be designed to run a gun on every hydrant of a given trail simultaneously. This allows you to cover a complete trail overnight efficiently without continuously moving guns. This also makes smaller guns more practical.
Tower mounted guns can be more efficient than guns on the ground – if they are not too close to a tree line. The greater height allows more time for the droplets to freeze, therefor more water can be put to the gun and a better air/water ratio obtained. The first 10-12’ (3-4 meters) is by far the most important for cooling purposes. You can gain about 25% on the air/water ratio with short towers. Taller towers are only necessary if the output of a gun is poorly nucleated. You also lose more snow off trail with tall towers when the wind is adverse.
Placement of the towers is important. If a tower is poorly placed, the snow lost into the woods will be greater than the gain in air/water ratio. Some tower designs are so poorly nucleated that they need to be significantly higher than 10-12’ (3-4 meters) in order to work at all. You may need to incur the cost of injecting a nucleating agent like Snomax™ to make them work at any temperature above your water’s natural nucleation temperature.
Whisperguns™ do not require a nucleating agent, making better snow at lower cost. Test your water to find out what its natural nucleation temperature is. Natural bacteria, clay particles, spores, and other particles have a significant impact on the temperature your water is likely to freeze at quickly enough for snowmaking. The test should NOT be done indoors during the winter as indoor humidity is consistently low during the winter. It should be done outdoors in mild weather under both high humidity and low humidity conditions.
We have found that most nuclei work better in low humidity conditions and often not at all in high humidity conditions. This is where a self-nucleated gun like a Whispergun™ excels. When you do the test, pay particular attention to the amount of time it takes for identical sized droplets to freeze at a constant temperature, as this is the parameter affected by humidity. In snowmaking you need those droplets to freeze quickly. This means that when you test your water with a commercial nucleating agent, you may find, for example, that you do not want to inject the agent during the night when the humidity is high, but instead inject just at sunrise when temperatures start to rise (and the relative humidity starts to fall). You could save real money by paying attention to the humidity. There is more about nucleation in the NUCLEATION section.
Have an extra set of guns. Frequently this improves system efficiency by 25% if a ski area desires to change trails frequently. The second set of guns can be set up in advance of a trail change, greatly reducing downtime while you change trails.
A good air-to-air aftercooler/demister for the compressed air is worth its weight in gold. The dew point of the compressed air should be reduced to within a few degrees of the outside air to minimize rime ice within the air hoses. Changing hoses is not fun and very inefficient in the long run. Choose an aftercooler with at least twice the capacity you think you need.
Fergus S. Smith