Simple Snow Part 1

Basic Snowmaking

Yaroslav Stanchak (revised 2/26/09)

The purpose of this article is to review the snowmaking process that everyone involved in the ski industry is familiar with and to add some additional insights that may be of benefit to knowledgeable snowmakers. For the newly minted snowmaking people this will be an introduction to an essentially simple process that has increased in complexity over the years. The following descriptions are simplified to minimize the haze produced by complex mathematical functions and technical jargon.

The basic snowmaking process essentially requires pressurized water, which is broken into a fine spray by mechanical means and ejected into the surrounding atmosphere. The spray of water particles is mixed with cold air. In order for an ice particle to form a nucleation point is required - normally a dust particle or very small ice crystal. These small water particles are then transformed into ice particles through heat conduction, evaporation and sublimation if the minimum thermodynamic thresholds of the snowmaking variables are met. When a water particle is being transformed into an ice crystal its temperature stabilizes at 32 F throughout the freezing process irrespective of the surrounding temperature. The practical maximum temperature limits for manmade snowmaking range from 28 F to 36 F on the high end and are very dependent on relative humidity values. The temperature and relative humidity values required for snowmaking caused much confusion to early snowmakers until the adoption of the wet bulb temperature methodology. This methodology, while well known in the heating/cooling and weather industries, generally remained unrecognized within the ski industry until the late 1970's and early 1980's when knowledgeable snowmakers and the bigger snowmaking areas slowly adopted it. The wet bulb temperature allowed the confusing combination of multiple temperatures and relative humidities to be consolidated into a logical and technical numerical value for temperature. This term also allowed a reasonably consistent and repeatable means to describe overall snowmaking equipment and system performance with respect to various snowmaking temperatures. It created a common and understandable language framework for the snowmaking industry.

Delving deeper into the snowmaking thermodynamics, it is essential that the spray of water particles have an adequate airborne time period to at least partially freeze. This "hang time" is accomplished in a number of familiar ways. Air-water and electric fan snow guns project the water particles in a high arc allowing time for the freezing process. The low air tower snow guns emit the water spray from a 15 ft to 35 ft. height. Of course, there are many variations and combinations of the above - yet the one common element in all the methods is to extend for as long as possible the water particle "hang time". During this period nucleation, evaporation, sublimation and heat conduction all play their key roles in the formation of small ice particles. If the amount of water spray particles in a given snowmaking area is too great (rich mixture), then the necessary cooling for ice formation will not be adequate and the resultant snow mixture becoming too wet/damp. The snow particles under this condition tend to become larger. On the other hand, if the amount of water is too little (lean mixture) the overabundance of cooling then will form small or fine snow particles with the resultant snow too dry for good adhesion to deposited snow or ground surface. The above examples point out that there are many collisions between water particles and newly formed ice particles - more collisions equate to larger snow particles and fewer collisions smaller snow particles.

The above leads to examining the influence of water and snow particle size on snowmaking performance. The normal measurement ruler for particles is the micron - defined as 0.001 millimeters or 0.00004 inches. The size diameter distribution of water spray particles varies from 100 microns (0.004in.) to perhaps 1200 microns (0.048 in.) for most snowmaking applications, with the majority of useful normal snow particles ranging from about 500 microns to 1000 microns. The smaller snow particles give a fine and smooth textured snow, while the larger snow particles yield a grainier textured snow. There is a direct correlation between particle size and required "hang time". Smaller water particles will freeze much more rapidly than the larger particles and therefore need less hang time. This is particularly evident during marginal temperature snowmaking. However, there can be too much of a good thing - snow particle sizes below a certain diameter threshhold are much more susceptible to wind and drifting. Under some conditions, these very small snow particles will create a fog. More on this below.

Under normal snowmaking conditions as the ambient temperature drops at a steady rate the overall cooling affect increases steadily. For a given piece of snowmaking equipment set to a constant water flow the snow particles will decrease in size as the temperature drops. An easily observable fact! As the snow particles become smaller the amount of very fine particles increases dramatically (a very lean mixture). At this point the overabundance of cooling and small snow particle formation become a snowmaking liability. Under cold and very cold temperature conditions the water particles freeze almost instantly. If the snowmaking equipment produces very small water particles there will be very few collisions between water and ice particles and it is possible that the majority of formed snow particles will become too small to land on the ground anywhere near the desired trail. The "hang time" becomes too great for practical use below a minimum snow crystal diameter. Adding appreciable wind to this condition exacerbates the drift and fog. It is intuitively obvious that adding more water to the mixture (richen mixture), and thereby increasing the snow particle size, will moderate this circumstance under most conditions. In essence the snowmaking process, as described above in simplified form, points out the interplay of the cooling effect from the ambient temperature and relative humidity in combination with snow crystal size and required drop rate (hang time). The process from a technical view is relatively complex and for the reader enticed into further discovery about snowmaking and the essential thermodynamics a beginning engineering text on this subject can be obtained at any university library.

Other references are; 'On the Feasibility of Generating and Storing Winter Ice to Meet Water Demands in the Summer' Thesis by Moshe Alamaro 1999 MIT; "The History and Technology of Man-Made Snow in Winter Recreation Areas" paper by William A. Walsh, Jr., PE. 1972. Careful research on the Internet will also produce a wealth of information.