Heat Exchange Process
Snowmaking is a heat exchange process. Heat is removed from snowmaking water by evaporative and convective cooling and released into the surrounding environment. This heat creates a micro-climate inside the snowmaking plume that is very different from ambient conditions. Understanding this process can lead to practical benefits to the snowmaker.

There are many variables that affect snowmaking. Three of the most important variables are wet bulb temperature, nucleation temperature and droplet size.

Wet Bulb Temperature
The temperature of a water droplet exiting a snow gun is typically between 34 F and 44 F. Once a water droplet passes the nozzle and is released into the air, its temperature falls rapidly due to expansive and convective cooling and evaporative effects. The droplet's temperature will continue to fall until equilibrium is reached. this is the wet bulb temperature and it is as important as dry bulb (ambient) temperature in predicting snowmaking success. For example, snowmaking temperatures at 28 F and 10% humidity are equivalent to those at 20 F and 90% humidity.

Nucleation Temperature
Once the wet bulb temperature is known, there must be a way to predict whether water droplets will actually freeze at that temperature.

Ice is the result of a liquid (water) becoming a solid (ice) by an event called nucleation. In order to freeze, a water droplet must first reach its nucleation temperature. There are two types of nucleation, homogeneous nucleation and heterogeneous nucleation.

Water molecules in Liquid Form		(Solid Form) Molecules Form A Hexogonal Array

Homogeneous nucleation
Homogeneous nucleation occurs in pure water in which there is no contact with any other foreign substance or surface. With homogeneous nucleation, the conversion of the liquid state to solid state is done by either lowering temperatures or by changes in pressure. However, temperature is the primary influence on the conversion of water to ice or ice to water is temperature.

In homogeneous nucleation, the nucleation begins when a very small volume of water molecules reaches the solid state. This small volume of molecules, is called the embryo and becomes the basis for further growth until all of the water is coverted. The growth process is controlled by the rate of removal of the latent heat being released. Molecules are attaching and detaching from the embryo at roughly equal and very rapid rates. As more molecules attach to the embryo, energy is released causing the temperature of the attached molecules to be lower than the temperature of the unattached molecules. The growth rate continues until all the molecules are attached. At this point, you have the solid state (ice).

Most of us think that pure water freezes at 0 C or 32 F. In fact, the nucleation event (freezing) for pure water will take place as low as minus 40 C or minus 40 F. This is most likely to occur in laboratory experiments or high in the upper atmosphere (upper troposophere).

Heterogeneous nucleation
Hetergeneous nucleation occurs when ice forms at temperatures above minus 40 C or minus 40 F due to the presence of a foreign material in the water. This foreign material acts as the embryo and grows more rapidly than embryos of pure water. The location at which an ice embryo is formed is called an ice-nucleating site. As with homogeneous nucleation, heterogeneous nucleation is governed by two major factors: the free energy change involved in forming the embryo and the dynamics of fluctuating embryo growth. In heterogeneous nucleation, the configuration of molecules and energy of interaction at the nucleating site become the dominating influence in the conversion of water to ice.

Snowmaking involves the process of heterogeneous nucleation. There are many materials and substances which act as nucleators; each one promotes freezing at a specific temperature or nucleation temperature. These nucleators are generally categorized as high-temperature (i.e., soliver iodide, dry ice and ice nucleating proteins) or low-temperature (i.e., calcium, magnesium, dust and silt) nucleators. It is the low-temperature nucleators that are found in large numbers in untreated snowmaking water. The nucleation temperature of snowmaking water is between 15 F and 20F.

Why do you hear freezing warnings at temperatures around 32 F? The answer is that near factor is coming into play with the freezing process. That factor is called surface (i.e., roads, highways, trees). There is an energy interaction between the ice-nucleating site in the water with the surface. This causes the water droplets to freeze very near 32 F or 0 C.

In snowmaking it is the nucleator having the highest nucleation temperature that determines when a water droplet will freeze. Research has demonstrated that 95% of natural, untreated water droplets will freeze at widely different temperatures. The average temperature being 18.2 F. Introduing a consistent high-temperature nucleator into the water will raise the freezing point. With Snomax, the freezing point rises to 26.8 F.

As a water droplet cools, heat energy is released into the atmosphere at a rate of one calorie per gram of water. As it freezes into an ice crystal, the water droplet will release additional energy at a rate of 80 calories per gram of water. This quick release of energy raises the water droplet temperature to 32 F, where it will remain while freezing continues. This is one reason why we are accustomed to thinking that water freezes at 32 F. The water will continue to freeze as long as it remains at or below 32 F, but only after it has first cooled to its nucleation temperature. Any excess energy will be dissipated into the atmosphere.

Droplet Size
Since the distribution of various nucleators in a given volume of water is totally random, the size of the water droplet or the number of high-temperature nucleators has a significant effect on the temperature at which freezing occurs (nucleation temperature). In natural water, as the size of the water droplet decreases, the likeihood that the droplet will contain a high-temperature nucleator also decreases. Conversely, larger water droplets stand a better chance of containing high-temperature nucleators. The optimum situation for snowmakers is one in which every droplet of water passing through the snow gun nozzle contains at least one high-temperature nucleator and freezes in the plume.

Numbers in the droplets represent various ice-nucleating sites with different nucleation temperatures. The highest number nucleator in the droplet will determine at what temperature the water droplet will freeze.

The relationship between the variables of nucleation temperature and droplet size is summarized in two statistically valid conclusions. First, a 50% increase in the droplet size results in a one-degree F increase in nucleation temperature. Second, a 50% decrease in droplet size results in a three-degree F decrease in nucleation temperature. These conclusions are based on an average droplet size of 300 microns, and indicate that decreasing the droplet size can be counter-productive to promoting high-temperature nucleation, unless enough high-temperature nucleators are present.

Looking at the relationship between droplet size and evaporation, research in cloud seeding shows that:

• A 50% decrease in droplet size produces a four-fold increase in the evaporation rate.
• A droplet that is 50% smaller will evaporate to nothing after falling just one-eighth the distance that the average 300 micron droplet falls.

These conclusions further point out the undesireable results from using very small droplet, especially in areas where water loss is a critical issue.

Relating droplets size to nucleation temperature, it is possible to increase snowmaking production and efficiency by using high-temperature nucleators with larger water droplets. This method frequently allows for increased water flow, reduces evaporation, and yields more snow on the ground. In fact, studies indicate that a 20% increase in water flow can increase snow volume up to 40% if droplet size and nucleation temperature areoptimized.

In conclusion, a better understanding of the dynamics between wet bulb temperatures, nucleation temperature, and droplet size, together with a practical application of the science involved, can help improve the efficiency of the snow manufacturing process.