Without nucleation, a water droplet would never freeze before it hit the ground. Snowmaking requires crystallization of water droplets in the air before they hit the ground, otherwise you get a sloppy mess and clear ice. Nucleation is essential to that crystallization. When water is very pure, it is very slow to freeze.
We can see that clearly from the icing that occurs on airplanes flying through clouds at temperatures like 28 F/ -2 C. The water droplets in those clouds are frequently ready to freeze on contact with the airplane wing. If they are sparse and small enough, they can flow around the wing harmlessly. If they are a little larger, they will hit the wing ballistically and freeze instantly forming pebbly rime ice. If they are yet larger and abundant, they will splat on the surface and produce clear ice. Those supercooled (subcooled) droplets can be stable in clouds as liquid droplets for hours, even days. Large jet aircraft have heat from the engines to melt it, so it isn’t dangerous to them, but light aircraft can have a serious problem. If they can’t get out of the icing layer of clouds, they are going to go down. In the mountains that can be a particularly serious problem.
Nucleation is a common phenomenon throughout Chemistry and Physics. It is important in many fields from meteorology to metallurgy to glass, ceramics and beyond. Many things we take for granted would not work without it.
It is simply faster and easier to start with a significant chunk of something you want to produce, than to start with the individual molecules one by one. If you want to build a house, don’t start with grains of sand.
Water has unusual characteristics and water molecules are of course very small. Water behaves like a much larger molecule than it is. Methane (natural gas) is a molecule about the same size as water, yet it is a gas at room temperature and doesn’t condense until you reach -258 F (-162 C).
So why does water behave like this?
It is due to a phenomenon called “Hydrogen Bonding“. Water molecules are very strongly attracted to one another. The hydrogen atoms on each water molecule are attracted to the oxygen atoms on neighboring water molecules. They tend to form clusters of about 7 water molecules on average. This is why water boils at 212 F/100 C instead of -250 F/-160 C. This clustering also delays water molecules from rearranging easily into the neat rows and columns of a crystal. At the same time, it is hydrogen bonding that allows water to be a liquid at room temperature and to crystallize below 32 F/0 C in the first place. Otherwise it would be a gas like methane.
There is another reason water droplets don’t freeze quickly. If water is very pure, you can cool it to -40 F/-40 C and it can still be a liquid. The reason is simply due to the incredible number of water molecules in a droplet of water. There are about 10,000,000,000,000,000,000 water molecules in a single droplet of water. When individual water molecules start to form a crystal, they are so small that it can take a very long time for the crystal to grow to a significant size.
This is why, if you want a droplet to freeze quickly, you need to start the crystallization on a small bit of something with the right shape and surface chemistry. If that “bit of something” is visible under a microscope, it is vastly larger than a water molecule and has far greater area upon which crystallization can occur.
However, once crystallization starts, crystallinity is the favored structure below 32 F/0 C. Taking that favorable crystalline structure saves a tremendous amount of energy. When that energy is released it has to be transferred out of the water droplet to keep it below 32 F/0 C to continue the freezing process.
There are basically two ways to nucleate a water droplet to cause it to freeze within a reasonable amount of time. Nucleation can occur due to foreign particles dispersed WITHIN the droplet, OR a nucleus like another ice crystal can become ATTACHED to the outside of the droplet.
The ideal nucleus to freeze water is ice itself. It can nucleate crystallization when the water droplet is just below 32 F/0 C. It has exactly the right shape and surface chemistry for water molecules to attach to (water likes itself). If the water droplet is being cooled adequately and the crystal nucleus is large enough to begin with, the seed crystal can grow fast enough to freeze the droplet before it hits the ground.
There are natural nuclei in the environment that can initiate crystallization of water at temperatures a few degrees below 32 F/0 C. Natural nuclei are very abundant at 12 F/-11 C. Even testosterone is effective as a nucleating agent for water but it is too expensive to be practical and you really wouldn’t want to spray people with testosterone.
Some years ago a fellow at the National Weather Service became curious why there was often hail in a mountain range in Canada. He found that there was debris from rotting leaves that contained the natural bacteria Pseudomonas Syringae. That debris was being blown up into the air, frequently causing hail.
It is actually the proteins in the bacteria that help form water crystals. That’s why you can kill the bacteria and activate it by freeze drying. Proteins are composed of amino acids. Some amino acid sequences can exhibit hydrogen bonding in a pattern that is easy for water molecules to crystallize onto. Commercial nucleating agents take advantage of the ease and low cost of growing the bacteria. It isn’t as effective as actual ice nuclei, but it can work. More about Pseudomonas Syringae can be found here. https://en.wikipedia.org/wiki/Pseudomonas_syringae
Some people think that cloudy water with clay particles is good for snowmaking. It actually isn’t the clay. Most clay particles are rather poor nucleating agents and of little value. Using dirty water also has the side effect that when Spring comes, the dark clay particles absorb sunlight and cause the snow to melt faster. It is mostly organic matter that is helpful for nucleation. Even when water looks clean, it really isn’t.
The actual mechanism of nucleation in a compressed air/water snowgun depends upon the surface of a water droplet cooling to 32 F/0 C before the ice nucleus bumps into the droplet. If the surface of the water droplet is above that temperature the ice nucleus will melt. If it is below that temperature, the crystal will grow. To be sure that you get crystallization, you want a lot of ice nuclei. Fortunately that’s easy if you do it right.
If you could see inside the compressed air hose, you would see a sort of fog flowing toward the gun. That fog of droplets is caused by the cooling from the outside of the hose. When that enormous amount of tiny water droplets goes through the snowgun nozzle, the compressed air expands and cools, flash freezing those tiny droplets and water vapor. Those ice crystals aren’t noticeable, but they are what cause the much larger droplets to freeze. They actually amount to a very small fraction of the total water volume, but without them you get rain.
Nuclei that are not composed of ice use a different mechanism to nucleate a water droplet. It is why synthetic nuclei dispersed inside the water droplet do not help much in high humidity. They need to float outward to the surface of the water droplet and be exposed to the atmosphere. If there is a thick film of water on the surface of the protein nuclei, the clustering of liquid water molecules interferes with the initiation of crystallization.
Clustering of water molecules is why synthetic nucleating agents dispersed within the water do not work well in high humidity. The surface water needs to first evaporate before nucleation can be effective. If the rate of water flow is low enough and the droplets are dropped from enough height, they will perch on top of existing ice and freeze due to nucleation by the existing snow cover on the ground. This is why they often produce a styrofoam like surface. Fortunately that surface can be groomed to make an adequate ski surface, but grooming also costs money.
It is the same reason frost forms so easily on objects like your car windshield. It crystallizes directly from gaseous water vapor onto the surface. Liquid water clusters do not get a chance to interfere with crystallization and the RADIANT temperature of the sky above is also very cold, typically -15 F/-26 C even when the AIR temperature can be above freezing. On a clear, calm night, it is that cold sky that causes the top surface of leaves facing up to become much colder than the air temperature and freeze. The bottom side of the leaf is facing the ground and bathed in air, so it stays warmer. That cold sky is why your tomato plants freeze so easily. That’s why placing a cloth over them can prevent them from freezing. The top side of the cloth gets cold, but the bottom side facing the warmer ground remains warmer. You can see it clearly with an infrared thermometer.
Proteins can also have the opposite effect of nucleation, acting like antifreeze. Some fish contain proteins that allow them to survive being trapped in frozen water. Note that the needles of evergreens stay green through the Winter. When it is cloudy in the Fall, the foliage develops less red in its display of colors. That red pigment complex is proteinaceous, likely delays nucleation, and protects the leaf from freezing on a clear, cold night. That may be surprising, but plant and bacterial proteins are undoubtedly a factor in poor nucleation of surface water used for snowmaking. There is a segment about windshield ice explaining that very cold sky in the Eureka! section.
There’s a lot more to be said about nucleation, but those are the highlights.
Fergus S. Smith