Are no two ice-crystals alike?
Clouds are made up of water droplets, water vapour and suspended impurities such as tiny dust particles. If the temperature of the cloud then decreases the water molecules can begin to crystallise, arranging themselves around the dust particles in a hexagonal lattice structure known as Ice Ih (see figure 1).
There are fourteen known forms of ice, but ice Ih, an abbreviation for "form 1 hexagonal", is stable between -100°C and 0°C and so it is the form seen in snowflakes. The hexagonally arranged water molecules stack in sheets with sides that are perfectly straight and angled at 120° to each other, called 'facets' (see figure 2).
Facets arise because it is easier for molecules to stick permanently to a rough surface than a smooth one. So initially the rough regions fill in, and then the slower-growing smooth facets, which then define the shape of the crystal. Consequently, the initial form of any snow crystal is a perfect hexagonal prism. This is shown in figure 3. Later, as the hexagonal prism grows, it becomes less able to keep its shape and branching instabilities cause legs to emerge at the corners. The final form of the crystal will always have 6-fold symmetry because of the hexagonal way water molecules crystallise. In fact, as figure 4 reveals, you can still see the early hexagonal prism in the centre of many snowflakes.
Snowflakes that fall to the ground can vary in size from nearly perfect hexagonal prisms just 0.2 mm wide (called diamond dust) up to large ‘dendritic’ snow crystals of width 5 mm. Crystals that are smaller than diamond dust are too llight to fall to the ground so they remain aloft, whilst larger crystals are fragile, so they tend to break up in the slightest breeze.
But what causes snowflakes to begin to form in the first place? This occurs when two particular conditions are found in the clouds: supersaturation and supercooling, and the ultimate shape of a snow crystal can reveal how these conditions varied.
Supersaturation occurs when there is more water vapour in the air than the ordinary humidity limit (which is 100%). At every temperature, there is a maximum amount of water vapour that can be supported in the air. The higher the temperature, the more water vapour can be accommodated. But if you cool air that is already at 100% relative humidity then it becomes supersaturated, and this situation is unstable. As a result the excess water vapour crystallises out, either into water droplets or directly into ice.
The existence of liquid water below 0°C is called supercooling. Surprisingly, if you cool a drop of pure water to below its freezing point it won't freeze, and this is because the molecules in that drop of liquid have thermal motion which prevents them from crystallising. In fact the temperature has to fall to -42°C before freezing occurs. By comparison, a drop of tap water readily freezes at 0°C and this is because the liquid contains impurities that provide a surface to which the water molecules can cling (known as nucleation), and this reduces the effect of the thermal motion, raising the freezing point. If you could watch the process under a microscope you would see progressively more water molecules linking up to form tiny ice crystals. If a crystal is larger than a critical size then it will grow but if it's too small the molecules will break apart again. The same thing happens in clouds, and consequently the structure and composition of a snowflake can tell scientists about the temperature conditions experienced inside the cloud when the snowflake was forming.
Also, snowflakes are blown around inside clouds as they grow, meaning that they experience different conditions at different times, which causes the shape of the growth to change. Figure 5 illustrates how the environmental history of the snowflake is recorded in its structure and how this can be used to gather information about the snowflake's parent cloud. And as figure 6 shows, understanding this process mans that scientists can create designer snowflakes, which provide information about the physics of crystal growth and pattern formation.
Is it true that no two snowflakes are ever alike?
Every schoolchild learns the phrase that "no two snowflakes are alike" and for the large dendritic snowflakes that is certainly true. Why? Imagine you are organising your spice rack. You have five spices and five spaces in which to place them. There are five ways to position the first jar on the rack, four ways to place the second jar, three for the third and so on. In total, there are 120 ways to arrange five jars on a rack.
Now for a snow crystal there are more like 100 different features that can be identified, which gives 10158 (that's a 1 followed by 158 zeros) ways to build a snowflake. That's about twice as many possible arrangements as there are atoms in the universe, so it's pretty unlikely that you'll find two the same! And since snowflake growth is a result of the environment, even the smallest temperature and humidity difference will affect the shape of the snowflake. If the changes occur on a length-scale smaller than the snow crystal itself, then the snow crystal will be irregular (as many of the snowflakes in nature are). Radially symmetric snow crystals form because every part of the snowflake was subjected to the same conditions at the same time, so seeing similar snowflakes is more common when they are very small.
But Why Study Snowflakes?
Water itself has many unusual and counter-intuitive properties, some of which we are all familiar with such as the fact that ice (solid water) is less dense than liquid water so it floats. Cloud science is one of the most active aspects of climate change research today and because the patterns that make snowflakes so beautiful contain the story of the clouds they came from, then by its very nature, the humble little snow crystal is doing its part to help us understand the science of our environment.