Why do snowflakes have six sides?
In 1611, people did not know about molecules, but when philosopher Johannes Kepler saw a six-pointed snowflake on his sleeve, he thought its symmetry had to be due to it being made up of minute hexagonal (six-sided) particles. Kepler raised some fascinating questions about the fundamental nature of matter, and he was largely correct. The exquisite six-sided forms seen over and over again in snowflakes falling from the sky and in the snowpack are due to the nature and shape of the water molecule.
Ice versus Water
Earth, the “Water Planet,” could easily be called the “Half-Ice Planet,” because temperatures in winter over a much of the Earth are low enough that water freezes, which is why there are huge ice caps in the polar regions. Both water and ice are composed of tetrahedral molecules made up of two hydrogen atoms and one oxygen atom (H20) linked together by strong bonds. Ice has a more rigid and organized structure than water, which consists of looser strings of bonded molecules that can slide closer together. Substances in which molecules are more closely packed have a higher density than those in which the molecules have more space between them. That is why ice (less dense) floats in water (more dense).
Right: Liquid water (on left in image), H20 molecules move around quickly, because they have more energy than they do in the solid (ice) form (on right in image). When H20 freezes, the molecules slow down, forming an organized, lattice structure. The hexagonal arrangement of H20 molecules in the solid form is the reason that snow crystals have hexagonal symmetry. Image adapted from Encyclopedia Brittanica by Andrew Stark on Quora (Quora Terms of Service)
What’s so special about H20? Some important characteristics of water
Humans are 60% water, so we tend to take ice and water properties for granted, but they are extreme in ways that impact all life and the climate of Earth.
Water is denser than ice.
In most substances, the solid state is more dense than the liquid state, but water is different! Ice is less dense (917 kg per cubic meter) than liquid water (1000 kg per cubic meter). Another way of thinking about this is that when a certain amount of liquid water freezes into ice, it increases in volume. This is why:
Ice floats, so there are no glaciers at the bottom of the ocean, and we can skate on frozen ponds.
When pipes freeze they burst, and freezing water that freezes in cracks splits rocks.
Water has a high heat capacity and latent heat
Of all of the substances on Earth, water has the highest heat capacity, which is the amount of energy (heat) that it takes to increase its temperature. It also has the highest latent heat, which is the amount of energy it takes to melt a solid substance into a liquid or a liquid substance into a gas. This is why:
Snowbanks linger in the mountains, where it is cooler than it is at lower elevations, through the summer.
Snow in the mountains melts gradually, flowing into streams and rivers throughout the spring and early summer.
Water and ice can coexist.
At low temperatures, liquid water still coexists with ice as a thin layer. This is why:
Snow and and ice are slippery, allowing us to sled and ice skate.
Snow reflects sunlight
Snow reflects visible sunlight better than any other natural substance on Earth, often sending 85% back toward space. This is why:
Snow helps cool the planet by reflecting rather than absorbing solar radiation.