Ice, Water, and Molecules

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.

Portrait of Johannes Kepler - brown hair and beard with white collar, holding a compass

Philosopher Johannes Kepler’s observations of the 6-sided symmetry seen in snowflakes hinted at the underlying molecular structure of water. (Public Domain {{PD-old-100}})

close up of a snowflake (white) on a black background

The first successful photo of a snowflake was taken in 1885 by Wilson A. “Snowflake” Bentley. Based on more than 5,000 snowflake photographs, Bentley came up with the idea that no two snowflakes are exactly alike. Photo by Wilson A. Bentley (Public Domain {{PD-US-old}})

Hexagonal prism showing axes of snow crystal growth

Snow crystals can grow horizontally along their basal face, resulting in a flat hexagonal plate or dendrite. They can also grow vertically along their prism faces, resulting in column- and needle-shaped crystals. Most snow crystals grow in both ways, resulting in a stunning array of different snow crystal types. Illustration by Matthew Sturm

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).

Above: In most of the continental U.S., winters get cold enough for it to snow. The map key indicates 30 year average temperatures for December-January-February in degrees Celsius. Note that that 0 degrees Celsius/32 degrees Fahrenheit (“freezing” temperature) is shown in light green. Image Credit: National Center for Atmospheric Research Climate Data Guide / D. Schneider

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)

A water molecule (H₂0) is made up of one oxygen (O) atom and two hydrogen (H) atoms. H and O atoms from different molecules are attracted to each other, too. The connections between neighboring H₂0 molecules are called hydrogen bonds. Illustration by Matthew Sturm

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:

Large white iceberg floating on dark blue water

Photo by Lavivm on Pixabay (Pixabay License)

large boulder split down the middle with smaller rocks stuck inside the crack

Photo by Dominicus Johannes Bergsma from Wikimedia Commons (CC-BY-SA-4.0)

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:

Snow patches in mountains above a lake and conifer forest — Federal Highway Administration Photo / Martin Gyorgyfalvy (Public Domain)

Snow, icicle formations, and creek — Photo by Matthew Sturm

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:

Person ice skating, photo from the knees down — Photo by Gantas Vaičiulènas from Pexels (Pexels License)

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:

Sunny day in snow-covered forest clearing, snow on trees and ground — Photo by Kordi Vahle on Pixabay (Pixabay License)

Snow helps cool the planet by reflecting rather than absorbing solar radiation.

Snow in Science, Culture, and Climate