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Physical Science: Session 6

A Closer Look: Why Do Snowflakes Have Six Sides?

How does snow form?

When temperature and humidity conditions are conducive, individual water molecules from small droplets of liquid water in the atmosphere, particularly in clouds, can condense slowly to form solid water or ice.

water molecule

What is the shape of a water molecule?

A snowflake is built up molecule by molecule. Each time a growing snowflake moves past water droplets, several molecules of water are added to it. As we mentioned in the video for Session 6, in order to explain this behavior, we must refine our model of the water “particle” from a simple sphere to a small “V” shape, as shown in the following picture:

The central atom is an oxygen atom, which has hydrogen atoms bound to it on either side. The angle between the two arms of the molecule is 104.5 degrees, first determined around 1930 by x-ray diffraction techniques.

How does this lead to six-sided snowflakes?

Several snowflake formations.

The oxygen atom has a particularly strong attraction to the electron clouds of the two hydrogen atoms and pulls them closer. This leaves the two hydrogen ends more positively charged, and the center of the “V” more negatively charged. When other water molecules “brush up” against this growing snowflake, strong forces between the negatively charged and positively charged parts of different particles cause them to join together in a very specific three-dimensional pattern with a six-sided symmetry. Each water molecule that joins the snowflake reflects this pattern until eventually we can see its macroscopic six-sided shape.

If snowflakes are built from a particular pattern, why aren’t all snowflakes identical?

As a snowflake moves up and down in the atmosphere, slight changes in temperature and humidity cause the exact pattern to change as it is built. The overall shape of each flake, however, remains six-sided.

prev: a closer look intro