Saturday, January 15, 2005

How Do Snowflakes Form?

Snowflakes are a beautiful part of winter. You probably know that snow is a frozen version of rain, but why does snow fall in flakes rather than in drops? In other words, how do snowflakes form?

Like rain, snowflakes are formed in clouds (which are made of water vapor and tiny drops of liquid water). Since water freezes at 32 F, the water in clouds can freeze if the temperature is 32 F or below.

You may have noticed that snowflakes can have various shapes. Some look like typical intricate snowflakes (called dendrites), others look like long needles or tubes, while others look like hexagonal (six-sided) plates. If these plates have indentations (notches) in them, then they are called sector plates.

What determines the shape of snowflakes? The major factor is temperature, but snowflake shape is also affected by wind, humidity (how much water vapor is in the air), the amount of dust in the clouds, and the altitude (height) of the clouds.

These conditions are important because they determine how the water molecules (H2O) in a snowflake will be arranged. Whichever arrangement forms the most easily under a given set of conditions is the one that will happen in most of the snowflakes that day. This explains why, on a given day, most snowflakes look similar, but you can always find a few odd ones.

Since some conditions (such as wind currents) change so quickly, each snowflake is usually a tiny bit different than its neighbors. It is possible for two snowflakes to be identical, but this doesn't happen very often. Even if two snowflakes look identical, they are probably a tiny bit different. For example, one snowflake might have a few more water molecules in it than the other snowflake does, but your eyes can't see such a small difference.

As a general rule, there are five different snowflake shapes. Each one is found most commonly within a certain temperature range, as shown below:

Thin hexagonal plates 32-25 F
Needles 25-21 F
Hollow columns 21-14 F
Sector plates 14-10 F
Dendrites 10-3 F

Table adapted from

Think about the first snowfall of the winter. It usually looks like hexagonal plates or needles. This makes sense since the first snow usually falls when the temperature outside has barely dropped below freezing. The prettiest snowflakes (dendrites) tend to fall in January and February since these months are the coldest.

Little Lion Experiment:

On a day when it is snowing, put a piece of black construction paper in the freezer for 15 minutes or more in order to chill it. Then, go outside and hold the piece of paper so that snowflakes land on it.

The black color of the paper should allow you to easily see the shapes of the white snowflakes. However, the paper may begin to warm up after a while. If the snowflakes are melting on the paper, then you can cool the paper down by simply setting it on the ground against some snow, or standing over it so that it lies in your shadow.

Examine the snowflake shapes and try to figure out which of the five major types you have in front of you (there may be more than one shape on the paper). Repeat this experiment on other days when it is snowing. Try to do it on days with different temperatures (like 10 F, 20 F, and 30 F) so that you can see the different snowflake shapes that form at various temperatures.