How does snow form? -Susan, 8, Lake City, South Carolina

Dear Susan,

It just so happens that when I looked out the window here in Pullman, Wash., everything was covered in glittering snow. I watched it fall from the sky and wondered how exactly it formed, too.

So I put on my favorite red mittens and went to visit my friend Nic Loyd, a meteorologist here at Washington State University. He studies what’s going on up in the skies.

He explained that water moves through our atmosphere in different forms all the time. Clouds are made up of tiny water droplets that have turned into a gas called water vapor. It comes from evaporated water that rises from Earth’s surface.

So, in a single snowball, you might actually find traces of water from rivers, lakes, or oceans around the world.

Sometimes cool air up in the sky will cause water drops to hang onto pieces of dust, tiny bacteria, or other things floating in the air.

When the temperature plunges, the now heavier water drops will freeze into tiny ice crystals.

“Snow occurs when lots of tiny ice crystals in clouds stick together to form snowflakes,” Loyd said.

The flakes can be made up of anywhere from two to more than 200 ice crystals.

The hydrogen and oxygen building blocks that make up water will also freeze into particular patterns that give nearly all snowflakes six arms.

While snowflakes share this trait, they can come in all kinds of shapes and sizes. In fact, you may have heard the phrase “no two snowflakes are alike.”

Some of the first humans who took pictures of snowflakes under a microscope realized snowflakes came in lots of beautiful and different patterns.

After further research, it turns out some snowflakes actually are identical. It’s pretty rare to find two that are exactly alike. But the odds of finding them go up when you consider that a block of snow, just a foot tall by a foot wide, contains an estimated million billion snowflakes.

Once snowflakes have formed up in the clouds, gravity brings them down to Earth’s surface. It’s a nearly 20,000-foot fall.

Typically, it takes about an hour for a snowflake to fall from a cloud to the ground. That is, if we don’t catch them on our tongues first.

Snowflakes are lighter than rain and they are easily blown in the wind, so the journey is longer than a raindrop’s, which takes just about three minutes.

“Snow can only reach the ground if the temperature is below freezing everywhere in the atmosphere,” Loyd added. “If snow reaches the ground that means that it was never rain at any point during its journey from the cloud.”

After I left Loyd’s lab, I plopped down in a drift to make a snow cat angel. Then I looked up to the sky again. This time knowing that no matter where you go, somewhere in the world countless tiny snowflakes are forming up in the clouds.


Dr. Universe

Got a science question? E-mail Dr. Wendy Sue Universe at Ask Dr. Universe is a science-education project from Washington State University.