Saturday, December 11, 2010

How to Grow a Snowflake

"Snowflakes are made of ice, yet ice alone does not a snowflake make."  
Photo: Wilson Bentley

A cloud is a snowflake super-factory. Snowflakes, if you can believe it, are made at a rate of approximately 1 million billion (give or take a few) crystals a second. Someone smarter than me determined that at this rate, enough snow falls every 10 minutes to make one snowman for every single person on the earth.

Every 10 minutes.


That's a lot of snow crystals.

Do you ever just look at the clouds and wonder if it will snow? I do. All the time. Okay, so rather than asking if it will snow, let's ask why it snows at all? So that seems kind of simple, right? The water in the clouds freeze and fall to the earth as pretty little flakes. Well, kind of. But there is a bit more to it than that.

The Snowflake Recipe:

A million tons or so of water vapor
Cold air
Microscopic dust particles

This photo was taken by Patricia Rasmussen. Many people attribute these wonderful photos to Kenneth Libbrecht. Libbrecht is the brains behind the formation of a snowfake. Rasmussen captures the art.

* Take lots and lots of water VAPOR (not rain drops).

* Cool the air down around it - a lot. When a section of air is cooled down, its relative humidity increases. (When I first learned this, I had to ask myself why that was. So I went and looked it up, and learned. So if you are wondering why, then you, too, should go look it up and learn). With enough cooling, you end up with air that is supersaturated with water vapor (more than 100% humidity).

* Once you have supersaturated air, the water vapor condenses to form a cloud made of itsy, bitsy, minuscule water droplets.

* One very important thing needed to form a water droplet from water vapor is a nucleus, something for the little water vapor to hold onto. This is usually a microscopic dust particle. This dust particle is critical in not only forming the water droplet, but in forming an ice crystal later. See next point.

* Water droplets do not readily freeze (requiring temps well below freezing - between 5 and 21 degrees F) because their molecules are all jumbled up due to the jostling effect of thermal motion. In order to freeze, those molecules must be ordered somehow into a crystalline lattice, but they have a hard time settling down on their own. Hence the second benefit of that captive dust particle. It actually provides the template needed to help move the freezing process along.

* Once that tiny water droplet freezes with the guidance of the dust particle (and when I say tiny water droplet, I mean really, really, tiny. Minuscule. Microscopic) you have the very tiny beginnings of a snowflake. It continues to grow by condensing water VAPOR from the air, creating a lovely, full-fledged snowflake.

* It will continue to grow and form in the cloud until it gets big enough (typically 0.04 - 0.2 inches) for gravity to pull it to the earth.

Phew, there you have it. The making of a snowflake. Lots of work, actually.


Jenny said...

I have a book you might like to read. It is a children's book called Snowflake Bentley. Nice post!

Wendy said...

I'll look it up. Thanks.