Dear 1st graders,
I love bright, snowy days. That’s when you’ll find me building snowcats, throwing snowballs and watching the sun sparkle on the freshly fallen snow.
I asked my friend Von Walden why that sparkle happens. He’s an atmospheric scientist at Washington State University. Some of his research takes place near the North Pole and the South Pole.
He told me that snow sparkles because of the way snowflakes interact with the sun.
It turns out snowflakes are ice crystals. On Earth, snowflakes are hexagonal. That means they’re crystals with six sides.
Snowflakes can be long prisms or rounded grains like these taken at the South Pole by Walden’s team (left). Or they can be dendritic snowflakes with branch-like arms (right). But all snowflakes are crystals with six sides.
Images: (left) VP Walden et al., Atmospheric Ice Crystals Over the Antarctic Plateau in Winter; (right) NSF/Kenneth Libbrecht, Caltech
When a snowflake falls, it can land in lots of different positions.
“Some might fall on their edge and get jammed into the snow,” Walden said. “Some will fall flat. If you really got down there and looked, you’d see how uneven the surface of the snow is. So, the sunlight is hitting this uneven surface.”
That means light strikes all the parts of the fallen snowflake that stick out.
The light is also traveling from one kind of thing to another kind of thing. It’s moving from the air—which is gas—to the snow—which is solid. Any time light does that, it slows down and bends. Scientists call the bending refraction.
That bent light keeps traveling through the snowflake. Some of the light will hit different parts of the snowflake and bounce off. Scientist call light bouncing off something reflection.
As the light bounces off all the different bits of ice crystals on the surface of the snow and zooms back to our eyes, we see sparkles.
But that’s not all you can see if you examine snow.
“If you look carefully at a snow surface, it’s not only sparkling,” Walden said. “There are also different colors. The different wavelengths of sunlight—the red, the blue, the green, the yellow and the purple—all get refracted at different angles.”
When we see white light, it’s made of all those other colors smooshed together. If that white light enters an ice crystal and gets bent, the different colors in the light will each bend a little bit differently. That separates the colors so we can see them. It makes a rainbow in the snow.
Walden told me that some snow will be extra sparkly. Old snow packs down, and the surface becomes more even. So, the sparkliest snow happens on sunny days right after a snowfall with big snowflakes. Those ice crystals will have crisp edges. They’ll stick out of the surface in different ways, ready to bend and bounce lots of light.
You just need to s-know when and where to look.
Sincerely,
Dr. Universe