Now that fall has faded I've been trying to focus on the subtilty of light play on snow. I was hiking at Boulder's Chautauqua after a recent snowstorm enjoying nature's snow drift sculptures when I noticed a gem-like sparkle glinting off of the surface of the snow banks...
Gem Covered Snow Sculpture
The snow banks were covered with feathery crystals...
Crystal Covered Snow Banks
Closer inspection revealed that the sparkle was caused by the reflection of light from the crystals...
Snow Gems
When I got really close I discovered a micro-environment of delicate crystalline beauty...
Feather Like Crystals
These beautiful crystals are a type of frost called surface hoar. Here's the physics: just as frost forms on the outside of a cold glass on a warm humid day, surface hoar forms when a snow bank cools overnight. During a cold clear night, the snow bank radiates heat causing its surface to become colder than the surrounding air. Since warm air holds more water vapor than cold air, the vapor from the warmer air above the snow will condense onto the surface. The surface of the snow bank is also colder than its interior, which encourages evaporation and releases additional crystal growing moisture. By morning the snow bank can be covered with a layer of faceted ice crystals. The best conditions for formation of hoarfrost are: cold clear nights and a very light wind to circulate humid ground air around the snow surface.
As with my earlier discovery of snow fleas, I've discovered that little things can have big consequences. In the case of surface hoar there is a darker side which turns these benign crystals into killers...
This Crystal Has A Dark Side
This miniscule crystal is responsible for over 150 human deaths per year because surface hoar makes a perfect avalanche trigger. When layers of surface hoar are interlaced with subsequent layers of fresh snow, thin sheets of slippery ice crystals form within the snow. This laminated snow is hard to detect and can be very unstable...
Laminated Snow Bank
These sheets tend to slip resulting in the transformation of lots of little independent snowflakes into an angry crystalline flash mob travelling at 80 mph (130 km/h). Physics has consequences...
So Many High Speed Snowflakes (Photo Credit :http://uisevereweather.pbworks.com/)