WEDNESDAY FEBRUARY 28 - SNOW GLARE AND SUNGLASSES


I've never been one for sunglasses, but when I do break out the shades, it's usually on a sunny winter day when snow is on the ground. A fresh coat of dry snow can reflect 80 percent or more of the sunlight that strikes it. So on sunny days with a snowcover, not only is sunlight coming at you from the sky, but it's also coming at you from the ground.

As for snow's white color - sunlight is sometimes called "white light" because it contains the entire color spectrum - you can prove that to yourself by using a prism. How we perceive an object depends on which colors of the "white light" the object reflects back to our eyes. For example, a red apple is red because when sunlight strikes it, red dominates what reflects back. Snow looks white because when sunlight strikes it, all of the colors in the sunlight are reflected back about equally, so what your eye sees is essentially white light.

Snow's reflective nature has a cooling effect on air temperature. With snow reflecting all that energy back to space, the surface of the earth doesn't heat up much when snow is on the ground. Since it's the ground that heats the air, the air above the snowpack doesn't heat up much either. That can keep the air temperature above a deep snowpack perhaps 5 to 10oF lower than on a day when the ground is free of snow.

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