What Causes "Flat" Light

The quality of light strongly affects one's confidence as a skier.  On a sunny day, most of the solar energy (about 85% of it when the sun is high in the sky) is direct, meaning that it is traveling along a straight line from the sun to the Earth's surface.  This results in large brightness contrasts between directly illuminated surfaces and those in shadows.  Even subtle textures in the surface of the snow are easily seen.  

Descending a groomed run at Ischgl, Austria on a sunny day dominated by "direct" solar energy.

All else being equal, it is much easier to ski on such days.  It is easier to see the steepness of the terrain, variations in the snow surface, and subtle changes in the snow conditions.  

However, on day with thick overcast, even high overcast, the light can be "flat." Flat light is a colloquialism for situation in which there are no shadows, as was was the case at times at Alta today (Sunday).  

Flat light at Alta on Sunday, March 30th

Flat light is produced when most of the light from the sun isn't direct but is instead diffuse.  When skies are covered by thick clouds, the sunlight is not direct because it has been scattered by the liquid water and ice particles in the clouds.  As a result, the Earth's surface receives sunlight from all parts of the sky, resulting in a lack of shadows.  This is particularly problematic for seeing contrasts in a white surface like snow.  

High clouds sometimes vary in depth, so on a day like today, there can be variations in the flatness of the light.  Below is a photo I took looking up upper Sleepy Hollow off the Supreme Chair at 11:07 AM.  At that time, the sun was only partially obscured and there was enough direct radiation for the trees to cast shadows and for one to see some texture in the snow, as evident in the foreground of the picture.  

On our next run, at 11:33 AM, the sun was nearly obscured and there was considerably less direct radiation.  Shadows were less apparent and the texture and variability of the snow were harder to see.  

Such conditions though are not anywhere near as bad as they can get.  Add fog and one can have a hard time telling up from down.  Balance becomes difficult and one can even suffer from vertigo.  

Goggles can help some.  Color tints with higher light transmission that enhance contrast are usually best on flat light days.  Skiing closer to trees and other darker objects can also help.  

One of Bruce Tremper's ten commandments of avalanche safety is "thou shalt never go first."  This commandment is especially important on powder days with flat light, as a set of tracks helps to provide some contrast in an otherwise featureless landscape.  Be generous to your friends on these days and let them have first tracks.