Why is snow so bright ?

Snow seen against an overcast sky typically appears much brighter than the clouds that provide the illumination. This raises the problem of how a reflecting surface may appear brighter than its diffuse illuminant. We show that three factors are largely responsible for this visually striking effect: the law of darkening for the cloud cover, the reflectivity of the snow and the average landscape albedo, and the observer's contrast sensitivity function. Thus the explanation is of a curiously mixed type: Both physical and psychophysical factors play a decisive role.