KM and others,
My understanding of the Sector whiteout phenomenon goes a little further than just the sun angle.
One form of this white out occurs during high sun angles and a layer of (higher) cloud.
Those wavelengths of light not filtered out during the light's passage through the cloud layer are able to be refelcted from the (white) ground almost in their entirety. (Normally only a percentage of light is reflected by the snow). The upshot of that is that sky and ground merge into one with obvious consequences.