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1. Introduction: Skiing

Key words:
timber line | track surface structure | overcast sky | invisibility

Most of us are fond of skiing, especially downhill skiing.
As the name implies, this kind of fun is to be found in the mountains, often beyond the TIMBER LINE.
And though it is most wonderful in sunny weather, we often do it when the sky is overcast.

Right here an optical phenomenon comes up, the description of which I would like to hand over to a poet:

"When the fog is heavy and new snow falls so that the skier hardly sees his ski-tips, not to talk about the colored posts that mark the track, then it can happen that you get dizzy while you feel like suspended in a silent world -- instead of feeling how you run downhill.
I got dizzy. I drifted through a world of wadding. I didn't know whether I stood or ran. I felt sea-sick. Though I knew the skiing-ground and though the snow had seized falling, I anyway lost my way."
Taken from Michael Wildenhain (* 1958 Berlin): "Der Kalender". Cited according to "Draussen gibt's ein Schneegestoeber", Hrsg. Hannelore Westhoff, dtv, Muenchen 2000, ISBN 3-423-62039-0

The TRACK SURFACE STRUCTURE gets invisible even in clear air at OVERCAST SKY (fig. 1_a and 1_b).

fig. 1_a: snow surface visible; 39 kByte fig. 1_b: snow surface invisible; 22 kByte
fig. 1_a: snow surface visible: sunny fig. 1_b: snow surface invisible: overcast

Now what is the reason for this strange kind of vision or INVISIBILITY?
Even more astonishing: Why is visibilty restored as soon as you enter a forest?
These are the main questions to deal with in the following chapters.
We are going to look at the mechanisms of "white" appearance and at the prerequisites of structure perception.
And also, we will look at the different effects that diffused illumination and direct illumination have on special scenes.

Continued: 2. Optical Material  --  How Snow Turns White

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Last modified Jan. 21st, 2009