As now well established by the map, the surface wind is from the area
of high barometer to that of low--from the atmospheric hill to the
atmospheric valley.
The tendency of this is to free "high" of all clouds and moisture; but
then it is impossible to free "high" entirely of moisture; a little
will remain, and it is just this little, which is highly rarefied, that
produces the result. We look around us and above, we see little or no
evidence of evaporation, yet it is the while going on. When the sun is
immediately below the horizon, where it will shine horizontally through
the mass of light, suspended moisture, the delicate presence of vapor
heretofore unnoticed is revealed. The action of the sun's rays is the
same as when illuminating a well formed cloud--it is an embodiment
of the same principle, but the material is much more expanded. The
particles of suspended moisture are very fine, few and far between,
therefore the effect of the light upon it is more diffused and
transparent. It is much like looking through a piece of window glass
flatwise and endwise; flatwise we do not perceive any color; endwise,
from seeing through a greater mass, the glass has a very perceptible
green color.
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