This old saying actually has a scientific explanation. It relates to moving high and low surface-pressure weather systems, and the way that the colors in sunlight are scattered differently by dirty and clean atmospheres. This is the explanation of how these phenomena combine to color our sunrises and sunsets.
If
you look at a global map of surface pressure, you will see a string of
alternating high-pressure and low-pressure areas. That is because
pressure patterns are relative; i.e., if a region of lower pressure exists,
it must have higher pressure on either side.
Low pressure is associated with bad weather (sailor take
warning), and high pressure with good weather (sailor's delight). Low pressure
causes air to converge (to try to "fill" the low), and converging air causes
upward motion, which in turn produces clouds and precipitation.
In contrast, air diverges from the center of a high-pressure area. This causes downward motion, which suppresses cloud formation.
A temperature inversion (temperature increasing with height)
forms at the level of the troposphere where this downward motion is strongest.
This happens because downward-moving air experiences higher pressure as
it descends and thus is compressed. According to the laws of physics, air
heats when it is compressed. Vertical motion is inhibited at the level
of the inversion; thus dirty air containing suspensions of
soot, dust, and other particles (known as aerosols) is trapped near the surface.
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