How high is the sun when you see a rainbow?
How much water has to be in the air to make a rainbow?
Typically, the sun is fairly low when you see a rainbow. This is
because the sun is behind you as you view the rainbow, on a line from the center
of the rainbow to your eye. (The "center" of the rainbow is the center of
the partial circles in the rainbow.) When the sun is high, the rainbow
has to be almost at your feet. But this is possible, perhaps with the
water from a lawn sprinkler.
How much water? More water makes a brighter rainbow, up to a point. Then
you start to have a problem that the sun cannot illuminate enough of the
drops. The best situation is when a shower is localized in the near distant
in a direction opposite to the sun.
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Update: June 2012