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Falling Mirrors
Fri Jul 11, 2003
Listen in RealAudio 
Have you ever watched the sky darken when a thunderstorm approaches only to see things
brighten up when the rain begins? That's due to falling mirrors. Hi, I'm Bryan Yeaton and this
is The Weather Notebook.
We all expect that the pre-rain darkness of an approaching thunderstorm will deepen when the
rain finally begins to fall. But very often, a sudden downpour from a towering cumulonimbus
cloud, comes with an unexpected brightening of the sky.
The intensity of the brightening is determined by the number, the size, and the distribution
of the plummeting raindrops.
During a heavy downpour, countless rapidly falling raindrops surround us. Each one can
reflect, refract and scatter light rays. Reflected light bounces and refracted light bends.
Scattered light rays sort of explode. So, light rays filtering through clouds from all around
are caught within the downpour and therefore bounce around all over the place and then reach
our eyes from all directions. We are, in effect, caught in the fall of a large number of
little mirrors. It's like that trick of putting mirrors on the wall to brighten up a room
instead of buying new lights.
So back outside in the downpour, the number of large drops is much greater than in a light
rainfall. These big drops offer much more potential reflecting surface area for any bit of
sunlight they fall through. As these liquid mirrors get bigger, the air gets brighter, simply
because light rays - reflected, refracted, and scattered come to our eyes from seemingly
everywhere.
Thanks to contributing writer Dr. Keith Heidorn - the Weather Doctor from Victoria, British
Columbia. Support for the Weather Notebook comes from Subaru, Driven By What's Inside, and The
National Science Foundation. We are a production of the Mount Washington Observatory in
beautiful North Conway, New Hampshire.
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