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Falling Mirrors
Fri Mar 25, 2005
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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