Falling Mirrors
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Have you ever watched the sky darken when a thunderstorm approaches only to see things brighten up when the rain begins? Well it's due to falling mirrors. Hi, I'm Dave Thurlow and this is The Weather Notebook.

I think we all expect that the pre-rain darkness of an approaching thunderstorm will deepen when the rain finally begins. 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 all directions.

Thanks to contributing writer Dr. Keith Heidorn - the Weather Doctor from Victoria, British Columbia. Support for the Weather Notebook comes from Subaru, the beauty of all wheel drive, and is a production of the Mount Washington Observatory in beautiful North Conway New Hampshire.