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Have you ever watched the sky darken with the approach of a thunderstorm and then be surprised at how bright the day becomes during the heaviest downpour?
I think we all expect that the pre-rain darkness of an approaching thunderstorm will deepen when the rain finally begins to fall and that it should become darker under a heavy rainfall. But very often, a sudden downpour from a towering cumulonimbus cloud, sparking lightning and booming thunder, comes with an unexpected brightness to the surroundings.
The most important factors that produce a brightening around us are the raindrop numbers and their size distribution.
During a heavy downpour, a large number of rapidly falling raindrops surround us. Each one can reflect, refract and scatter light rays. So, light rays caught within the downpour may undergo multiple reflections rather than just pass us by, and thus, the light reaches our eyes from all directions. We are, in effect, caught in the fall of a large number of little mirrors. And as any interior decorator knows, you can brighten a room with a minimum of light by putting mirrors on walls or ceilings.
In a downpour, the number of large drops is generally much greater than in a light rainfall. Since the surface area of a drop increases as the square of its radius, a drop twice as large has four times the potential reflecting surface area. As these liquid mirrors get bigger, we become more effectively surrounded by reflecting surfaces and this too makes it appear brighter.
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