|MadSci Network: Environment/Ecology|
As it turns out, it doesn't matter. Any container with a flat bottom and vertical walls will serve as a rain gauge. If a skinny can and a wide bucket --- both with vertical walls --- are left outdoors in a storm that pours, for example, "one inch" of rain, each will get one inch of water in it. This is not so surprising if you consider that you could cover most of the bottom of the bucket with skinny cans.
To get more accurate measurements of small amounts of rainfall, many rain gauges have tapered walls, so that they are thinner at the bottom than at the top. If you see such a rain gauge, you'll notice that the markings are not evenly spaced. That's because the relationship between the volume of water and the depth is not so simple as when the walls are straight. The number marked at any particular point on the gauge is simply the volume of the gauge below that level divided by the area of the hole at the top of the gauge.
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