# How Is Rainfall Measured?

Rainfall is measured with an instrument called a rain gauge. It measures the amount of rain that falls on an area during a given period of time. The simplest kind of rain gauge is a glass tube closed at one end, with a funnel at the top.

Rain enters the funnel and falls into the tube where it can be easily measured. In the United States, rainfall is measured in inches. When you hear the weatherperson say we have had one inch of rain, it means that enough rain has fallen to make a layer of water one inch deep on level ground if none sank in.

How Do Rain Gauges Work?

It’s very easy to build a rain gauge. In fact, there’s a good chance you did it in school at some point. All you have to do is take a straight-sided can with a flat bottom (like a cylinder), make marks up its side every inch, set it out in the open, and wait for rain. If you see that the can has filled with water up to the 1 inch mark, then you know it has rained 1 inch. Up to the 2 inch mark, it has rained 2 inches.

But, you say, what size should the can be? In other words, does its diameter matter? No, any can or container will do.

Why? Think about it: the height of the water in the can is equal to the volume of water in the can divided by the area of the opening. And the volume of water in the can is proportional to the rate of rainfall (measured in something like drops per square inch) times the area of the can. If we put these together, we find that the area of the can cancels from the top and bottom of the ratio—meaning the area doesn’t matter!