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How Is Rainfall Measured?

How is rainfall measured? What does it mean when it rains an inch? And how many gallons of water fall on your roof in a storm? Keep on reading to learn all about the math behind April showers and May flowers.

By
Jason Marshall, PhD
April 25, 2014
Episode #196

Page 1 of 2

Red UmbrellaYou know the old saying about April showers bringing May flowers? Well, it looks like that ain't happening in my neck of the California woods this year. In fact, we're entrenched in a rather undrenched rut—it's hardly rained at all this season, and thus far April has been a complete shut-out.

But there's always hope that things may yet turn around and bring us those proverbial April showers and May flowers. And although I may be living through a dryer than usual April, I'm sure that many of you have been enjoying inches and inches of rain. 

Which might make you wonder: What does it mean when it rains an inch? How is rainfall measured? How can you measure it yourself? And how much rain falls on your roof in a big storm? Those are exactly the questions we'll be talking about today.

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How Is Rainfall Measured?

As you probably know, rainfall amounts in the United States are typically measured in inches. Actually, although we usually just say "inches," we really mean "inches in the storm" or "inches in the last 24 hours" or "inches in some time period." Why does that matter? Well, obviously 1 inch of rain in a 15 minute period is a lot more water than 1 inch of rain in the last month. So we really don't know how much it's rained if we don't know the time period we're talking about.

So what does 1 inch, 2 inches, or whatever number of inches of rain in some time period mean? Well, if all the rain that falls stays right where it lands—meaning it doesn't run off and accumulate in streams and rivers and eventually in lakes and oceans, and it isn't absorbed into the ground—then 1 inch of rain in an area is enough to evenly cover the ground in that area with a layer of water 1 inch deep. Of course, water typically does run into streams and is absorbed into the ground, so 1 inch of rain rarely means an inch of standing water. 

But whether or not we actually see 1 inch of water on the ground for each inch of rain, we can use this definition to construct a device that measures how much it has rained—a so-called rain gauge.

How Do Rain Gauges Work?

Rain GaugeIt'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? Nope, any can 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!

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