When I hear people talk about so many "inches of rainfall" it drives me crazy. "How can you measure rainfall in inches? It's not one dimensional!", I first thought. Now I understand how it's done, I still think it's dumb.
It's done like this. You collect rainwater. You use a funnel of a certain size and it feeds into a tube. The side of the tube has a reading of so many inches. You can then say that there were so many inches of rainfall in a certain time.
This is dumb because you have to know that the container is of a specific size to understand what an 'inch of water' is. If you want to compare notes with someone else in another country, for example, you have to make sure the containers are exactly the same size. You also then have to make sure that your funnels were the same size, otherwise you have to do some calculations.
The better way is to simply measure rainfall per square metre. That way you can say, "Well, my garden is so many square metres, then it will get so much rain." Instead of saying, "Well, my garden is so many square metres, and I expect so many inches of rain, therefore that works out as...um...I'm sure it'll be fine."
How to calculate rainfall per square metre:
1) Figure the area of the top of your funnel in square centimetres. (A = πr²)
2) Catch some rain!
3) Measure how much rain(R) you catch in millilitres.
4) The amount of rain you catch per square metre is 10R/A litres per square metre.
Example: The top of my funnel has a 5cm radius. It's area is 78.54 square cms.
I catch 100ml of rain. Per square metre, that's (10 x 100)/78.54 litres = 12.7 litres.
Easy, right? Obviously to really be of any use, you'll have to measure rainfall daily or weekly for about a year and use the results to estimate what the rainfall might be like the following year. Personally, that's way too boring for me. I just work with whatever the weather throws at me. But a rant against measuring rain in inches was deserved. Because it's dumb. And it's dumb.