The question is pretty self-explanatory, I guess. Outside right now a gentle summer rain is falling, and I was in the living room talking to my baby daughter. I was telling her that the soft sound she hear outside was the falling of “hundreds…hundreds of thousands…millions, I guess, of raindrops!”

I realized I had no idea how many raindrops fell in an average storm, much less on an average day. Yet this question seems to me as if, given our current state of knowledge, it should be answerable.

Probably the question will need further definition. I’m not really sure how to define it, so if you know how, feel free to set up some reasonable limits. (For example, it may be necessary to define it as an average over the past ten years of raindrops at least 1 gram in weight that hit the ground or a body of water–I dunno).

Well a gram-sized raindrop would be a huge rain drop. A gram of water is one cubic centimeter. Estimates I’ve seen say the average raindrop is somewhere from half a milligram to 100 milligrams

The most important thing you’d want to specify is where, I think, – in your yard, your town, your country, the world? To keep it simple let’s say your yard.

Find out the annual rainfall in your area. (Note you may get precipitation rather than rain so you’ll have to decide if you want to count snow or not. If not figure what fraction of the precipitation comes as rain.) If you’re in the US you’ll probably get that in inches. About 30 inches precipitation is typical for the US. Say maybe 20 inches of rain.

Multiply by 2.54 to get that in centimeters. About 50 cm or half a meter

Multiply by the size of your yard. Say about a quarter acre which is 1000 sq meters so we have 500 cubic meters of rain in your yard. That’s a half billion cubic centimeters which is a half billion grams.

At the largest size estimate a raindrop is a tenth of a gram so it’s more than 5 billion raindrops in your yard in a year.

Nice, Oldguy! That’s the kind of thing I’m looking for.

The one definition I’d like to make myself would be the scope: I intended to ask about the number of raindrops that fall every day on Earth. How would your calculations extend to that, do you know?

Well we’d divide by 365 to get the average for one day. We used 1000 sq meters as the size of your yard. The total area of the earth is 531 million sq kilometers or 53110^12 sq meters which is 53110^9 times as big as your yard.

So if your yard were typical there would be 510^953110^9/365 = 710^18 or 7 million trillion (7 quintillion if you prefer) raindrops. I suspect 30 inches per year is too high for the average rainfall all over earth, but we’ve already used the biggest estimate of a rain drop size so that may not be too bad.