My idea is this: set up a means of detecting how many raindrops are illuminated by a beam over X distance, let's say 10 meters. Let us further assume that the diameter of a raindrop for this experiment is 2 millimeters, about the largest it can get and still fall as a near-perfect sphere. That gives us a maximum of 5000 raindrops over a distance of 10 meters, which wouldn't ever happen at once. So, for every 2mm, if we treat it as either illuminated or unilluminated, we can have a single binary digit, say 0 for unlit, and 1 for lit. Thus, at any one instant, we'd have a random binary string of 5000 characters (that is, 5000 random 0s and 1s in a big long row) which would most likely change several dozen times a second, let's call it twenty times per second.
If there were a means of detection/counting which binary bit is illuminated at any given instant, and updating that data twenty times a second, a minute's worth of rainfall at a constant rate would give us 6 million bits of binary data over a single minute's span. Whether you can create an algorithm of that data and encrypt something with cipher that changes 20 times a second, or use a mammoth cipher that's about 6 million binary characters (all of these numbers are by my admittedly crude reckoning), you could have a decent means of encryption either way. And it's scaleable, by extending or shortening the monitored portion of the laser beam. You could have resolution narrowed to 0.5 meters, or extended to a kilometer with a powerful enough laser and a reliable enough means of detecting the illumination of the raindrops along it's path.
This is the random shit I think of. Any math/computer/crypto geeks out there who can tell me if this has any conceivable use whatsoever? Or know anyone who does? Oh, and if you sell it to the military, I expect a cut of the dough.