According to those who study the microphysics of raindrops, there is an amazing complexity in the size, shape and velocity of rainfall. This kind of information can be found in meteorological journals. I became interested in recording the size and location of raindrops on my own. I found a water sensitive paper available online. The one I use is manufactured by a Swiss company named Syngenta. I believe there are other makers as well. The paper arrives a beautiful vibrant yellow, and maintains this color as long as it is dry. When it is exposed to water, the paper turns blue through chemical reaction. After doing some tests with a DSLR, I realized that I could use video to record the paper as I exposed it to rain. From this information, it is possible to work backwards and hypothesize about how big each drop is, when it landed, how fast it was going, and where it might have been in the air before it landed. The instructions here show how I made a recording instrument that works with GoPro cameras and makes 2" x 36" strips of exposed paper.
Step 1: Step 1: Aluminum Legs / Arms
I worked with a GoPro to figure out the minimum focal length, and maximum width I could record on the Narrow setting. This happened to be about 13 inches for the focal length, and 12" for the width. From this information, I sketched by hand and worked in Autocad to come up with a profile I liked. I then send the dwg file to a waterjet company who cut the pieces for me.
Step 2: Step 2: Acrylic Box
I wanted to maximize the length of the recorded paper strip, but also keep it dry. I built a clear acrylic box to hold the paper, and keep it in a correct orientation to the cameras. I used a table saw, and mill to cut notches in the sides to receive the aluminum assembly, and to be able to slide in strips of the water sensitive paper mounted to aluminum.
Step 3: Step 3: Test!
Wait for the rain, and give it a try. I get the video rolling, open the lid for a few seconds, wait for some blue to show up, and close the lid. The recordings need to be kept dry and photographed or scanned as soon as possible. Now, for the fun data processing part!