In this instructable I discuss a simple rain gauge that is is essentially made from trash.
This is a wonderful project for any group of people. We, possibly, cannot claim that this would be a substitute for professional instrument but as you will see that it is quite accurate. And at places where there are no weather instruments this will give some reasonable indication of rain fall.
Having said this this instrument has lots of educational value. Children learn to be disciplined observers, take readings and learn about the rainfall at their own location.
Step 1: Understanding how Rain Gauge works
What the rainfall measure is telling us is the height of rain water collected in a vessel that has vertical walls and has uniform cross section from bottom to the top.
Therefore, in principle any vessel that has a uniform cross section can be used as a rain gauge. All the vessels will collect rain water to the same height 'h' as shown below. For example you can keep an aquarium in open with a scale pasted to it as a rain gauge. However, from an open rain gauge collected water will evaporate quickly when the rain stops affecting the final reading. Also a lot of dirt, like leafs etc. can fall into it.
The rain fall can be measured in different ways. As said above one can simply measure height 'h' of collected water in a vessel of uniform cross section. One can measure volume of collected water and convert this value into rain fall. More sophisticated rain gauges have electronic devices to automatically record rain fall.