A: Steve, Scotts Valley asked us, "how do you measure one-hundredths of an inch of rain fall when rain forms droplets and is not flat?"
Chief Meteorologist Norm Hoffmann has the answer.
The easiest way to measure rainfall is to put a can out in your backyard, in a clear area, and when the rain has stopped drop a ruler or measuring stick into the can and measure how much rain has fallen. However that will get you to the closest tenth of an inch and sometimes a quarter of an inch depending on your measuring stick or ruler.
The standard rain gauge, developed around the start of the 20th century, consists of a funnel attached to a graduated cylinder (2 cm in diameter) that fits inside a larger outside container (20 cm in diameter and 50 cm tall). When measurements are taken, the height of the water in the small graduated cylinder is measured and the excess overflow in the large container is carefully poured into another graduated cylinder and measured to give the total rainfall (see the attached pictures). Each horizontal line on the cylinder represents 0.01 inch. So the cylinder on the top has 10 times more area than the top of the cylinder inside so we can measure down to .01 of an inch. The image is attached above.
So if raindrops are small droplets and not flat when they hit the larger cylinder at the top they eventually collect together and then run down the funnel into the smaller cylinder which can measure .01 of an inch of rainfall.
The other rain gauge that is used is called a tipping bucket rain gauge. Rain is captured in a large funnel (the same as above) and the water runs into a small bucket which can measure .01 of an inch of rainfall. Each time .01 of an inch falls the bucket tips and dumps the water and then starts to refill again. That tip sends a signal and it is recorded in a computer. We add up the tips and measure to total rainfall for 6, 12 or 24 hours. So we can measure down to on hundredth of an inch of rainfall.
If the drops are small, they collect together into larger drops (over time) and then gradually run down into the cylinder or into the tipping bucket and gradually add up to a rainfall total. If the drops are larger drops they just run down the funnel and drop into the smaller cylinder and gradually add to the water in the cylinder.
I hope that answers your question and thanks for asking.