The funnel is 8 inches across but the tube is much thinner. How does that help an observer read a tiny amount of rain, and what happens when the rain will not fit?

collector funnel, 8 in acrossoverflow canmeasuring tube, 20 in tallsupportmeasuring stick, 24 inClose-up of the stick1 mark = 0.01 in of rain0.100.20water is at 0.000 in0.00 in deep in the tube
storm time
0:00:00
rain rate
0.60 in/hr · heavy rain
drops
2.05 mm · fall 22.0 ft/s
rain fallen
0.000 in
water in tube
0.00 in deep
area ratio
50.27 ÷ 5.03 in² = 10.0
overflow can
0.000 in of rain
stick reading
not read yet

Press the steps in order, or press Auto. The storm runs fast: the clock shows storm time. The gauge is cut open at the front so you can see the water inside.

Gauge facts from the National Weather Service (Northern Indiana), The 8-inch Standard Rain Gauge: 8 in funnel with 10 times the area of the tube, 20 in tube that holds 2.00 in, 0.005 in to wet the funnel, overflow can, 24 in stick marked in hundredths, support 10 to 15 in above the ground. Rain rates and drop sizes from the USGS Water Science School (table from Lull, 1959). Example values: the storm times, the 12 in support height, the can height, wall thicknesses, funnel depth and hole size, and the stick width. Drawn to scale in inches with the Anatomy kit (MIT license).