How Does Your Rain Gauge Work?

When it rains, your gauge measures the amount of precipitation that falls through the area at the top of the gauge. When you read the gauge, you measure the depth of water that has fallen through the area and accumulated in the bottom, that is the depth of water.

How, you may ask, can this gauge work properly if the inner tube is 10 inches long but only records 1 inch depth of water? The reason has to do with accuracy. The National Weather Service (NWS) has adopted the criteria that the gauge should be able to measure to an accuracy of 0.01". The problem with a gauge that's 4" (CoCoRaHS) or 8" (NWS standard) in diameter, is that it's nearly impossible to read the depth to an accuracy of 0.01". That's where the funnel and inner tube come in.

The funnel of the CoCoRaHS gauge squeezes the water into the area of the inner tube, which is 1/10th of the area of outer cylinder (The NWS gauge has a similar funnel and inner tube). By reducing the area that the water falls into, the depth can be stretched by the same factor of 10. In this way, the total volume of water (area times depth) that fell through the top of the gauge and the total volume in the inner tube are the same. This stretching allows us to read the depth of water an accuracy of 0.01".

Incidentally, the Fort Collins, Colorado weather station has a CoCoRaHS and NWS gauge side-by-side and has been keeping track of their measurements for a number of years. The results show that both gauges record very similar amounts of precipitation. View the abstract from the AMS 15th Conference on Applied Climatology/13th Symposium on Meteorological Observations and Instrumentation (2005): "GAUGE".