Rain and drizzle are the easiest forms of precipitation to measure.  Rain gauges are used to measure liquid water depth and can be as simple as an open bucket with a consistent cross section throughout.  Meteorologists however, use more accurate instruments and slightly more sophisticated gauges to measure rainfall.
 
 

• Remember that a measurement of precipitation is taken on a flat surface, so that if 1 inch of rain has fallen, it would leave a puddle 1 inch deep in the bottom of a flat open pan or bucket with a consistent cross sectional area.  [This means the bucket can not be tapered].

• Generally amounts of rainfall less than 1 hundredth of an inch (.01") are reported as a trace of rainfall.

 
 

• Below is a standard bucket rain gauge, which magnifies the amount of rainfall through a simple cross sectional area conversion so that more precise measurements can be made, especially for very small amounts of rain.

 
• Typically the cross sectional area of the collector funnel is 10 times the cross sectional area of the inside tube.  This allows for rainfall to be magnified 10 times.
EXAMPLE:  0.1 inches of rain would fill the inside tube with 1 whole inch of water, and a scaled ruler would measure this as 0.1 inches of actual rainfall.
• The larger cross section on the outer can allows for a large amount of rain to be collected and then the smaller cross section on the inside cylinder allows a scaled ruler to be used to measure the exact amount of rainfall down to the nearest hundredth of an inch.
• If more than 6 inches of rain were to fall, the overflow would fall into the larger gray cylinder and would have to be poured into the smaller cylinder to be measured and added to the 6 inches that filled the inner cylinder.

*Imagine how hard it would be to measure to the nearest .01 of an inch in a regular bucket of water with a standard ruler!
 
 
 
 

[The diagram below is not drawn with the standar 10:1 ratio, and is meant to simply illustrate the magnification effect of the larger funnel.]

Another form of rain gauge is the tipping bucket gauge.  This gauge uses a double sided scoop or bucket that pivots when full to dump out the rainfall.  An electronic sensor feels every time the bucket tips each way and records the number of times that the buckets tip.  The buckets hold a known amount of rainfall so that the amount of precipitation can be recorded along with the time of occurence and intensity (amount of rain per unit of time).

• It is often useful to know when the most intense rainfall occured and exactly how intense it was.  For example this information can allow meteorologists to get a feel for how much rain in a short amount of time will cause a flash flood.

 
 

• Yet another form of rain gauge measures the weight of rain that has fallen and translates this to a vertical depth.  This is called a weighing gauge and is very similar in design to the standard bucket rain gauge except for a weighing-scale located at the base.

Click here to continue in sequence and learn about measurement techniques for snow and sleet!

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Click here for the Main Precipitation Page!