Rain Gauges

A rain gauge is a type of instrument used by meteorologists and hydrologists to gather and measure the amount of liquid or freezing precipitation over a set period of time. more...

Most rain gauges generally measure the precipitation in millimetres. The level of rainfall is sometimes reported as inches or centimetres.

Types of rain gauges include graduated cylinders, weighing gauges, tipping bucket gauge, and simple buried pit collectors. Each type has it advantages and disadvantages for collecting rain data.

Rain gauges have their limitations. Attempting to collect rain data in a hurricane can be nearly impossible and unreliable (even if the equipment survives) due to wind extremes. Also, rain gauges only indicate rainfall in a localized area. An extreme example of this is the annual rainfall in Seattle: the official weather station for the city is at Seattle-Tacoma International Airport, the driest part of the city, and the gauge itself was placed, by chance, at the driest part of the airport. Actual annual rainfall for the city is around 254 mm (10 in) greater than the official records indicate.

Another problem encountered is when the temperature is close to or below zero. Rain may fall on the funnel and freeze or snow may collect in the gauge and not permit any subsequent rain to pass through.

Rain gauge amounts are read either manually or by AWS (Automatic Weather Station). The frequency of readings will depend on the requirements of the collection agency. Some countries will supplement the paid weather observer with a network of volunteers to obtain precipitation data (and other types of weather) for sparsely populated areas.

In most cases the precipitation is not retained, however some stations do submit rainfall (and snowfall) for testing, which is done to obtain levels of pollutants.

Rain gauges, like most meteorological instruments, should be placed far enough away from structures and trees to ensure that any effects caused are minimised.

Standard Rain Gauge

The standard rain gauge consists of a funnel attached to a graduated cylinder that fits into a larger container. In most cases the cylinder is marked in mm and in the picture above will measure up to 25 mm (0.98 in) of rainfall. Each horizontal line on the cylinder is 0.2 mm (0.007 in). The larger container collects any rainfall amounts over 25 mm that flows from a small hole near the top of the cylinder. A metal pipe is attached to the container and can be adjusted to ensure the rain gauge is level. This pipe then fits over a metal rod that has been placed in the ground.

Weighing Rain Gauge

One kind of recording rain gauge, the weighing-type recording gauge consists of a storage bin atop a pen recording the changes of weight on a rotating drum. These do not have the disadvantage of underestimating intense rain like tipping bucket recording gauges do, but they are more expensive.

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