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Standard Rain Gauge
Tipping Bucket Rain Gauge Recorder
Close up of a Tipping Bucket Rain Gauge Recorder chart
The exterior of a Tipping Bucket Rain Gauge
The interior of a Tipping Bucket Rain GaugeA 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.
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 guage 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.
Tipping Bucket Rain Gauge
The tipping bucket rain gauge consists of a large copper cylinder set into the ground. At the top of the cylinder is a funnel that collects and channels the precipitation. The precipitation falls onto on of two levers which are balanced in same manner as a scale. After an amount of precipitation equal to 0.2 mm (0.007 in) falls the lever tips and an electrical signal is sent to the recorder. The recorder consists of a pen mounted on an arm attached to a geared wheel that moves once with each signal sent from the collector. When the wheel turns the pen arm moves either up or down leaving a trace on the graph and at the same time making a loud click. Each jump of the arm is sometimes referred to as a 'click' in reference to the noise. The chart is measured in 10 minute periods (vertical lines) and 0.4 mm (0.015 in) (horizontal lines) and rotates once every 24 hours and is powered by a clockwork motor that must be manually wound.
The tipping bucket rain gauge is not as accurate as the standard rain gauge because the rainfall may stop before the lever has tipped. When the next period of rain begins it may take no more than one or two drops to tip the lever. This would then indicate that 0.2 mm (0.007 in) has fallen when in fact on a minute amount has. The advantage of the tipping bucket rain gauge is that the character of the rain (light, medium or heavy) may be easily obtained. Rainfall character is decided by the total amount of rain that has fallen in a set period (usually 1 hour) and by counting the number of 'clicks' in a 10 minute period the observer can decide the character of the rain.
See also: disdrometer, millimetre