Rain gauge

A rain gauge is a type of instrument used by meteorologists and hydrologists to gather and measure the amount of liquid or solid (snow, sleet, hail) 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 freezing. 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.

History

The world's first Rain Gauge is said to be have developed in 1441 in Joseon Dynasty of Korea, by King Munjong. He sent a rain gauge to every village, and they were used as an official tool to measure the farm's potential harvest. Munjong also used these measurements to determine what the farmer's land taxes should be. Development of Rain Gauge came two hundred years earlier before inventor Christopher Wren created a rain gauge in Europe. [1] [2]

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.

The weighing-type recording gauge also contains a device to measure the quantity of chemicals contained in the locations atmosphere. This is extremely helpful for scientits studying the effects of greenhouse gases released into the atmophere and their effects on the levels of acid rain.

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 one 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 only 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.

Modern tipping rain gauges consist of a plastic collector balanced over a pivot. When it tips, it actuates a switch (such as a magnet/reed switch) which is then electronically recorded or transmitted to a remote collection station.

Note that tipping gauges can incorporate weighing gauges for the best of both. In these, a strain gauge is fixed to the collection bucket so that the exact rainfall can be read at any moment. Each time the collector tips, the strain gauge (weight sensor) is re-zeroed to null out any drift.

Optical Rain Gauge

These have a row of collection funnels. In an enclosed space below each is a laser diode and a phototransistor detector. When enough water is collected to make a single drop, it drips from the bottom, falling into the laser beam path. The sensor is set at right angles to the laser so that enough light is scattered to be detected as a sudden flash of light. The flashes from these photodetectors are then read and transmitted or recorded.