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A rain gauge (also known as udometer, ombrometer, pluviometer and hyetometer) is an instrument used by and Hydrology to gather and measure the amount of liquid precipitation in a predefined area, over a set period of time. It is used to determine the depth of precipitation (usually in mm) that occurs over a unit area and measure rainfall amount.
In 1441, the Cheugugi was invented during the reign of Sejong the Great of the Joseon of Korea as the first standardized rain gauge. 측우기 測雨器, Naver encyclopedia In 1662, Christopher Wren created the first tipping-bucket rain gauge in United Kingdom in collaboration with Robert Hooke. Robert Hooke also designed a manual gauge with a funnel that made measurements throughout 1695. Richard Towneley was the first to make systematic rainfall measurements over a period of 15 years from 1677 to 1694, publishing his records in the Philosophical Transactions of the Royal Society. Towneley called for more measurements elsewhere in the country to compare the rainfall in different regions, although only William Derham appears to have taken up Towneley's challenge. They jointly published the rainfall measurements for Towneley Park and Upminster in Essex for the years 1697 to 1704.
The naturalist Gilbert White took measurements to determine the mean rainfall from 1779 to 1786, although it was his brother-in-law, Thomas Barker, who made regular and meticulous measurements for 59 years, recording temperature, wind, barometric pressure, rainfall and clouds. His meteorological records are a valuable resource for knowledge of the 18th-century British climate. He was able to demonstrate that the average rainfall varied greatly from year to year with little discernible pattern.
Due to the ever-increasing numbers of observers, standardisation of the gauges became necessary. Symons began experimenting with new gauges in his own garden. He tried different models with variations in size, shape, and height. In 1863 he began a collaboration with Michael Foster Ward Colonel Michael Foster Ward from Calne, Wiltshire, who undertook more extensive investigations. By including Ward and various others around Britain, the investigations continued until 1890. The experiments were remarkable for their planning, execution, and drawing of conclusions. The results of these experiments led to the progressive adoption of the well-known standard gauge, still used by the UK Met Office today, namely, one made of "... copper, with a five-inch funnel having its brass rim one foot above the ground ...":A short history of the British Rainfall Organisation by DE Pedgley, Sept 2002, published by The Royal Meteorological Society
Most modern rain gauges generally measure the precipitation in in height collected during a certain period, equivalent to per square metre. Previously rain was recorded as inches or points, where one point is equal to 0.254 mm or 0.01 of an inch.
Rain gauge amounts are read either manually or by automatic weather station (AWS). 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, but some stations do submit rainfall and snowfall for testing, which is done to obtain levels of pollutants.
Rain gauges have their limitations. Attempting to collect rain data in a tropical cyclone 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. For virtually any gauge, drops will stick to the sides or funnel of the collecting device, such that amounts are very slightly underestimated, and those of .01 inches or .25 mm may be recorded as a "trace".
Another problem encountered is when the temperature is close to or below freezing. Rain may fall on the funnel and ice or snow may collect in the gauge, blocking subsequent rain. To alleviate this, a gauge may be equipped with an automatic electric heater to keep its moisture-collecting surfaces and sensor slightly above freezing.
Rain gauges should be placed in an open area where there are no buildings, trees, or other obstacles to block the rain. This is also to prevent the water collected on the roofs of buildings or the leaves of trees from dripping into the rain gauge after a rain, resulting in inaccurate readings. Rain gauges can help people comprehend the amount of precipitation fallen down in a certain period of time.
Each horizontal line on the cylinder is . In designs made for areas using Imperial units, each horizontal line represents inches.
It consists of a rotating drum that rotates at constant speed, this drum drags a graduated sheet of cardboard, which has the time at the abscissa while the y-axis indicates the height of rainfall in rain. This height is recorded with a pen that moves vertically, driven by a buoy, marking on the paper the over time. Each cardboard sheet is usually used for one day.
As the rain falls, the water collected by the funnel falls into the container and raises the buoy that makes the pen arm rise in the vertical axis, marking the cardboard accordingly. If the rainfall does not vary, the water level in the container remains constant, and while the drum rotates, the pen's mark is more or less a horizontal line, proportional to the amount of water that has fallen. When the pen reaches the top edge of the recording paper, it means that the buoy is "up high in the tank" leaving the tip of the conical needle in a way that uncovers the regulating hole, i.e., the maximum flow that the apparatus is able to record. If the rain suddenly decreases, making the container (as it empties) quickly lower the buoy, that movement corresponds to a steep slope line that can reach the bottom of the recorded cardboard if it stops raining.
The rain gauge of intensities allowed precipitation to be recorded over many years, particularly in Barcelona (95 years), apart from many other places around the world, such as Hong Kong.
The weighing-type recording gauge may also contain a device to measure the number of chemicals contained in the location's atmosphere. This is extremely helpful for scientists studying the effects of greenhouse gases released into the atmosphere and their effects on the levels of the acid rain. Some Automated Surface Observing System (ASOS) units use an automated weighing gauge called the AWPAG (All Weather Precipitation Accumulation Gauge).
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 a pre-set amount has fallen when only a fraction of that amount has actually fallen. Tipping buckets also tend to underestimate the amount of rainfall, particularly in snowfall and heavy rainfall events.Groisman, P.Y. (1994): " The Accuracy of United States Precipitation Data" Bulletin of the American Meteorological Society 75(2): 215–227. 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) by counting the number of pulses during that period. Algorithms may be applied to the data as a method of correcting the data for high-intensity rainfall.
Modern tipping rain gauges consist of a plastic collector balanced over a pivot. When it tips, it actuates a switch (such as a reed switch) which is then electronically recorded or transmitted to a remote collection station.
Tipping gauges can also incorporate elements of weighing gauges whereby 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.
To measure the water equivalent of frozen precipitation, a tipping bucket may be heated to melt any ice and snow that is caught in its funnel. Without a heating mechanism, the funnel often becomes clogged during a frozen precipitation event, and thus no precipitation can be measured. Many Automated Surface Observing System (ASOS) units use heated tipping buckets to measure precipitation." The Tipping Bucket Rain Gauge ." National Weather Service.
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