Rain Gauges
A rain gauge (also known as a udometer or a pluviometer or an ombrometer) is a type of instrument used by meteorologists to gather and measure the amount of liquid precipitation over a set period of time.
History The first known rainfall records were kept by the Ancient Greeks, about 500 B.C. This was followed 100 years later by people in India using bowls to record the rainfall. The readings from these were correlated against expected growth, and used as a basis for land taxes. In the Arthashastra, used for example in Magadha, precise standards were set as to grain production. Each of the state storehouses were equipped with a standardised rain gauge to classify land for taxation purposes.
Principles Most rain gauges generally measure the precipitation in millimeters.The level of rainfall is sometimes reported as inches or centimeters.
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. 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 freeze or snow may collect in the gauge and not permit any subsequent rain to pass through.
Rain gauges should be placed in an open area where there are no obstructions, such as building or trees, 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.
Types Types of rain gauges include graduated cylinders, weighing gauges, tipping bucket gauges, and simple buried pit collectors. Each type has its advantages and disadvantages for collecting rain data.
History The first known rainfall records were kept by the Ancient Greeks, about 500 B.C. This was followed 100 years later by people in India using bowls to record the rainfall. The readings from these were correlated against expected growth, and used as a basis for land taxes. In the Arthashastra, used for example in Magadha, precise standards were set as to grain production. Each of the state storehouses were equipped with a standardised rain gauge to classify land for taxation purposes.
Principles Most rain gauges generally measure the precipitation in millimeters.The level of rainfall is sometimes reported as inches or centimeters.
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. 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 freeze or snow may collect in the gauge and not permit any subsequent rain to pass through.
Rain gauges should be placed in an open area where there are no obstructions, such as building or trees, 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.
Types Types of rain gauges include graduated cylinders, weighing gauges, tipping bucket gauges, and simple buried pit collectors. Each type has its advantages and disadvantages for collecting rain data.
The standard NOAA rain gauge, developed around the start of the 20th century, consists of a funnel attached to a graduated cylinder (2 cm in diameter) that fits inside a larger outside container (20 cm in diameter and 50 cm tall).
A weighing-type precipitation gauge consists of a storage bin, which is weighed to record the mass.The advantages of this type of gauge are that it does not underestimate intense rain, and it can measure other forms of precipitation, including rain, hail and snow. These gauges are, however, more expensive and require more maintenance than tipping bucket gauges.
The tipping bucket rain gauge consists of a funnel that collects and channels the precipitation into a small seesaw-like container. After a pre-set amount of precipitation falls, the lever tips, dumping the collected water and sending an electrical signal. An old-style recording device may consist of a pen mounted on an arm attached to a geared wheel that moves once with each signal sent from the collector. In this design, 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.
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.
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.