Meteorological instrumentation

Galileo thermometer

Meteorological instrumentation is the equipment used to sample the state of the atmosphere at a given time. Each science has its own unique sets of laboratory equipment. However, meteorology is a science which does not use much lab equipment but relies more on field-mode observation equipment. In science, an observation, or observable, is an abstract idea that can be measured and for which data can be taken. Rain was one of the first quantities to be measured historically. Two other accurately measured weather-related variables are wind and humidity. Many attempts had been made prior to the 15th century to construct adequate equipment to measure atmospheric variables. The devices to measure these three sprang up in the mid-15th century and were respectively the rain gauge, the anemometer, and the hygrometer. The 17th century saw the development of the barometer and the Galileo thermometer, while the 18th century saw the development of the thermometer with the Fahrenheit and Celsius scales. The 20th century developed new remote sensing tools, such as weather radars and weather satellites, which provide better sampling both regionally and globally. Remote sensing instruments collect data from remote weather events and subsequently producing weather information. Each remote sensing instruments collects data about the atmosphere from a remote location and, usually, stores the data where the instrument is located.

Contents

[hide] 1 History of measurement and scales 2 Types 3 Weather stations 4 Surface weather observations

5 References

[edit] History of measurement and scales

A hemispherical cup anemometer

In 1441, King Sejongs son, Prince Munjong, invented the first standardized rain gauge. These were sent throughout the Joseon Dynasty of Korea as an official tool to assess land taxes based upon a farmer's potential harvest. In 1450, Leone Battista Alberti developed a swinging-plate anemometer, and is known as the first anemometer.[1] In 1607, Galileo Galilei constructs a thermoscope. In 1643, Evangelista Torricelli invents the mercury barometer.[1] In 1662, Sir Christopher Wren invented the mechanical, self-emptying, tipping bucket rain gauge. In 1714, Gabriel Fahrenheit creates a reliable scale for measuring temperature with a mercury-type thermometer.[2] In 1742, Anders Celsius, a Swedish astronomer, proposed the 'centigrade' temperature scale, the predecessor of the current Celsius scale.[3] In 1783, the first hair hygrometer is demonstrated by Horace-Bénédict de Saussure. In 1806, Francis Beaufort introduced his system for classifying wind speeds.[4] The April 1960 launch of the first successful weather satellite, TIROS-1, marked the beginning of the age where weather information became available globally.

[edit] Types

Modern aneroid barometerSee also: List of weather instruments

A thermometer measures air temperature, or the kinetic energy of the molecules within air. A barometer measures atmospheric pressure, or the pressure exerted by the weight of the Earth's atmosphere above a particular location. An anemometer measures the wind speed and the direction the wind is blowing from at the site where it is mounted. A hygrometer measures the relative humidity at a location, which can then be used to compute the dew point. Radiosondes directly measure most of these quantities, except for wind, which is determined by tracking the radiosonde signal with an antenna or theodolite. Supplementing the radiosondes a network of aircraft collection is organized by the World Meteorological Organization, which also use these instruments to report weather conditions at their respective locations. A sounding rocket, sometimes called a research rocket, is an instrument-carrying rocket designed to take measurements and perform scientific experiments during its sub-orbital flight.

A pyranometer is a type of actinometer used to measure broadband solar irradiance on a planar surface and is a sensor that is designed to measure the solar radiation flux density (in watts per metre square) from a field of view of 180 degrees. A ceilometer is a device that uses a laser or other light source to determine the height of a cloud base. Ceilometers can also be used to measure the aerosol concentration within the atmosphere. A ceiling balloon is used by meteorologists to determine the height of the base of clouds above ground level during daylight hours. The principle behind the ceiling balloon is a balloon with a known ascent rate (how fast it climbs) and determining how long the balloon rises until it disappears into the cloud. Ascent rate times ascent time yields the ceiling height. A disdrometer is an instrument used to measure the drop size distribution and velocity of falling hydrometeors. Some disdrometers can distinguish between rain, graupel, and hail. Rain gages are used to measure the precipitation which falls at any point on the Earth's landmass.

Remote sensing, as used in meteorology, is the concept of collecting data from remote weather events and subsequently producing weather information. Each remote sensing instruments collects data about the atmosphere from a remote location and, usually, stores the data where the instrument is located. The common types of remote sensing are Radar, Lidar, and satellites (or photogrammetry). The main uses of radar are to collect information concerning the coverage of precipitation and wind. Satellites are chiefly used to determine cloud cover, as well as wind. SODAR (SOnic Detection And Ranging) is a meteorological instrument also known as a wind profiler which measures the scattering of sound waves by atmospheric turbulence. SODAR systems are used to measure wind speed at various heights above the ground, and the thermodynamic structure of the lower layer of the atmosphere. RADAR and LIDAR are not passive because both use EM radiation to illuminate a specific portion of the atmosphere.[5] Weather satellites along with more general-purpose Earth-observing satellites circling the earth at various altitudes have become an indispensable tool for studying a wide range of phenomena from forest fires to El Niño.

[edit] Weather stations

A weather station is a facility with instruments and equipment to make observations of atmospheric conditions in order to provide information to make weather forecasts and to study the weather and climate. The measurements taken include temperature, barometric pressure, humidity, wind speed, wind direction, and precipitation amounts. Wind measurements are taken as free of other obstructions as possible, while temperature and humidity measurements are kept free from direct solar radiation, or insolation. Manual observations are taken at least once daily, while automated observations are taken at least once an hour.

[edit] Surface weather observations

Main article: Surface weather observation

Weather station at Mildura Airport, Victoria, Australia.

Surface weather observations are the fundamental data used for safety as well as climatological reasons to forecast weather and issue warnings worldwide.[6] They can be taken manually, by a weather observer, by computer through the use of automated weather stations, or in a hybrid scheme using weather observers to augment the otherwise automated weather station. The ICAO defines the International Standard Atmosphere, which is the model of the standard variation of pressure, temperature, density, and viscosity with altitude in the Earth's atmosphere, and is used to reduce a station pressure to sea level pressure. Airport observations can be transmitted worldwide through the use of the METAR observing code. Personal weather stations taking automated observations can transmit their data to the United States mesonet through the use of the Citizen Weather Observer Program (CWOP), or internationally through the Weather Underground Internet site.[7] A thirty-year average of a location's weather observations is traditionally used to determine the station's climate.[8]

[edit] References

1. ^ a b Jacobson, Mark Z. (June 2005) (paperback). Fundamentals of Atmospheric Modeling (2nd ed.). New York: Cambridge University Press. pp. 828. ISBN 9780521548656.

2. ̂ Grigull, U., Fahrenheit, a Pioneer of Exact Thermometry. Heat Transfer, 1966, The Proceedings of the 8th International Heat Transfer Conference, San Francisco, 1966, Vol. 1.

3. ̂ Beckman, Olof, History of the Celsius temperature scale., translated, Anders Celsius (Elementa,84:4,2001); English

4. ̂ Bill Giles O.B.E. (2009). Beaufort Scale. BBC. Retrieved on 2009-05-12. 5. ̂ Peebles, Peyton, [1998], Radar Principles, John Wiley & Sons, Inc., New York, ISBN

0-471-25205-0. 6. ̂ Office of the Federal Coordinator of Meteorology. Surface Weather Observation

Program. Retrieved on 2008-01-12. 7. ̂ Weather Underground. Personal Weather Station. Retrieved on 2008-03-09. 8. ̂ MetOffice. Climate Averages. Retrieved on 2008-03-09.

Retrieved from "http://en.wikipedia.org/wiki/Meteorological_instrumentation"

SIX'S MAXIMUM AND MINIMUM THERMOMETER

Six's maximum and minimum thermometer is a popular thermometer among gardeners for use in greenhouses. Its purpose is to record the maximum and minimum temperatures reached since the thermometer was last read. Generally speaking a minimum temperature occurs during the night and a maximum during the day. It was invented by James Six towards the end of the eighteenth century, and consists of a fairly large cylindrical bulb full of alcohol, or oil of creosote, connected by a U- shaped stem to a second bulb nearly full of alcohol or oil of creosote. The bend of the U contains a thread of mercury. Two scales are provided, one against each limb of the tube so that the temperature may be read against either of the mercury levels. Resting on each of the mercury surfaces are small steel indexes provided with light springs to hold them in position in the stem. Expansion or contraction of the fluid in the larger bulb causes a movement of the mercury thread. Consequently, one or other index is pushed forward by the mercury and left in the extreme position reached. Thus, the lower end of the index on the left indicates the minimum and that on the right the maximum temperature attained. It is interesting to note that Six's maximum and minimum thermometers were still being used in 2000 of exactly the same design and construction as ones produced over 100 years ago.

Mercury barometers

A mercury barometer has a glass tube of at least 33 inches (about 84 cm) in height, closed at one end, with an open mercury-filled reservoir at the base. The weight of the mercury

actually creates a vacuum in the top of the tube. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir. Since higher temperature at the instrument will reduce the density of the mercury, the scale for reading the height of the mercury is adjusted to compensate for this effect.

Torricelli documented that the height of the mercury in a barometer changed slightly each day and concluded that this was due to the changing pressure in the atmosphere [9] . He wrote: "We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight".

The mercury barometer's design gives rise to the expression of atmospheric pressure in inches or millimeters (torr): the pressure is quoted as the level of the mercury's height in the vertical column. 1 atmosphere is equivalent to about 29.9 inches, or 760 millimeters, of mercury. The use of this unit is still popular in the United States, although it has been disused in favor of SI or metric units in other parts of the world. Barometers of this type normally measure atmospheric pressures between 28 and 31 inches of mercury.

Design changes to make the instrument more sensitive, simpler to read, and easier to transport resulted in variations such as the basin, siphon, wheel, cistern, Fortin, multiple folded, stereometric, and balance barometers. Fitzroy barometers combine the standard mercury barometer with a thermometer, as well as a guide of how to interpret pressure changes. Fortin barometers use a variable displacement mercury cistern, usually constructed with a thumbscrew pressing on a leather diaphragm bottom. This compensates for displacement of mercury in the column with varying pressure. To use a Fortin barometer, the level of mercury is set to the zero level before the pressure is read on the column. Some models also employ a valve for closing the cistern, enabling the mercury column to be forced to the top of the column for transport. This prevents water-hammer damage to the column in transit.

On June 5, 2007, a European Union directive was enacted to restrict the sale of mercury, thus effectively ending the production of new mercury barometers in Europe.

[edit] Aneroid barometers

See also: Barograph

Old aneroid barometer

Modern aneroid barometer

An aneroid barometer uses a small, flexible metal box called an aneroid cell. This aneroid capsule (cell) is made from an alloy of beryllium and copper.[10] The evacuated capsule (or usually more capsules) is prevented from collapsing by a strong spring. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer. Many models include a manually set needle which is used to mark the current measurement so a change can be seen. In addition, the mechanism is made deliberately 'stiff' so that tapping the barometer reveals whether the pressure is rising or falling as the pointer moves. It also was invented by Blaise Pascal.

Weather vaneFrom Wikipedia, the free encyclopedia

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Weather vane and bell on the roof of the Cathedral Saint-Étienne of Bourges (France)

A weather vane is an instrument for showing the direction of the wind. They are typically used as an architectural ornament to the highest point of a building.

Although partly functional, weather vanes are generally decorative, often featuring the traditional cockerel design with letters indicating the points of the compass. Other common motifs include ships, arrows and horses. Not all weather vanes have pointers.

The word 'vane' comes from the Anglo-Saxon word 'fane' meaning 'flag'.

Contents

[hide] 1 Operation 2 History 3 Slang term 4 See also 5 References 6 Further reading

7 External links

[edit] Operation

A pig weather vane on a barbecue restaurant in North Carolina

"Sighting the Course for Seacoast New Hampshire" NH Welcome Center, Seabrook, New Hampshire, 1-96; By William Barth Osmundsen, a NH Percent for Art Award.

Wind vane on a church in Norway

The design of a wind vane is such that the weight is evenly distributed each side of the surface, but the surface area is unequally divided, so that the pointer can move freely on its axis. The side with the larger area is blown away from the wind direction. The pointer is therefore always on the smaller side (a north wind is one that blows from the north). Most wind vanes have directional markers beneath the arrow, aligned with the geographic directions.

Wind vanes, especially those with fanciful shapes, do not always show the real direction of a very gentle wind. This is because the figures do not achieve the necessary design balance: an unequal surface area but balanced in weight.

To obtain an accurate reading, the wind vane must be located well above the ground and away from buildings, trees, and other objects which interfere with the true wind direction. Changing wind direction can be meaningful when coordinated with other apparent sky conditions, enabling the user to make simple short range forecasts. From the street level the size of many weathercocks is deceptive. [1]

[edit] History

The Tower of the Winds

The Tower of the Winds on the ancient Roman agora in Athens once bore on its roof a wind vane in the form of a bronze Triton holding a rod in his outstretched hand, rotating as the wind changed direction. Below, the frieze was adorned with the eight wind deities. The eight metre high structure also featured sundials, and a water clock inside dates from around 50 BC.[2]

The wind vane evolved from a Triton to a weathercock as the Roman Empire converted to Christianity. Many churches have a weathercock on the tower or spire. The cock refers to the fall of St Peter and to intimate the necessity for watchfulness and humility.

Functional modern wind vane

Early weather vanes had very ornamental pointers, but modern wind vanes are usually simple arrows that dispense with the directionals because the instrument is connected to a remote reading station. Modern aerovanes combine the directional vane with an anemometer (a device for measuring wind speed). Co-locating both instruments allows them to use the same axis (a vertical rod) and provides a coordinated readout.

World's largest weather vane in Jerez, Spain

Another wind direction device is the windsock used at airports to show wind direction and strength. The wind fills the sock and makes it blow away from the prevailing wind. Strong winds make the sock point almost horizontally, while light airs allow the sock to hang limply. Because of its size, the windsock can often be seen from the air as well as the ground. Even the most technologically-advanced airports still use windsocks.

According to the Guinness World Records, the world's largest weather vane is located in Jerez, Spain. A challenger for the title of world's largest weather vane is located in Whitehorse, Yukon. The weather vane is a retired Douglas DC-3 atop a swiveling support. Located beside Whitehorse International Airport, the weather vane is used mainly by pilots to determine wind direction. The weather vane only requires a 5 km/hour wind to rotate. [3]

An anemometer is a device for measuring the wind speed, and is one instrument used in a weather station. The term is derived from the Greek word anemos, meaning wind. The first known description of an anemometer was given by Leon Battista Alberti in around 1450[1].

Anemometers can be divided into two classes: those that measure the wind's velocity, and those that measure the wind's pressure; but as there is a close connection between the pressure and the velocity, an anemometer designed for one will give information about both.

Contents

[hide] 1 Velocity anemometers

o 1.1 Cup anemometers o 1.2 Windmill anemometers o 1.3 Hot-wire anemometers o 1.4 Laser Doppler anemometers o 1.5 Sonic anemometers o 1.6 Ping-pong ball anemometers

2 Pressure anemometers o 2.1 Plate anemometers o 2.2 Tube anemometers o 2.3 Effect of density on measurements

3 See also 4 Notes 5 References

6 External links

[edit] Velocity anemometers

[edit] Cup anemometers

A simple type of anemometer is the cup anemometer, invented (1846) by Dr. John Thomas Romney Robinson, of Armagh Observatory. It consisted of four hemispherical cups each mounted on one end of four horizontal arms, which in turn were mounted at equal angles to each other on a vertical shaft. The air flow past the cups in any horizontal direction turned the cups in a manner that was proportional to the wind speed. Therefore, counting the turns of the cups over a set time period produced the average wind speed for a wide range of speeds. On an anemometer with four cups it is easy to see that since the cups are arranged symmetrically on the end of the arms, the wind always has the hollow of one cup presented to it and is blowing on the back of the cup on the opposite end of the cross.

When Robinson first designed his anemometer, he wrongly claimed that no matter how big the cups or how long the arms, the cups always moved with one-third of the speed of the wind. This was apparently confirmed by some early independent experiments, but it was very far from the truth. It was later discovered that the actual relationship between the speed of the wind and that of the cups, called the anemometer factor, depended on the

dimensions of the cups and arms, and may have a value between two and a little over three. Every single experiment involving an anemometer had to be done all over again.

The three cup anemometer developed by the Canadian John Patterson in 1926 and subsequent cup improvements by Brevoort & Joiner of the USA in 1935 led to a cupwheel design which was linear and had an error of less than 3% up to 60 mph. Patterson found that each cup produced maximum torque when it was at 45 degrees to the wind flow. The three cup anemometer also had a more constant torque and responded more quickly to gusts than the four cup anemometer.

The three cup anemometer was further modified by the Australian Derek Weston in 1991 to measure both wind direction and wind speed. Weston added a tag to one cup, which causes the cupwheel speed to increase and decrease as the tag moves alternately with and against the wind. Wind direction is calculated from these cyclical changes in cupwheel speed, while wind speed is as usual determined from the average cupwheel speed.

Three cup anemometers are currently used as the industry standard for wind resource assessment studies

A windmill style of anemometer

[edit] Windmill anemometers

The other forms of mechanical velocity anemometer may be described as belonging to the windmill type or propeller anemometer. In the Robinson anemometer the axis of rotation is vertical, but with this subdivision the axis of rotation must be parallel to the direction of the wind and therefore horizontal. Furthermore, since the wind varies in direction and the axis has to follow its changes, a wind vane or some other contrivance to fulfill the same purpose must be employed. An aerovane combines a propeller and a tail on the same axis to obtain accurate and precise wind speed and direction measurements from the same instrument. In cases where the direction of the air motion is always the same, as in the ventilating shafts of mines and buildings for instance, wind vanes, known as air meters are employed, and give most satisfactory results.

[edit] Hot-wire anemometers

Hot-wire sensor

Hot wire anemometers use a very fine wire (on the order of several micrometers) electrically heated up to some temperature above the ambient. Air flowing past the wire has a cooling effect on the wire. As the electrical resistance of most metals is dependent upon the temperature of the metal (tungsten is a popular choice for hot-wires), a relationship can be obtained between the resistance of the wire and the flow velocity.[2]

Several ways of implementing this exist, and hot-wire devices can be further classified as CCA (Constant-Current Anemometer), CVA (Constant-Voltage Anemometer) and CTA (Constant-Temperature Anemometer). The voltage output from these anemometers is thus the result of some sort of circuit within the device trying to maintain the specific variable (current, voltage or temperature) constant.

Additionally, PWM (pulse-width modulation) anemometers are also used, wherein the velocity is inferred by the time length of a repeating pulse of current that brings the wire up to a specified resistance and then stops until a threshold "floor" is reached, at which time the pulse is sent again.

Hot-wire anemometers, while extremely delicate, have extremely high frequency-response and fine spatial resolution compared to other measurement methods, and as such are almost universally employed for the detailed study of turbulent flows, or any flow in which rapid velocity fluctuations are of interest.

[edit] Laser Doppler anemometers

Drawing of a laser anemometer. The laser is emitted (1) through the front lens (6) of the anemometer and is backscattered off the air molecules (7). The backscattered radiation (dots) re-enter the device and are reflected and directed into a detector (12).

Laser Doppler anemometers use a beam of light from a laser that is split into two beams, with one propagated out of the anemometer. Particulates (or deliberately introduced seed material) flowing along with air molecules near where the beam exits reflect, or backscatter, the light back into a detector, where it is measured relative to the original laser beam. When the particles are in great motion, they produce a Doppler shift for

measuring wind speed in the laser light, which is used to calculate the speed of the particles, and therefore the air around the anemometer.[3]

[edit] Sonic anemometers

3D ultrasonic anemometer

Sonic anemometers, first developed in the 1970s, use ultrasonic sound waves to measure wind speed and direction. They measure wind velocity based on the time of flight of sonic pulses between pairs of transducers. Measurements from pairs of transducers can be combined to yield a measurement of 1-, 2-, or 3-dimensional flow. The spatial resolution is given by the path length between transducers, which is typically 10 to 20 cm. Sonic anemometers can take measurements with very fine temporal resolution, 20 Hz or better, which make them well suited for turbulence measurements. The lack of moving parts makes them appropriate for long term use in exposed automated weather stations and weather buoys where the accuracy and reliability of traditional cup-and-vane anemometers is adversely affected by salty air or large amounts of dust. Their main disadvantage is the distortion of the flow itself by the structure supporting the transducers, which requires a correction based upon wind tunnel measurements to minimize the effect. An international standard for this process, ISO 16622 Meteorology—Sonic anemometers/thermometers—Acceptance test methods for mean wind measurements is in general circulation.

Two-dimensional (wind speed and wind direction) sonic anemometers are used in applications such as weather stations, ship navigation, wind turbines, aviation and weather buoys.

[edit] Ping-pong ball anemometers

A common anemometer for basic use is constructed from a ping-pong ball attached to a string. When the wind blows horizontally, it presses on and moves the ball; because ping-pong balls are very lightweight, they move easily in light winds. Measuring the angle

between the string-ball apparatus and the line normal to the ground gives an estimate of the wind speed.

This type of anemometer is mostly used for middle-school level instruction which most students make themselves, but a similar device was also flown on Phoenix Mars Lander.

[edit] Pressure anemometers

The first designs of anemometers which measure the pressure were divided into plate and tube classes.

[edit] Plate anemometers

These are the earliest anemometers and are simply a flat plate suspended from the top so that the wind deflects the plate. In 1450, the Italian art architect Leon Battista Alberti invented the first mechanical anemometer; in 1664 it was re-invented by Robert Hooke (who is often mistakenly considered the inventor of the first anemometer). Later versions of this form consisted of a flat plate, either square or circular, which is kept normal to the wind by a wind vane. The pressure of the wind on its face is balanced by a spring. The compression of the spring determines the actual force which the wind is exerting on the plate, and this is either read off on a suitable gauge, or on a recorder. Instruments of this kind do not respond to light winds, are inaccurate for high wind readings, and are slow at responding to variable winds. Plate anemometers have been used to trigger high wind alarms on bridges.

[edit] Tube anemometers

Helicoid propeller anemometer incorporating a wind vane for orientation.

James Lind's anemometer of 1775 consisted simply of a glass U tube containing liquid, a manometer, with one end bent in a horizontal direction to face the wind and the other vertical end remains parallel to the wind flow. Though the Lind was not the first it was

the most practical and best known anemometer of this type. If the wind blows into the mouth of a tube it causes an increase of pressure on one side of the manometer. The wind over the open end of a vertical tube causes little change in pressure on the other side of the manometer. The resulting liquid change in the U tube is an indication of the wind speed. Small departures from the true direction of the wind causes large variations in the magnitude.

The highly successful metal pressure tube anemometer of William Henry Dines in 1892 utilized the same pressure difference between the open mouth of a straight tube facing the wind and a ring of small holes in a vertical tube which is closed at the upper end. Both are mounted at the same height. The pressure differences on which the action depends are very small, and special means are required to register them. The recorder consists of a float in a sealed chamber partially filled with water. The pipe from the straight tube is connected to the top of the sealed chamber and the pipe from the small tubes is directed into the bottom inside the float. Since the pressure difference determines the vertical position of the float this is a measure of the wind speed.

The great advantage of the tube anemometer lies in the fact that the exposed part can be mounted on a high pole, and requires no oiling or attention for years; and the registering part can be placed in any convenient position. Two connecting tubes are required. It might appear at first sight as though one connection would serve, but the differences in pressure on which these instruments depend are so minute, that the pressure of the air in the room where the recording part is placed has to be considered. Thus if the instrument depends on the pressure or suction effect alone, and this pressure or suction is measured against the air pressure in an ordinary room, in which the doors and windows are carefully closed and a newspaper is then burnt up the chimney, an effect may be produced equal to a wind of 10 mi/h (16 km/h); and the opening of a window in rough weather, or the opening of a door, may entirely alter the registration.

While the Dines anemometer had an error of only 1% at 10 mph it did not respond very well to low winds due to the poor response of the flat plate vane required to turn the head into the wind. In 1918 an aerodynamic vane with eight times the torque of the flat plate overcame this problem.

[edit] Effect of density on measurements

In the tube anemometer the pressure is measured, although the scale is usually graduated as a velocity scale. In cases where the density of the air is significantly different from the calibration value (as on a high mountain, or with an exceptionally low barometer) an allowance must be made. Approximately 1½% should be added to the velocity recorded by a tube anemometer for each 1000 ft (5% for each kilometer) above sea-level.

Rain gaugeFrom Wikipedia, the free encyclopedia

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Standard Rain Gauge

Tipping Bucket Rain Gauge Recorder

Close up of a Tipping Bucket Rain Gauge Recorder chart

A rain gauge (also known as a udometer or a pluviometer [Pluviograph ] or an ombrometer or a cup) is a type of instrument used by meteorologists and hydrologists to

gather and measure the amount of liquid precipitation (as opposed to solid precipitation that is measured by a snow gauge) over a set period of time.

Contents

[hide] 1 History 2 Principles 3 Types

o 3.1 Standard rain gauge o 3.2 Weighing precipitation gauge o 3.3 Tipping bucket rain gauge o 3.4 Optical rain gauge

4 See also 5 References

6 External links

[edit] History

The first known records of rainfalls 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.[1]

In 1662 AD, Christopher Wren created the first tipping-bucket rain gauge in Britain.[2]

Garden rain gauge

[edit] Principles

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

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

[edit] 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.

[edit] Standard rain gauge

The standard rain gauge, developed around the start of the 20th century, consists of a funnel attached to a graduated cylinder that fits into a larger container. If the water overflows from the graduated cylinder the outside container will catch it. When measurements are taken, the cylinder will be measured and then the excess will be put in another cylinder and measured. 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.

[edit] Weighing precipitation gauge

A weighing-type precipitation gauge consists of a storage bin, which is weighed to record the mass. Certain models measure the mass using a pen on a rotating drum, or by using a vibrating wire attached to a data logger. The advantages of this type of gauge over tipping buckets 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 weighing-type recording gauge may also contain a device to measure the quantity 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.

[edit] Tipping bucket rain gauge

The interior of a 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 small buckets or levers which are balanced in same manner as a scale (or child's seesaw). 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 exterior of a tipping bucket rain 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 0.2 mm (0.007 in) has fallen when in fact only a minute amount has. Tipping buckets also tend to underestimate the amount of rainfall, particularly in snowfall and heavy rainfall events[3]

[4]. 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 reed switch) which is then electronically recorded or transmitted to a remote collection station.

Tipping gauges can also incorporate weighing gauges. In these gauges, 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[citation needed]. The Automated Surface Observing System (ASOS) uses heated tipping buckets to measure precipitation [5]

[edit] 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.

Hydrometer

A hydrometer is an instrument used to measure the specific gravity (or relative density) of liquids; that is, the ratio of the density of the liquid to the density of water.

A hydrometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. The liquid to be tested is poured into a tall jar, and the hydrometer is gently lowered into the liquid until it floats freely. The point at which the surface of the liquid touches the stem of the hydrometer is noted. Hydrometers usually contain a paper scale inside the stem, so that the specific gravity can be read directly. The scales may be Plato, Oechsle, or Brix, depending on the purpose.

Hydrometers may be calibrated for different uses, such as a lactometer for measuring the density (creaminess) of milk, a saccharometer for measuring the density of sugar in a liquid, or an alcoholometer for measuring higher levels of alcohol in spirits.

Contents

[hide] 1 Principle 2 History 3 Ranges 4 Scales 5 Commercial uses

o 5.1 Lactometer o 5.2 Alcoholometer o 5.3 Saccharometer o 5.4 Thermohydrometer o 5.5 Barkometer

6 Soil analysis 7 See also 8 References

9 Sources

[edit] Principle

The operation of the hydrometer is based on the Archimedes principle that a solid suspended in a fluid will be buoyed up by a force equal to the weight of the fluid displaced. Thus, the lower the density of the substance, the further the hydrometer will sink. (See also Relative density and hydrometers.)

[edit] History

An early description of a hydrometer appears in a letter from Synesius of Cyrene to Hypatia of Alexandria. In Synesius' fifteen letter, he requests Hypatia to make a hydrometer for him:[1]

The instrument in question is a cylindrical tube, which has the shape of a flute and is about the same size. It has notches in a perpendicular line, by means of which we are able to test the weight of the waters. A cone forms a lid at one of the extremities, closely fitted to the tube. The cone and the tube have one base only. This is called the baryllium. Whenever you place the tube in water, it remains erect. You can then count the notches at your ease, and in this way ascertain the weight of the water.

[edit] Ranges

In low density liquids such as kerosene, gasoline, and alcohol, the hydrometer will sink deeper, and in high density liquids such as brine, milk, and acids it will not sink so far. In fact, it is usual to have two separate instruments, one for heavy liquids, on which the mark 1.000 for water is near the top of the stem, and one for light liquids, on which the mark 1.000 is near the bottom. In many industries a set of hydrometers is used — covering specific gravity ranges of 1.0–0.95, 0.95–0.9 etc — to provide more precise measurements.

[edit] Scales

Modern hydrometers usually measure specific gravity but different scales were (and sometimes still are) used in certain industries. Examples include:

Baumé scale , formerly used in industrial chemistry and pharmacology Brix scale, primarily used in fruit juice, wine making and the sugar industry Oechsle scale , used for measuring the density of grape must Plato scale , primarily used in brewing Twaddell scale, formerly used in the bleaching and dyeing industries [2]

[edit] Commercial uses

A modern hydrometer in a sugar solution

Because the commercial value of many liquids, including sugar solutions, sulfuric acid, and alcohol beverages such as beer and wine, depends directly on the specific gravity, hydrometers are used extensively.

[edit] Lactometer

A lactometer (or galactometer) is a hydrometer used to test milk. The specific gravity of milk does not give a conclusive indication of its composition since milk contains a variety of substances that are either heavier or lighter than water. Additional tests for fat content are necessary to determine overall composition. The instrument is graduated into a hundred parts. Milk is poured in and allowed to stand until the cream has formed, then the depth of the cream deposit in degrees determines the quality of the milk. Another instrument, invented by Doeffel, is two inches long, divided into 40 parts, beginning at the point to which it sinks when placed in water. Milk unadulterated is shown at 14°.[3]

[edit] Alcoholometer

An alcoholometer is a hydrometer which is used for determining the alcoholic strength of liquids. It is also known as a proof and traille hydrometer. It only measures the density of the fluid. Certain assumptions are made to estimate the amount of alcohol present in the fluid. Alcoholometers have scales marked with volume percents of "potential alcohol", based on a pre-calculated specific gravity. A higher "potential alcohol" reading on this scale is caused by a greater specific gravity, assumed to be caused by the introduction of dissolved sugars. A reading is taken before and after fermentation and approximate alcohol content is determined by subtracting the post fermentation reading from the pre-fermentation reading. [4]

[edit] Saccharometer

A saccharometer is a hydrometer used for determining the amount of sugar in a solution. It is used primarily by winemakers and brewers,[5] and it can also be used in making sorbets and ice-creams.[6] The first brewers' saccharometer was constructed by John Richardson in 1784.[7]

It consists of a large weighted glass bulb with a thin stem rising from the top with calibrated markings. The sugar level can be determined by reading the value where the surface of the liquid crosses the scale. It works by the principle of buoyancy. A solution with a higher sugar content is denser, causing the bulb to float higher. Less sugar results in a lower density and a lower floating bulb.

[edit] Thermohydrometer

A thermohydrometer is a hydrometer that has a thermometer enclosed in the float section. For measuring the density of petroleum products, like fuel oils, the specimen is usually heated in a temperature jacket with a thermometer placed behind it since density is dependent on temperature. Light oils are placed in cooling jackets, typically at 15oC. Very light oils with many volatile components are measured in a variable volume container using a floating piston sampling device to minimize light end losses.

As a battery test it measures the temperature compensated specific gravity and electrolyte temperature.

[edit] Barkometer

A barkometer is calibrated to test the strength of tanning liquors used in tanning leather.[8]