Weather buoys are instruments which collect weather and ocean data within the world's oceans, as well as aid during emergency response to chemical spills, legal proceedings, engineering design. Moored buoys have been in use since 1951, while drifting buoys have been used since 1979. Moored buoys are connected with the ocean bottom using either chains, nylon, or buoyant polypropylene. With the decline of the weather ship, they have taken a more primary role in measuring conditions over the open seas since the 1970s. During the 1980s and 1990s, a network of buoys in the central and eastern tropical Pacific Ocean helped study the El Niño-Southern Oscillation. Moored weather buoys range from 1.5–12 metres in diameter, while drifting buoys are smaller, with diameters of 30–40 centimetres. Drifting buoys are the dominant form of weather buoy with 1250 located worldwide. Wind data from buoys has smaller error than that from ships. There are differences in the values of sea surface temperature measurements between the two platforms as well, relating to the depth of the measurement and whether or not the water is heated by the ship which measures the quantity.
The first known proposal for surface weather observations at sea occurred in connection with aviation in August 1927, when Grover Loening stated that "weather stations along the ocean coupled with the development of the seaplane to have an long range, would result in regular ocean flights within ten years." Starting in 1939, United States Coast Guard vessels were being used as weather ships to protect transatlantic air commerce. During World War II The German Navy deployed weather buoys at fifteen fixed positions in the North Atlantic and Barents Sea, they were launched from U-boats into a maximum depth of ocean of 1000 fathoms, limited by the length of the anchor cable. Overall height of the body was 10.5 metres, surmounted by a extendible aerial of 9 metres. Data were transmitted four times a day; when the batteries were exhausted, after about eight to ten weeks, the unit self-destructed. The Navy Oceanographic Meteorological Automatic Device buoy's 6-metre hull was designed in the 1940s for the United States Navy’s offshore data collection program.
The United States Navy tested marine automatic weather stations for hurricane conditions between 1956 and 1958, though radio transmission range and battery life was limited. Between 1951 and 1970, a total of 21 NOMAD buoys were deployed at sea. Since the 1970s, weather buoy use has superseded the role of weather ships, as they are cheaper to operate and maintain; the earliest reported use of drifting buoys was to study the behavior of ocean currents within the Sargasso Sea in 1972 and 1973. Drifting buoys have been used since 1979, as of 2005, 1250 drifting buoys roamed the Earth's oceans. Between 1985 and 1994, an extensive array of moored and drifting buoys was deployed across the equatorial Pacific Ocean to monitor and help predict the El Niño phenomenon. Hurricane Katrina capsized a 10 m buoy for the first time in the history of the National Data Buoy Center on August 28, 2005. On June 13, 2006, drifting buoy 26028 ended its long-term data collection of sea surface temperature after transmitting for 10 years, 4 months, 16 days, the longest known data collection time for any drifting buoy.
The first weather buoy in the Southern Ocean was deployed by the Integrated Marine Observing System on March 17, 2010. Weather buoys, like other types of weather stations, measure parameters such as air temperature above the ocean surface, wind speed, barometric pressure, wind direction. Since they lie in oceans and lakes, they measure water temperature, wave height, dominant wave period. Raw data is processed and can be logged on board the buoy and transmitted via radio, cellular, or satellite communications to meteorological centers for use in weather forecasting and climate study. Both moored buoys and drifting buoys are used. Fixed buoys measure the water temperature at a depth of 3 metres. Many different drifting buoys exist around the world that vary in design and the location of reliable temperature sensors varies; these measurements are beamed to satellites for immediate data distribution. Other than their use as a source of meteorological data, their data is used within research programs, emergency response to chemical spills, legal proceedings, engineering design.
Moored weather buoys can act as a navigational aid, like other types of buoys. Weather buoys range in diameter from 1.5–12 metres. Those that are placed in shallow waters are smaller in size and moored using only chains, while those in deeper waters use a combination of chains and buoyant polypropylene. Since they do not have direct navigational significance, moored weather buoys are classed as special marks under the IALA scheme, are coloured yellow, display a yellow flashing light at night. Discus buoys are moored in deep ocean locations, with a diameter of 10 -- 12 metres; the aluminum 3-metre buoy is a rugged meteorological ocean platform that has long term survivability. The expected service life of the 3-metre platform is in excess of 20 years and properly maintained, these buoys have not been retired due to corrosion; the NOMAD is a unique moored aluminum environmental monitoring buoy designed for deployments in extreme conditions near the coast and a
Diamond dust is a ground-level cloud composed of tiny ice crystals. This meteorological phenomenon is referred to as ice crystals and is reported in the METAR code as IC. Diamond dust forms under otherwise clear or nearly clear skies, so it is sometimes referred to as clear-sky precipitation. Diamond dust is most observed in Antarctica and the Arctic, but can occur anywhere with a temperature well below freezing. In the polar regions of Earth, diamond dust may persist for several days without interruption. Diamond dust is similar to fog in. Fog refers to a cloud composed of liquid water. Fog is a dense enough cloud to reduce visibility, while diamond dust is very thin and may not have any effect on visibility. However, diamond dust can reduce the visibility, in some cases to under 600 m; the depth of the diamond dust layer can vary from as little as 20 to 30 m to 300 metres. Because diamond dust does not always reduce visibility it is first noticed by the brief flashes caused when the tiny crystals, tumbling through the air, reflect sunlight to the eye.
This glittering effect gives the phenomenon its name since it looks like many tiny diamonds are flashing in the air. Serial photos of Diamond Dust These ice crystals form when a temperature inversion is present at the surface and the warmer air above the ground mixes with the colder air near the surface. Since warmer air contains more water vapor than colder air, this mixing will also transport water vapor into the air near the surface, causing the relative humidity of the near-surface air to increase. If the relative humidity increase near the surface is large enough ice crystals may form. To form diamond dust the temperature must be below the freezing point of water, 0 °C, or the ice cannot form or would melt. However, diamond dust is not observed at temperatures near 0 °C. At temperatures between 0 °C and about −39 °C increasing the relative humidity can cause either fog or diamond dust; this is because small droplets of water can remain liquid well below the freezing point, a state known as supercooled water.
In areas with a lot of small particles in the air, from human pollution or natural sources like dust, the water droplets are to be able to freeze at a temperature around −10 °C, but in clean areas, where there are no particles to help the droplets freeze, they can remain liquid to −39 °C, at which point very tiny, pure water droplets will freeze. In the interior of Antarctica diamond dust is common at temperatures below about −25 °C. Artificial diamond dust can form from snow machines; these are found at ski resorts. Diamond dust is associated with halos, such as sun dogs, light pillars, etc. Like the ice crystals in cirrus or cirrostratus clouds, diamond dust crystals form directly as simple hexagonal ice crystals — as opposed to freezing drops — and form slowly; this combination results in crystals with well defined shapes - either hexagonal plates or columns - which, like a prism, can reflect and/or refract light in specific directions. While diamond dust can be seen in any area of the world that has cold winters, it is most frequent in the interior of Antarctica, where it is common year-round.
Schwerdtfeger shows that diamond dust was observed on average 316 days a year at Plateau Station in Antarctica, Radok and Lile estimate that over 70% of the precipitation that fell at Plateau Station in 1967 fell in the form of diamond dust. Once melted, the total precipitation for the year was only 25 mm. Diamond dust may sometimes cause a problem for automated airport weather stations; the ceilometer and visibility sensor do not always interpret the falling diamond dust and report the visibility and ceiling as zero. However, a human observer would notice clear skies and unrestricted visibility; the METAR identifier for diamond dust within international hourly weather reports is IC. Crepuscular rays Halo Light beam Light pillar Sun dog False sunrise False sunset Greenler, R.. Rainbows and Glories. Milwaukee: Peanut Butter Publishing. Pp. 195 pp. ISBN 0-89716-926-3. — An excellent reference for optical phenomena including photos of displays in Antarctica caused by diamond dust. Schwerdtfeger, W..
"The climate of the Antarctic". In S. Orvig. Climates of the Polar Regions. World Survey of Climatology. Vol. 14. Elsevier. Pp. 253–355. ISBN 0-444-40828-2. Radok, U. and R. C. Lile. "A year of snow accumulation at Plateau Station". In J. A. Businger. Meteorological Studies at Plateau Station, Antarctica. Antarctic Research Series. Vol. 25. American Geophysical Union. Pp. 17–26. ISBN 0-87590-125-5. Manual of Surface Weather Observations. Atmospheric Environment Service of Canada. Photo of artificial Diamond Dust A remarkable video filmed in Japan. 1min 22sec HQ Longer version of the above video. 5min 10sec HD Note that images are different from naked eye in that they capture out-of-focus crystals which are shown as large, blurred objects. By naked eye, Diamond dust looks more like the photo below: A night photo that presents closer to naked eye observation
A rain gauge is an instrument used by meteorologists and hydrologists to gather and measure the amount of liquid precipitation over a set period of time. The first known rainfall records were kept by the Ancient Greeks, about 500 B. C. People living in India began to record rainfall in 400 B. C; the readings were correlated against expected growth. 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 rain gauge to classify land for taxation purposes. In 1247, the Song Chinese mathematician and inventor Qin Jiushao invented Tianchi basin rain and snow gauges to reference rain, snowfall measurements, as well as other forms of meteorological data. In 1441, the Cheugugi was invented during the reign of Sejong the Great of the Joseon Dynasty of Korea as the first standardized rain gauge. In 1662, Christopher Wren created the first tipping-bucket rain gauge in Britain in collaboration with Robert Hooke. Hooke designed a manual gauge with a funnel that made measurements throughout 1695.
It was Richard Towneley, 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, barometric pressure 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 from year to year with little discernible pattern; the meteorologist George James Symons published the first annual volume of British Rainfall in 1860.
This pioneering work contained rainfall records from 168 land stations in Wales. He was elected to the council of the British meteorological society in 1863 and made it his life's work to investigate rainfall within the British Isles, he set up a voluntary network of observers, who collected data which were returned to him for analysis. So successful was he in this endeavour that by 1866 he was able to show results that gave a fair representation of the distribution of rainfall, the number of recorders increased until the last volume of British Rainfall that which he lived to edit, for 1899, contained figures from 3,528 stations — 2,894 in England and Wales, 446 in Scotland, 188 in Ireland, he collected old rainfall records going back over a hundred years. In 1870 he produced an account of rainfall in the British Isles starting in 1725. Due to the ever-increasing numbers of observers, standardisation of the gauges became necessary. Symons began experimenting on new gauges in his own garden, he tried different models with variations in size and height.
In 1863 he began collaboration with Colonel Michael Foster Ward from Calne, 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 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 Meteorological Office today, one made of "... copper, with a five-inch funnel having its brass rim one foot above the ground..."Most modern rain gauges measure the precipitation in millimetres in height collected on each square meter during a certain period, equivalent to litres per square metre. 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; 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 for sparsely populated areas.
In most cases the precipitation is not retained, but some stations do submit rainfall and snowfall for testing, 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 due to wind extremes. Rain gauges only indicate rainfall in a localized area. For any gauge, drops will stick to the sides or funnel of the collecting device, such that amounts are slightly underestimated, those of.01 inches or.25 mm may be recorded as a "trace". Another problem encountered. 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 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 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 of rain gauges include graduated cylinders, weighing gauges, tipping bucket gauges, simple buried pit collectors. Each type has its advantages and disadvantages for collectin
A barograph is a barometer that records the barometric pressure over time in graphical form. Alexander Cumming, a watchmaker and mechanic, has a claim to having made the first effective recording barograph in the 1760s using an aneroid cell. Cumming created a series of barometrical clocks, including one for King George III. However, this type of design fell out of favour. Since the amount of movement that can be generated by a single aneroid is minuscule, up to seven aneroids are stacked "in series" to amplify their motion; this type of barograph was invented in 1844 by the Frenchman Lucien Vidi. In such barographs one or more aneroid cells act through a gear or lever train to drive a recording arm that has at its extreme end either a scribe or a pen. A scribe records on smoked foil; the recording material is mounted on a cylindrical drum, rotated by clockwork. The drum makes one revolution per day, per week, or per month and the rotation rate can be selected by the user. Various other types of barograph have been invented.
Karl Kreil described a machine in 1843 based on a syphon barometer, where a pencil marked a chart at uniform intervals. Francis Ronalds, the Honorary Director of the Kew Observatory, created the first successful barograph utilising photography in 1845; the changing height of the mercury in the barometer was recorded on a continuously moving photosensitive surface. By 1847, a sophisticated temperature-compensation mechanism was employed. Ronalds’ barograph was utilised by the UK Meteorological Office for many years to assist in weather forecasting and the machines were supplied to numerous observatories around the world. Today, traditional recording barographs for meteorological use have been superseded by electronic weather instruments that use computer methods to record the barometric pressure; these are not only less expensive than earlier barographs but they may offer both greater recording length and the ability to perform further data analysis on the captured data including automated use of the data to forecast the weather.
Older mechanical barographs are prized by collectors as they make good display items being made of high quality woods and brass. The most common weather Barograph found in homes and public buildings these days are the 8-day type; some important manufacturers of Barographs are Negretti and Zambra and Mason, Richard Ferris among others. The late Victorian to early 20th century is considered to be the heyday of Barograph manufacture, many important refinements were made at this time, including improved temperature compensation and modification of the pen arm, to allow less weight to be applied to the paper, allowing better registration of small pressure changes. Marine barographs include damping, this evens out the motion of the ship so that a more stable reading can be obtained, this can be either oil damping of the mechanism or simple coiled spring feet on the base. But, newer solid state, digital barographs eliminate this issue altogether, since they use no moving parts; as atmospheric pressure responds in a predictable manner to changes in altitude, barographs may be used to record elevation changes during an aircraft flight.
Barographs were required by the FAI to record certain tasks and record attempts associated with sailplanes. A continuously varying trace indicated that the sailplane had not landed during a task, while measurements from a calibrated trace could be used to establish the completion of altitude tasks or the setting of records. Examples of FAI approved sailplane barographs included the Replogle mechanical drum barograph and the EW electronic barograph. Mechanical barographs are not used for flight documentation now, having been displaced by GNSS Flight Recorders. On the top right of the picture of the three-day barograph can be seen a silver knurled knob; this is to adjust the barograph so that it reflects the station pressure. Visible below the knob is a small silver plunger; this is pressed every three hours to leave a time mark on the paper. The line between two of these marks is called the'characteristic of barometric tendency' and is used by weather forecasters; the observer would first note if the pressure was higher than three hours prior.
Next, a code number would be chosen. There are nine possible choices and no single code has preference over another. In the case of the graph on the barograph, one of two codes could be picked. An 8 or 6; the observer should pick the 6 because it represents the last part of the trace and is thus most representative of the pressure change. In the bottom centre is the aneroid; as the pressure increases, the aneroid is pushed down causing the arm to move up and leave a trace on the paper. As the pressure decreases, the spring lifts the arm moves down. After three days the drum to which the graph is attached is removed. At this point the clockwork motor is wound and if necessary corrections can be made to increase or decrease the speed and new chart is attached. Thermo-hygrograph
SODAR written as sodar, is a meteorological instrument used as a wind profiler to measure the scattering of sound waves by atmospheric turbulence. SODAR systems are used to measure wind speed at various heights above the ground, the thermodynamic structure of the lower layer of the atmosphere. Sodar systems are in fact nothing more than sonar systems used in the air rather than in water. Other names used for sodar systems include sounder and acoustic radar. Commercial sodars operated for the purpose of collecting upper-air wind measurements consist of antennas that transmit and receive acoustic signals. A mono-static system uses the same antenna for transmitting and receiving, while a bi-static system uses separate antennas; the difference between the two antenna systems determines whether atmospheric scattering is by temperature fluctuations, or by both temperature and wind velocity fluctuations. Mono-static antenna systems can be divided into two categories: those using multiple axis, individual antennas and those using a single phased array antenna.
The multiple-axis systems use three individual antennas aimed in specific directions to steer the acoustic beam. Using three independent axes is enough to retrieve the three components of the wind speed, although using more axes would add redundancy and increase robustness to noise when estimating the wind speed, using a least-squares approach. One antenna is aimed vertically, the other two are tilted from the vertical at an orthogonal angle; each of the individual antennas may use a single transducer focused into a parabolic reflector to form a parabolic loudspeaker, or an array of speaker drivers and horns all transmitting in-phase to form a single beam. Both the tilt angle from the vertical and the azimuth angle of each antenna are fixed when the system is set up. Phased-array antenna systems use a single array of speaker drivers and horns, the beams are electronically steered by phasing the transducers appropriately. To set up a phased-array antenna, the pointing direction of the array is either level, or oriented as specified by the manufacturer.
The horizontal components of the wind velocity are calculated from the radially measured Doppler shifts and the specified tilt angle from the vertical. The tilt angle, or zenith angle, is 15 to 30 degrees, the horizontal beams are oriented at right angles to one another. Since the Doppler shift of the radial components along the tilted beams includes the influence of both the horizontal and vertical components of the wind, a correction for the vertical velocity is needed in systems with zenith angles less than 20 degrees. In addition, if the system is located in a region where vertical velocities may be greater than about 0.2 m/s, corrections for the vertical velocity are needed, regardless of the beam's zenith angle. The vertical range of sodars is 0.2 to 2 kilometers and is a function of frequency, power output, atmospheric stability, and, most the noise environment in which a sodar is operated. Operating frequencies range from less than 1000 Hz to over 4000 Hz, with power levels up to several hundred watts.
Due to the attenuation characteristics of the atmosphere, high power, lower frequency sodars will produce greater height coverage. Some sodars can be operated in different modes to better match vertical resolution and range to the application; this is accomplished through a relaxation between maximum altitude. Traditionally used in atmospheric research, sodars are now being applied as an alternative to traditional wind monitoring for the development of wind power projects. Sodars used for wind power applications are focused on a measurement range from 50m to 200m above ground level, corresponding to the size of modern wind turbines; some sodar products, such as the REMTECH PA-XS Sodar and the AQ510 Sodar, have been developed for this market. Compact-beam sodars are more accurate in complex terrain where the wind vector can change across the measurement area of the sodar. By providing a more compact beam angle, these sodars reduce the effect of any change in the wind vector; this provides a more accurate estimate of wind flow and therefore energy production of a wind turbine.
Compact beam sodars reduce the effect of fixed echos and allow a more compact unit design. Multiple-axis sodars provide the capability for simultaneous firing of all three sound beams, unlike single-axis sodars which must fire each sound beam sequentially. Simultaneous firing can provide three times the number of sample points in any given period, resulting in a higher signal to noise ratio, higher data availability and greater accuracy. Sodars designed for the wind energy industry differ in important aspects such as the traceability of data as a number of manufacturers do not return full signal and noise spectrum data from the sodar unit, but rather, only return processed wind speed data; this means the raw data can not be reprocessed. The underlying physical principles behind the two devices are the same. Both devices use sound waves to determine remote properties of the environment. Both devices use the Doppler effect to measure radial speeds on at least three non-colinear beams, which after simple computations yield the three vector components of the speed of the transmitting medium at different altitudes.
Both sodars and ADCPs c
A dropsonde is an expendable weather reconnaissance device created by the National Center for Atmospheric Research, designed to be dropped from an aircraft at altitude over water to measure storm conditions as the device falls to the surface. The sonde contains a GPS receiver, along with pressure and humidity sensors to capture atmospheric profiles and thermodynamic data, it relays these data to a computer in the aircraft by radio transmission. Since the early 1970s, hurricane hunters have employed dropsondes while flying over the ocean to obtain meteorological data on the structure of hurricanes deemed to be of possible concern to land locations in the northern Atlantic and northeastern Pacific oceans. Dropsonde instruments are the only way to measure the wind and pressure near the sea surface within the core of such cyclones, allowing meteorologists to reliably establish the storm's intensity and size; the data obtained is fed into supercomputers for numerical weather prediction, enabling forecasters to better track and predict what will happen to the hurricane.
During a typical hurricane season, the 53d Weather Reconnaissance Squadron Hurricane Hunters deploys 1000 to 1500 sondes on training and storm missions. They measure humidity, wind speed, air pressure. Aircraft reconnaissance missions are sometimes requested to investigate the broader atmospheric structure over the ocean when cyclones may pose a significant threat to the United States; these interests include not only potential hurricanes, but possible snow events or significant tornado outbreaks. The dropsondes are used to supplement the large gaps over oceans within the global network of daily radiosonde launches. Satellite data provides an estimate of conditions in such areas, but the increased precision of sondes can improve forecasts of the storm path. Dropsondes may be employed during meteorological research projects.. The sonde is a lightweight system designed to be operated by one person and is launched through a chute installed in the measuring aircraft; the device's descent is slowed and stabilized by a small square-cone parachute, allowing for more readings to be taken before it reaches the ocean surface.
The parachute is designed to deploy after release so as to reduce or eliminate any pendulum effect, the device drops for three to five minutes. The sonde has a casing of stiff cardboard to protect electronics and form a more stable aerodynamic profile. To obtain data in a tropical cyclone, an aircraft flies into the system. A series of dropsondes are released as the plane passes through the storm launched with greatest frequency near the center of the storm, including into the eyewall and eye, if one exists. Most drops are performed at a flight level of around 10,000 feet; the dropsonde sends back coded data, which includes: The time of the drop. Time is always in UTC. Location of the drop, indicated by the latitude and Marsden square; the height, dewpoint depression, wind speed, wind direction recorded at any standard isobaric surfaces encountered as the dropsonde descends, which are from the set of: 1000, 925, 850, 700, 500, 400, 300, 250 hectopascals, at the sea surface. The temperature and dewpoint depression at all other atmospheric pressure deemed significant due to important changes or values in the atmospheric conditions found Air pressure, dewpoint depression, wind speed and wind direction of the tropopause.
Included in the report is information on the aircraft, the mission, the dropsonde itself, other remarks. A driftsonde is a high altitude, durable weather balloon holding a transmitter and a bank of miniature dropsonde capsules which can be dropped at automatic intervals or remotely; the water-bottle-sized transmitters in the dropsondes have enough power to send information to the balloon during their parachute-controlled fall. The balloon carries a larger transmitter powerful enough to relay readings to a satellite; the single-use sensor packages cost US$300 to $400 each. After being introduced in April 2007, around a thousand a year are expected to be used to track winds in hurricane breeding grounds off of West Africa, which are outside the operating region of Hurricane Hunter planes. Aerology Radiosonde Data from Expendable Probes NCAR GPS Dropsonde System Vaisala Dropsonde RD94
A pyranometer is a type of actinometer used for measuring solar irradiance on a planar surface and it is designed to measure the solar radiation flux density from the hemisphere above within a wavelength range 0.3 μm to 3 μm. The name pyranometer stems from the Greek words πῦρ, meaning "fire", ἄνω, meaning "above, sky". A typical pyranometer does not require any power to operate. However, recent technical development includes use of electronics in pyranometers, which do require external power; the solar radiation spectrum that reaches earth's surface extends its wavelength from 300 nm to 2800 nm. Depending on the type of pyranometer used, irradiance measurements with different degrees of spectral sensitivity will be obtained. To make a measurement of irradiance, it is required by definition that the response to “beam” radiation varies with the cosine of the angle of incidence; this ensures a full response when the solar radiation hits the sensor perpendicularly, zero response when the sun is at the horizon, 0.5 at a 60° angle of incidence.
It follows that a pyranometer should have a so-called “directional response” or “cosine response”, as close as possible to the ideal cosine characteristic. Following the classifications and definitions noted in the ISO 9060, three types of pyranometers can be recognized and grouped in two different technologies: thermopile technology and silicon semiconductor technology; the light sensitivity, known as'spectral response', depends on the type of pyranometer. The figure here above shows the spectral responses of the three types of pyranometer in relation to the Solar Radiation Spectrum; the Solar Radiation Spectrum represents the spectrum of sunlight that reaches the Earth’s surface at sea level, at midday with A. M. = 1.5. The latitude and altitude influence this spectrum; the spectrum is influenced by aerosol and pollution. A thermopile pyranometer is a sensor based on thermopiles designed to measure the broadband of the solar radiation flux density from a 180° field of view angle. A thermopile pyranometer thus measures 300 to 2800 nm with a flat spectral sensitivity The first generation of thermopile pyranometers had the active part of the sensor divided in black and white sectors.
Irradiation was calculated from the differential measure between the temperature of the black sectors, exposed to the sun, the temperature of the white sectors, sectors not exposed to the sun or better said in the shades. In all thermopile technology, irradiation is proportional to the difference between the temperature of the sun exposed area and the temperature of the shadow area. In order to attain the proper directional and spectral characteristics, a thermopile pyranometer is constructed with the following main components: A thermopile sensor with a black coating, it absorbs all solar radiation, has a flat spectrum covering the 300 to 50,000 nanometer range, has a near-perfect cosine response. A glass dome, it limits the spectral response from 300 to 2,800 nanometers, while preserving the 180° field of view. It shields the thermopile sensor from convection. For first class and secondary standard pyranometers a second glass dome is used; this construction provides an additional “radiation shield”, resulting in a better thermal equilibrium between the sensor and inner dome, compared to using a single dome.
The effect of having a second dome is a strong reduction of instrument offsets. In the modern thermopile pyranometers the active junctions of the thermopile are located beneath the black coating surface and are heated by the radiation absorbed from the black coating; the passive junctions of the thermopile are protected from solar radiation and in thermal contact with the pyranometer housing, which serves as a heat-sink. This prevents any alteration from yellowing or decay when measuring the temperature in the shade, thus impairing the measure of the solar irradiance; the thermopile generates a small voltage in proportion to the temperature difference between the black coating surface and the instrument housing. This is of the order of 10 µV per W/m2. On a sunny day the output is around 10 mV; each pyranometer has a unique sensitivity, unless otherwise equipped with electronics for signal calibration. Thermopile pyranometers are used in meteorology, climate change research, building engineering physics and in photovoltaic systems.
They are installed horizontally in meteorological stations and mounted in the'plane of array' when used for monitoring of photovoltaic systems. The solar energy industry, in a new standard, IEC 61724-1:2017, has defined what type of pyranometers should be used depending on the size and category of solar power plant. Known as a silicon pyranometer in the ISO 9060, a photodiode-based pyranometer can detect the portion of the solar spectrum between 400 nm and 900 nm, with the most performant detecting between 350 nm and 1100 nm; the photodiode converts the aforementioned solar spectrum frequencies into current at high speed, thanks to the photoelectric effect. The conversion is influenced by the temperature with a raise in current produced by the raise in temperature A photodiode-based pyranometer is composed by a housing dome, a photodiode, a diffuser or optical filters; the photodiode acts as a sensor. The current generated by the photodiode is proportional to irradiance.