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Aerosol Measurement in the Australian Outback: Intercomparison of Sun Photometers
Journal of Atmospheric and Oceanic Technology
Volume 20 2003
American Meteorological Society
R. M. Mitchell, B. W. Forgan

Field comparison of network Sun photometers
Journal of Geophysical Research
VOL. 108, NO. D19, 4596, doi:10.1029/2002JD002964, 2003
L. J. Bruce McArthur, David H. Halliwell, Ormanda J. Niebergall, Norm T. O’Neill, James R. Slusser, and Christoph Wehrli

 

Glossary of Solar Radiation Resource Terms

Aerosol Optical Depth - (technically known as the relative aerosol optical depth) usually considered to be synonymous with the airmass, is the approximate number of aerosols in a path through the atmosphere relative to the standard number of aerosols in a vertical path through a clean, dry atmosphere at sea level.

Albedo - the fraction of solar radiation that is reflected. The solar energy community defines albedo as the fraction of solar radiation that is reflected from the ground, ground cover, and bodies of water on the surface of the earth. Astronomers and meteorologists include reflectance by clouds and air. To reduce confusion, some solar researchers use the term ground reflectance.

Azimuth Angle - the angle between the horizontal direction (of the sun, for example) and a reference direction (usually North, although some solar scientists measure the solar azimuth angle from due South).

Bright Sunshine - when the sun casts an obvious shadow or when a Campbell-Stokes sunshine recorder is recording. The lower limit for bright sunshine (based on a Campbell-Stokes recorder) is between 70 W/m2 (very dry air) and 280 W/m2 (very humid air).

Broadband Solar Irradiance - theoretically the solar radiation arriving at the earth from all frequencies or wavelengths, in practice limited to the spectral range of radiometers, typically from 300 nm to 3000 nm wavelength. Meteorologists refer to this band as short-wave radiation.

Campbell-Stokes Sunshine Recorder - a clear glass sphere that focuses the sun's rays onto a special strip chart, producing a charred path when there is bright sunshine. The length of the path determines the bright sunshine duration. The lower limit for bright sunshine (based on a Campbell-Stokes recorder) is between 70 W/m2 (very dry air) and 280 W/m2 (very humid air).

Cosine Response - the effects of radiance incidence angle on pyranometer measurement performance. If a pyranometer is rotated while a beam of light is shined upon it, it will record the maximum energy when it is directly facing the beam, and the energy will fall to zero when it is sideways to (or facing away from) the beam. A graph of the energy reported by the pyranometer, as the angle it makes with the beam of light should look like the cosine of the angle, if the instrument were perfect. Pyranometers have imperfections that keep them from producing this curve.

Direct Normal Irradiance - synonym for beam radiation, the amount of solar radiation from the direction of the sun.

Global Radiation - total solar radiation; the sum of direct, diffuse, and ground-reflected radiation; however, because ground reflected radiation is usually insignificant compared to direct and diffuse, for all practical purposes global radiation is said to be the sum of direct and diffuse radiation only.

Ground-Reflected Radiation - the radiation from the sun which is reflected back into the atmosphere after striking the Earth.

Incident Angle - the angle that a ray (of solar energy, for example) makes with a line perpendicular to the surface. For example, a surface that directly faces the sun has a solar angle of incidence of zero, but if the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90°.

Infrared Radiation - radiation with wavelengths greater than those of the visible light (at about 800 nanometres (nm)) but shorter than those of microwaves (at about 800,000 nm). Infrared radiation is associated with heat energy.

Irradiance - the rate at which radiant energy arrives at a specific area of surface during a specific time interval. This is known as radiant flux density. A typical unit is W/m2.

longwave Radiation - see infrared radiation.

Minutes of Sunshine - a specific instance of bright sunshine duration, the number of minutes per day during which the sun casts an obvious shadow or when a Campbell-Stokes sunshine recorder is recording, usually above 210 W/m2. The World Meteorological Organization defines sunshine as solar direct irradiance exceeding 120 W/m2.

NIP - a Normal Incident Pyrheliometer, used to determine the amount of solar direct irradiance emitted from the direction of the sun.

Normal Radiation - radiation striking a surface that is facing the sun. Mathematically, the word normal is the vector (direction) that is perpendicular to a surface, and the direction of a normal radiation source is perpendicular to a radiation source. Global (total) normal solar irradiance is all radiation that strikes a flat surface that faces the sun, while direct normal solar irradiance excludes all radiation that does not come from the direction of the sun in the sky.

Optical Depth - (technically known as the relative aerosol optical depth) usually considered to be synonymous with the airmass, is the approximate number of aerosols in a path through the atmosphere relative to the standard number of aerosols in a vertical path through a clean, dry atmosphere at sea level.

Photometer - an instrument that measures illuminance.

PMOD/WRC - the Physikalisch-Meteorologisches Observatorium Davos / World Radiation Center, at Davos, Switzerland. PMOD/WRC determines and maintains worldwide standards for measurement of solar radiation, including the World Radiometric Reference (WRR), for the World Meteorological Organization.

Pyranometer - an instrument with a hemispherical field of view, used for measuring total or global solar radiation, specifically global horizontal radiation; a pyranometer with a shadow band or shading disk blocking the direct beam measures the diffuse sky radiation.

Pyrheliometer - instrument with a narrow (circumsolar) field of view which measures direct normal irradiance. Pyrheliometers are mounted on sun-following trackers so that the instrument is always aimed at the sun.

Rotating Shadow Band Radiometer - an instrument that determines total solar radiation and diffuse sky radiation by periodically shading the total sky sensor from the sun with a rotating shadow band.

Shading Disk - a disk on a tracking arm, which blocks the direct normal irradiance so as to allow a pyranometer to measure only the diffuse sky radiation.

Shadow Band - a metal strip that blocks the direct normal radiation so as to allow a pyranometer to measure only the diffuse sky radiation.

Short-wave Radiation - the principal portion of the solar spectrum that spans from approximately 300 nanometres (nm) to 3000 nm in the electromagnetic spectrum.

Silicon Sensor - a photovoltaic cell that is used to measure solar irradiance. Because its spectral response is not as exact as that of thermopile instruments, it has a greater uncertainty.

Solar Irradiance - the amount of solar energy that arrives at a specific area of a surface during a specific time interval (radiant flux density). A typical unit is W/m2.

Solar Radiation - the electromagnetic radiation emitted by the sun.

Sunshine - used interchangeably with the more precise term bright sunshine, when the sun casts an obvious shadow. The World Meteorological Organization defines sunshine as solar direct irradiance exceeding 120 W/m2.

Sunshine Duration - the length of time for which the sun casts an obvious shadow. The threshold tolerance for bright sunshine (based on World Meteorological Organization) is direct irradiance of 120 W/m plus or minus 20%.

Terrestrial Radiation - electromagnetic radiation that is emitted by the Earth, as opposed to solar radiation emitted by the sun.

Thermopile - a set of thermocouple junctions connected in series in order to boost the voltage to a meaningful amount (output usually measured in millivolts). A thermocouple is a metallic strip or wire that produces an electromagnetic potential (voltage) when the two ends (junctions) are at different temperatures. The "cold" junctions of thermopile radiometers are shielded from the sun and are at ambient temperature. The "hot" junctions are painted black to absorb the solar radiation and are at an elevated temperature.

Total Solar Radiation - solar radiation that is the sum of direct, diffuse, and ground-reflected radiation; however, because ground reflected radiation is usually insignificant compared to direct and diffuse, for all practical purposes global radiation is said to be the sum of direct and diffuse radiation only.

Ultraviolet Radiation - the range of radiation just beyond the violet in the visible spectrum (at about 400 nanometres (nm)) to the x-ray region (at about 4 nm). UV-A is in the 315-400 nm range, and UV-B is in the 280-315 nm range. UV-C is below 280 nm.

WMO - the World Meteorological Organization

WRR - the World Radiometric Reference, which provides the basis for all measurements by radiometers in the world. Every five years, many of the best absolute cavity radiometers undergo an intercomparison at PMOD/WRC (Davos, Switzerland). The most stable, accurate, and precise instruments provide the World Radiometric Reference for the coming years. Any credible radiometer measurement must be traceable to the WRR.

Zenith Angle - the angle between the direction of interest (of the sun, for example) and the zenith (directly overhead).

For further information on solar radiation resource terms please refer to Renewable Resource Data Center (RReDC). The RReDC is supported by the National Center for Photovoltaics (NCPV) and managed by the Department of Energy's Office of Energy Efficiency and Renewable Energy. The RReDC is maintained by the Electric and Hydrogen Systems Center at the National Renewable Energy Laboratory.

 

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