Glossary of Solar Radiation Resource Terms

Other relevant glossaries

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Absolute Cavity Radiometer - an instrument used for very accurate measurements of solar irradiance . Absolute cavity radiometers absorb radiation on a blackened conical receiver and are electrically self-calibrating. Absolute cavity radiometers determine the solar constant and provide the reference from which other radiometers are calibrated. Below is a picture of an absolute cavity radiometer used to transfer the World Radiometric Reference ( WRR ) to all radiometers calibrated at the Solar Radiation Research Laboratory.


Absolute Humidity - the mass (in grams) of water in a volume (cubic meter) of air; units are g/m3.

Absorption - when the substance of interest is captured by another substance, reducing the amount available. For examble, solar energy is absorbed by some atmospheric molecules, solar collectors, and the ocean.

Aerosol - excluding weather and clouds, any small particle that tends to stay in the air, such as smoke, dust, salt, and pollen.

Aerosol Optical Depth - (technically known as the relative aerosol optical depth) is the "extinction per unit path length due to aerosols alone"...the typical means of computing this is to account for the extinction of as many of the other constituents as possible.... (water vapor, ozone, mixed gases, and 'equivalent extinction' represented by Rayliegh scattering of atmospheric molecules, and what is 'left over' is the aerosol extinction. The transmission T and "optical depth" t of an atmospheric constituent are related as follows:

          T = Exp(-t*m) , or t = - ln(T)/m,

where m is the airmass, or equivalent path length through the atmosphere.

In simplest terms, for Io = extraterrestrial radiation, and I = radiation at surface

          I = Io * ( Tw*To*Tg*Tr*Ta)

where Tw=transmittance of water, To of ozone, Tg of gases, Tr Rayleigh effects, Ta aerosols

Strictly speaking, both extinction coefficients (see and transmission are a function of wavelength, so optical depths are defined at specific wavelengths (spectral optical depth) or Total (integrated over all wavelengths). The total optical depth is sometimes called the "opacity".

AES - Atmospheric Environment Service, the Canadian equivalent of USA's National Oceanic and Atmospheric Administration. AES operates the solar measurement network for Canada. They are also the equivalent of USA's National Climatic Data Center in that they respond to requests for weather data and maintain the data archives.

AHF - Automatic Hickey-Frieden absolute cavity radiometer. This is the model designation given by The Eppley Laboratory, Inc. for their commercial version of an electrically self-calibrating absolute cavity radiometer used to define and transfer the World Radiometric Reference (WRR) to pyrheliometers and pyranometers used for solar irradiance measurements. The WRR is maintained at the World Radiation Center, Davos, Switzerland for the World Meteorological Organization.

Airmass - the relative path length of the direct solar beam radiance through the atmosphere. When the sun is directly above a sea-level location the path length is defined as airmass 1 (AM 1.0). AM 1.0 is not synonymous with solar noon because the sun is usually not directly overhead at solar noon in most seasons and locations. When the angle of the sun from zenith (directly overhead) increases, the airmass increases approximately by the secant of the zenith angle. A better calculation (Kasten, F. and A. T. Young (1989). Revised optical air mass tables and approximation formula. Applied Optics 28 (22), 4735-4738 )   follows:

          m = 1.0 / [ cos(Z) + 0.50572 * (96.07995 - Z)-1.6364]

where Z is the solar zenith angle.

The figure below illustrates the concept of airmass.


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.

Algorithm - the set of simple instructions that combine to accomplish a task. Computer codes are algorithms.

Ambient Temperature - air temperature measured with a thermometer, similar to dry-bulb temperature.

Anemometer - an instrument that measures wind speed.

Angle of Incidence - 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°. The figure accompanying the description of airmass illustrates a solar angle of incidence of 48.2° to a horizontal surface.

Angular Response Characterization - quantifying 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. The determination of the true behavior of the pyranometer as the angle it makes with the light beam changes is called angular response characterization (see graph below for an example).

Click here for graph

ASHRAE - the American Society of Heating, Refrigerating, and Air-Conditioning Engineers.

Atmosphere - the zone of air that surrounds a planet.

Atmospheric Pressure - the pressure (force per area) created by the weight of the atmosphere. At higher elevations, the atmospheric pressure is lower because there is less air.

Atmospheric Turbidity - haziness in the atmosphere due to aerosols such as dust (particles ranging from 0.1 to 1+ microns in diameter). If turbidity is zero, the sky has no dust. A sun photometer is used to measure atmospheric turbidity.

Attenuation - loss of a substance as it is deflected, fragmented, or absorbed. For example, solar irradiance attenuates as it passes through the atmosphere to the surface of the earth.

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).

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