UO Solar Radiation Monitoring Laboratory

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SRML Glossary — "A"

Special thanks to NREL

We'd like to thank the National Renewable Energy Laboratory for making available to us their glossary, which is the basis of ours. We've edited and reformatted it, and linked it to our Web pages, and we'll continue to add our own specialized terms, illustrations, and examples. Please note that the Solar Radiation Monitoring Laboratory takes full responsibility for any inaccuracies that may occur.

Links to other glossary sections:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z


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. Here 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) 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.
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 Character-
ization
 
  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).


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).
   
Links to other glossary sections:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

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© 2002, UO Solar Radiation Monitoring Laboratory.
Last revised: March 17, 2002.


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