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Glossary of Solar Radiation Resource Terms |
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Other relevant glossaries
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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. 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 http://amsglossary.allenpress.com/glossary/search?id=extinction-coefficient1
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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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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