Glossary of Solar Radiation Resource Terms

Other relevant glossaries


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Barometer - an instrument that measures atmospheric pressure.


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


Beam Radiation - synonym for direct normal irradiance, the amount of solar radiation from the direction of the sun.

Click Shining On (Figure 3) to see solar components


Bias Limit - An indication of the average deviation of the predicted, or true values, from the measured values. Typically expressed as twice the mean bias error (MBE):

where:

    yi is the ith predicted, or true value
    xi is the ith measured value
    N is the number of observations.


Bidirectional Reflectance - A term for the amount of reflected radiation compared to the amount of incident radiation, or albedo, of a surface. The surface is not perfectly specular. That is, the reflected intensity is not at the same angle with respect to the surface normal as the incoming rays, nor are the two intensities necessarily equal (in crude terms, when light bounces off the Earth, some of it is absorbed and the rest of it bounces "funny", not as it would off a mirror).


Biomass - plants, crops, and trees; converted by solar fuel technologies into fuels and byproducts. See the Biomass Resource Information Clearinghouse.


Bird Clear Sky Model - named after Dr. Richard Bird, a scientist at SERI ( NREL), this physical model uses properties of the atmosphere such as albedo, turbidity, and precipitable water to determine the amount of solar radiation striking the earth's surface from a cloudless sky.


Blackbody - the theoretical "perfect" absorber of light at all wavelengths. As blackbodies heat up, they emit a characteristic double-exponential light frequency (energy) curve, which is imperfectly seen in nature. For example, Figure 2 from Shining On below shows the imperfect blackbody curve of the sun and the chunks that are ripped from it by the earth's atmosphere.


Bolometer - the most sensitive thermometer known to science. Invented in 1880 by astronomer Samuel P. Langley, the bolometer is used to measure light from the faintest stars and the sun's heat rays. It consists of a fine wire connected to an electric circuit. When radiation falls on the wire, it becomes very slightly warmer. This increases the electrical resistance of the wire. The difference in conductivity is proportional to the incident irradiance.


BORCAL - (Broadband Outdoor Radiometer CALibration)

A method of calibrating pyrheliometers and pyranometers based on the summation technique at the Solar Radiation Research Laboratory (SRRL). Up to three days of clear-sky solar irradiance measurements taken at 30-second intervals from sunrise to sunset are used to compute the individual radiometer responsivities.

For pyrheliometer calibrations, the reference direct normal irradiance is measured with an electrically self-calibrating absolute cavity radiometer traceable to the World Radiometric Reference (WRR). The individual pyrheliometer responsivity is computed as the mean ratio of the signal from the pyrheliometer (microvolts DC) to the reference irradiance (Watts per square meter) for each of the 30-second data samples.

Pyranometer responsivities are computed from the ratio of the signal from each pyranometer (microvolts DC) to the reference global horizontal irradiance determined by the direct normal irradiance (measured with an absolute cavity radiometer) and the simultaneous diffuse horizontal irradiance (measured by a reference pyranometer placed under a solar-tracking shading disk):

    Global Horizontal = Direct Normal x cos(Z) + Diffuse Horizontal

where

    Z = Solar Zenith Angle at the time of measurement.

The reference pyranometer is calibrated prior to use in a BORCAL event by means of the Shade Calibration Technique.


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.


BSRN - the worldwide Baseline Surface Radiation Network, or the program that manages it.


Btu - British Thermal Unit, the amount of energy required to raise one pound of water one degree Farenheit at 60°F, equivalent to 1055 joules or 252.1 calories.


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