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Valentia Observatory

Solar Radiation
Energy from the sun in the form of ultra-violet, visible and infra-red electromagnetic radiation is known as solar radiation. It has been measured by Met Éireann since 1954 when the first pyranometer was installed at Valentia Observatory. At present, observations of solar radiation are made at seven meteorological stations as well as by some Universities and other research institutes. Data are available from Met Éireann's climate database, located in Glasnevin, Dublin.

Why Solar Radiation is measured
Solar radiation is measured by Met Éireann as one of the many elements in its observational programme and is being added to its climatological database. It is used in agrometeorology where plant growth, ripening, drying conditions and crop yield are related to the solar energy available. Solar radiation has been supplied to users interested in studying the correlation between solar radiation and such topics as the growth of plants, algae and planktons; the efficiency of energy collectors; solar heating, and research into the measurement and analysis of solar radiation.

Solar Radiation spectrum Brief description of the nature of Solar Radiation
As mentioned above, solar radiation is a term used to describe visible and near-visible (ultraviolet and near-infrared) radiation emitted from the sun. Different regions of the solar spectrum are described by their wavelength range within the broad band range of 0.20 to 4.0 µm (microns). Infrared radiation emitted from the atmosphere is referred to as terrestrial radiation. Table 1 below gives the components of solar and terrestrial radiation together with their approximate wavelength ranges:

Table 1  
0.20 - 0.39µ
Visible 0.39 - 0.78µ
Near-Infrared 0.78 - 4.00µ
4.00 - 100.00µ

Solar radiation with wavelengths greater than 3.0µm (approximately 2% of the total energy reaching the earth) is absorbed by the atmosphere. Radiation from other sources with still longer wavelengths (infra-red, micro- and radio waves) is transmitted through the 'atmospheric window'. Water vapour and CO2 also absorb some radiation in the infra-red region while ozone absorbs much of the ultra-violet.

Outside the earth's atmosphere, solar radiation has an intensity of approximately 1370 watts/meter2. This is the value at mean earth-sun distance at the top of the atmosphere and is referred to as the Solar Constant. The availability of energy at the surface is affected by location (including latitude and elevation), season, and time of day, all of which can be readily determined. However, the biggest factors affecting the available energy are cloud cover and other meteorological conditions, which vary with location and time. In Ireland at the surface on a clear day, at noon, the direct beam radiation will be approximately 940 watts/meter2 for many locations.
Measurement of Solar/Terrestrial Radiation
Table 2 below gives a summary of the solar and terrestrial radiation measurements made by Met Éireann:

Table 2  
Type of measurement Description
Direct Solar Radiation Solar radiation at normal incidence in the direct beam from the sun
Diffuse Solar Radiation Scattered radiation on a horizontal surface
Global Solar Radiation Sum of the direct beam plus the diffuse component on a horizontal surface
Infra-red Radiation Terrestrial infra-red radiation emitted by the sky on the Earth's surface
Net Radiation balance Combined downward solar radiation and sky infra-red minus upward reflected solar and terrestrial radiation
Turbidity Measure of the amount of scattering in the atmosphere

Pyranometer Pyranometer Pyheliometer

This is the most frequently used sensor for measuring Global or diffuse radiation. It consists of a thermopile composed of a series of thermocouples with alternate 'warm' and 'cold' junctions. 'Warm' junctions are in thermal contact with a 'black' absorbing surface with low thermal capacity. 'Cold' junctions are in contact with a material with relatively high thermal capacity and remains at ambient temperature. When solar radiation falls on the 'black' absorbing surface almost all of the incident radiation is absorbed. This results in a rise in temperature at the 'warm' junction proportional to the intensity of the radiation. A voltage is then generated by the thermopile, which is proportional to the temperature difference between the 'warm' and 'cold' junctions. This output voltage is converted Watts/meter2 by use of a sensitivity factor (whose units are µV/(W/m2)) which can be established through sensor calibration with standard lamps in controlled conditions.


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