Global solar radiation
The flux of the solar short-wave radiation, in wavelengths of 0.3-3 micron (UV up to "near" infrared) that reaches the ground both directly from the sun disc (see direct radiation) and indirectly (see diffuse radiation) from the sky. Measurements of the flux of radiation are made using a pyranometer that is placed on a horizontal plane. The measurement units are watt per square m.
Direct solar radiation
The flux of the sun's radiation arrives directly from the sun disc. The measurements of the direct radiation are done using a pyrheliometer that is placed in a plane that is perpendicular to the sun’s rays. The pyrheliometer is attached to an equatorial chassis (solar tracker) that automatically points towards the sun. The chassis is equipped with a synchronous electrical circuit that follows after the expected movement of the sun. Any inaccuracy in adjusting for the sun's direction can cause a serious error in the measurement. Therefore, the tracker is calibrated manually at least once a day (in the last couple of years automatic solar trackers were developed that are equipped with a computer that adjust to the sun's position's anytime, even after a blackout). In this case the measurement units are also in watts per
square m.
Diffuse solar radiation
The diffuse solar radiation is the short wave radiation from the sky, except for the sun disc itself, after going through processes of dispersion and reflection by the molecules and particles in the air. The measurements of diffuse radiation are made using a pyranometer that is placed on a horizontal plane, equipped with a shading ring which blocks the direct radiation. The ring hides not only the sun but also the sky around it, an area whose contribution to radiation is greater than the relative size of the sky which it occupies. This necessitates some corrections in the measurement. The correction varies from place to place and takes into account the the cloud cover and concentration of aerosols. In the last few years, a pyranometer shaded by a metal ball is placed on a computer-controlled automatic tracker reducing considerable errors in the diffuse solar energy measurement.
Measurement units of the solar radiation
The flux of the solar radiation that reaches the ground is measured in units of watt per square m. ( 1 watt= 1 joule per second). MegaJoules (millions of joules) are conveniently used to for the hourly global radiation accumulations.
This unit is also available for the calculation of a daily average in a particular month (i.e. the cumulative radiation during one day in a particular month).
Ultra-violet radiation
The sun’s radiation in very short wave lengths, between 0.2 and 0.4 microns (between 200 and 400 nanometers, with one nanometer = one billionth of a m. A secondary division distinguishes between C-band ultra-violet radiation (280-200 nanometers), ultra-violet radiation in the biologic B-band (280-320 nanometers), and A-band ultra-violet radiation (320-400 nanometers). The shorter the wave length, the larger the energy carried by the photon, and the greater the ability of the radiation to penetrate body tissue. The stratospheric ozone layer absorbs the C-band radiation completely, as well as part of the A-band and B-band radiation. The thinning of the ozone layer (also known as the “hole in the ozone layer”) has aroused health concerns because the increased exposure to B-band ultra-violet radiation is linked to cancer, cataracts and more.
Instruments for measuring ultra-violet radiation
In Israel, measurements are usually carried out for the global B-band ultra-violet radiation. The Meteorological Service operates radiometers at a number of observation stations, measuring the entire spectrum of wave lengths that characterize ultra-violet radiation. At two additional observation stations, Ben-Gurion University operates radiometers for detecting A-band radiation.
All the instruments are broadband-type radiometers. The response of the radiometer “imitates” the skin’s response to radiation which is strongest at wave lengths in the 295-300 nanometer range.
Units for measuring ultra-violet radiation
The ultra-violet radiation flux can be expressed in units of watt/m2, but in order to illustrate the significance of the measurement, MED (Minimal Erythemal Dose) units are used. These units are defined as the amount of radiation that causes a minimal reddening of white, un-tanned skin. However, a special UVB index was developed, which is expressed in non-dimensional units (equal to the amount of the radiation flux in milliwatt divided by 25) The scale of the index ranges from 1 to 13 , with 1 also including all lower values, and a value of 11 or higher categorized as extreme.