Solar radiation quantities measured
Solar energy received at the Earth's surface can be separated into two basic components: direct solar energy and diffuse solar energy. Direct solar energy is the energy arriving at the Earth's surface with the Sun's beam. The Sun's beam is quite intense, and hence has also been described a 'shadow producing' radiation.
Diffuse solar energy is the result of the atmosphere attenuating, or reducing the magnitude of the Sun's beam. Some of the energy removed from the beam is redirected or scattered towards the ground - the rate at which this energy falls on a unit horizontal surface per second is called the diffuse solar irradiance.
The remaining energy from the beam is either scattered back into space, or absorbed by the atmosphere. Absorption only occurs at specific wavelengths, for example, UVB solar energy is absorbed by ozone in the stratosphere. Scattering occurs at all wavelengths; hence the mechanism by which solar energy is scattered from water droplets and ice particles makes possible those majestic satellite pictures of clouds. The combination of both forms of solar energy incident on a horizontal plane at the Earth's surface is referred to as global solar energy and all three quantities (specifically their rate or irradiance) are linked mathematically by the following expression:
Eg = Ed + Eb cos(z)
where: Eg = global irradiance on a horizontal surface, Ed = diffuse irradiance, Eb = direct beam irradiance on a surface perpendicular to the direct beam, z = Sun's zenith angle. By measuring the three components independently, a useful quality assurance test is immediately available by comparing the measured quantity with that calculated from the other two.
Radiation quantities are generally expressed in terms of either irradiance or radiant exposure. Irradiance is a measure of the rate of energy received per unit area, and has units of watts per square metre (W/m2), where 1 watt (W) is equal to 1 Joule (J) per second. Radiant exposure is a time integral (or sum) of irradiance. Thus a 1 minute radiant exposure is a measure of the energy received per square metre over a period of 1 minute. Therefore a 1-minute radiant exposure = mean irradiance (W/m2) x 60(s), and has units of joule(s) per square metre (J/m2). A half-hour radiant exposure would then be the sum of 30 one-minute (or 1800 one-second) radiant exposures. For example: a mean irradiance of 500 W/m2 over 1 minute yields a radiant exposure of 3000 J/m2 or 3 KJ/m2. The output of the Bureau of Meteorology's computer model, which estimates the daily global solar exposure from satellite data, provides irradiance integrated over a period of a day i.e. radiant or global exposure, with units of megajoule(s) per square metre. In terms of remote sensing by satellite, radiance refers to energy received by a satellite sensor and is the rate of energy received per unit area per unit of solid angle (with units of watt(s) per square metre per steradian).
Direct solar irradiance
Direct solar irradiance (also referred to as direct normal irradiance) is a measure of the rate of solar energy arriving at the Earth's surface from the Sun's direct beam, on a plane perpendicular to the beam, and is usually measured by a pyrheliometer mounted on a solar tracker. The tracker ensures that the Sun's beam is always directed into the instrument's field of view during the day. The pyrheliometer has a field of view of 5° . In order to use this measurement for comparison with global and diffuse irradiances, it is necessary to obtain the horizontal component of the direct solar irradiance. This is achieved by multiplying the direct solar irradiance by the cosine of the Sun's zenith angle.
Sunshine duration is defined to be the sum of all time periods during the day when the direct solar irradiance equals or exceeds 120 W/m2. This measurement is only obtained from configurations that measure direct solar irradiance.