(Solar) radiation - A generic term covering all forms of radiant energy falling on the surface of the earth and originating from the sun. (Sometimes insolation is used with this meaning). The far infrared "sky radiation" is not included.
(Solar) irradiation - The sum of the energy falling on a surface in a given time-period, originating from the sun, measured in MJ/m2.
(Solar) irradiance - The power or instantaneous rate of energy received by a surface, originating from the sun, measured in W/m2. In practice, the irradiance is usually measured by the average rate of accumulation of energy over one hour, for each hour of the day.
Global irradiance/irradiation - The power/energy received in the direct beam from the sun and that diffused by the hemisphere of the sky, onto a horizontal surface. Also referred to as the hemispherical solar irradiance on a horizontal plane, measured in W/m2.
Diffuse irradiance/irradiation - That portion of the power/energy received by a surface, not necessarily horizontal, from as much of the hemisphere of the sky as is exposed to the surface, and not including the radiation in direct beam from the sun.
Direct beam irradiance/irradiation - The power/energy falling in a direct line from the orb of the sun, without dispersion or reflection, onto a surface. For a horizontal surface, it is taken as the difference between the measured global and diffuse radiation.
Reflected irradiance/irradiation - The power/energy reflected from the surface of the earth which may fall onto a surface inclined to the horizontal.
Total irradiance/irradiation - The sum of direct beam, diffuse and reflected power/energy falling on an inclined surface. For a horizontal surface, it is the same as the global.
Extraterrestrial irradiance/irradiation - The total power/energy falling on a surface of the earth, which is conceived as a perfect spheroid without atmosphere. It is a function of the date and apparent time.
Clearness Index - The ratio between the global and the extraterrestrial irradiance at a given instant.
Solar heat gain factor - The solar irradiance transmitted through an unshaded standard clear glass window 3 mm thick, including that portion of the absorbed radiation that is transferred indoors.
Azimuth (of a vector) - The angle between true north and the projection of the vector onto a horizontal plane, measured clockwise.
Azimuth of a plane - The azimuth of the normal to the plane.
Solar zenith angle - The angle between the normal to a horizontal plane and the solar beam. Also referred to as the angular distance of the sun from the vertical (Θz).
Inclination of a plane - The angle between the normal to the plane and the vertical.
Mean Solar Time (MST) - This is Universal Time (UT or "Greenwich Mean Time") adjusted to a local longitude. It is the time recorded for all radiation measurements.
Local Time - This is the standard clock time (ignoring daylight saving) for the location. All recorded radiation measurements data provided in this website are adjusted to local time.