A little old image of direct normal solar radiation in South America with a resolution of 40 km x 40 km created by NREL fot SWERA project.
Better resolution images in pdf format can be found here.
The solar radiation that is emitted from the sun in the form of electromagnetic waves, entering the atmosphere is filtered by the gases it contains, mainly by clouds that are floating, which bounce the light to operate as mirrors. When passing a cloud, global radiation decreases dramatically and diffuse radiation increases as a result of the deviation in the direction of the aces of light that causes the cloud.
Figure 1: Solar radiation in a sunny day
Figure 2: Solar radiation in a partly cloudy day
Accurate computation of solar position plays a fundamental role in solar energy applications, especially for concentrating systems. The required accuracy varies over a wide range, depending on the application: flat systems tolerate errors of a few degrees without significant losses, while high-concentration systems can require an accuracy of the order of 0.01º. More specific applications, such as the calibration of pyranometers ( Reda and Andreas, 2004 ), require an even greater accuracy.
Most of the energy that it is emitted from the Sun is in form of electromagnetic radiation with a specific spectrum given by the temperature of his external layer. The little part of this energy that arrive to earth is our source of life and energy.
The radiation that arrives to the external layer of the earth to a normal plane, before been filtered by the atmosphere is called Extraterrestrial Radiation and can be approximately calculated by :
= Solar constant, 1367 [watt/m2]
dn = day number of year (1 … 365)
Then the radiation is filtered by the gases presents in the atmosfere like H2O, CO2, O3 and O2, and reflected by the clouds. The radiation that finally arrives to the lands and oceans it’s in part absorbed and part reflected.
The absorbed radiation is transformed in heat and emitted back to the space like infrared radiation.
Image from Kipp & Zonen
To measure the solar radiation that arrives on Earth requires an instrument with a thermopile, if this is design to measure infrared wavelength the instruments it’s call Pyrgeometer but if is design to measure the visible spectrum It’s call Pyranometer. A good thermopile must have a flat response to the whole spectrum that is measuring.
Image from Kipp & Zonen