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The sum of direct and diffuse radiation on a horizontal surface. The Earth's atmosphere reduces the radiative power of extraterrestrial solar radiation (solar constant) through absorption, reflection, and scattering, so that the irradiance on the Earth's surface in our latitudes decreases to approximately 1000 W/m² (summer, clear sky, midday). The availability of solar energy fluctuates depending on meteorological conditions and astronomical principles (which, among other things, determine the seasonal variations).