Solar radiation is the electromagnetic energy emitted by the sun, which travels across 150 million kilometres of space to reach Earth in about 8 minutes. The sun radiates across the entire spectrum — from gamma rays to radio waves — but the majority of energy (about 99 %) arrives as ultraviolet (UV), visible light, and near-infrared radiation. At the top of the atmosphere, Earth receives approximately 1,361 W/m² (the solar constant), though this varies slightly (~0.1 %) with the 11-year solar cycle.
The atmosphere filters incoming solar radiation substantially. Ozone (O₃) absorbs most harmful UV-B and all UV-C. Water vapour, CO₂, and other greenhouse gases absorb certain infrared wavelengths. Clouds reflect 20–70 % of incoming light depending on thickness. Aerosols (dust, pollution, volcanic particles) scatter and absorb radiation. After these interactions, roughly 48 % of the solar constant reaches the Earth's surface on a clear day. The surface absorbs this energy and re-emits it as longwave infrared radiation, which is partially trapped by the greenhouse effect.
Solar radiation varies enormously with latitude, season, time of day, altitude, and cloud cover. Spain, with its southern latitude and frequent clear skies, receives 1,600–2,000 kWh/m² annually — among the highest in Europe, making it ideal for solar energy. The UV index measures the biologically effective UV radiation reaching the surface. For meteorology, incoming solar radiation is the ultimate energy source driving all weather — differential heating between equator and poles, land and sea, surface and atmosphere creates the pressure gradients, winds, and circulation patterns that constitute our weather and climate systems.