Evaporation is the initial driver of the water cycle: the sun heats the surface of oceans, lakes, rivers, and moist soil, providing water molecules with the kinetic energy needed to transition from liquid to gas phase. Unlike boiling, evaporation occurs at any temperature below the boiling point, provided the surrounding air is not saturated with vapour. The evaporation rate depends on several factors: available solar radiation, water and air temperature, relative humidity (lower humidity means greater evaporation), wind speed (which renews air above the evaporating surface), and the exposed surface area. It is estimated that oceans contribute approximately 86% of global evaporation, equivalent to around 425,000 km³ of water per year. In meteorology, evaporation has a cooling effect on the surface—each gram of water evaporated absorbs 2,260 joules of energy—and constitutes the primary source of atmospheric moisture that subsequently forms clouds and precipitation. Evapotranspiration, which includes plant transpiration, amplifies this vapour input. In arid regions such as southeastern Spain, evaporation can greatly exceed annual precipitation, creating chronic water deficits.