The constant movement of water on Earth through evaporation, transportation and condensation. The amount of water on Earth remains approximately constant, but the energy of solar radiation reaching the surface keeps water molecules constantly moving. Water evaporates from water bodies, soils or through the transpiration of plants. In the atmosphere, it is carried away by air currents and condenses under the right conditions. Eventually it reaches the earth’s surface in the form of dew, rain, snow or hail. Liquid water is collected in watercourses or groundwater, from where it travels by gravity to lower-lying areas such as oceans.