Where does the energy to power the water cycle come from? Heat from the sun is responsible for initiating the various physical processes of the hydrologic cycle. Heat from the sun causes water molecules to evaporate from Earth's surface. Evaporation leads to an increase in atmospheric water vapor, which eventually triggers condensation and clouds form. Uneven heating of Earth's surface by the sun also generates wind. These air currents are responsible for the movement of clouds over mountains, which can create more precipitation. Heat from the sun can also contribute to surface runoff by melting snow and ice.