The hydrologic cycle or the water cycle as it is commonly known is driven by solar energy. Surface water gains heat energy from the sun and becomes water vapor. Evaporation brings this water to the atmosphere where condensation occurs. When temperatures decrease, it becomes precipitation and returns to Earth as rain or snow. Another important part of the hydrologic cycle is the water vapor released by living organisms via respiration. Plants release excess water by transpiration through their stomata. Groundwater flows from high elevations to low, eventually making it to sea level where this water is added to the ocean. Runoff from mountain streams, replenishes groundwater as well as rivers, streams and eventually the sea. The hydrologic cycle is vital to all life on Earth and without solar energy, would cease.