The water cycle describes how water changes between solid, liquid, and gas (water vapor) phases and changes location. Water can be evaporated, which is the process where a liquid is converted to a gas. Solar energy warms the water sufficiently to excite the water molecules to the point of vaporization. Evaporation occurs from surface water bodies such as oceans, lakes, and streams and the land surface. Plants contribute significant amounts of water vapor as a byproduct of photosynthesis in a process called transpiration. Geologists commonly combine these two sources of water entering the atmosphere in a term called evapotranspiration.
Water vapor in the atmosphere can migrate long-distance from ocean to over land by way of prevailing winds. Over the ocean or land, the air can cool and cause the water to condense back into liquid water. This usually happens in the form of very small water droplets that form around a microscopic piece of dust or salt called condensation nuclei. These small water droplets are visible as a cloud. Clouds build and once the water droplets are big enough, they fall to earth as precipitation. Precipitation can take the form of rain, snow, hail, or sleet.
Once it has reached the surface it does two important things relevant to the geology of this chapter. At the surface, liquid water can flow as runoff into streams, lakes, and eventually back to the oceans (in most cases). Water in streams and lakes is called surface water. In addition, water can also infiltrate into the soil and finally fill the pore spaces in the rock or sediment deep underground to become groundwater, the name given to all subsurface water. Groundwater slowly moves through rock and unconsolidated materials and some of it eventually reaches the surface again, where it discharges as springs and into streams, lakes, and the ocean. Also, surface water in streams and lakes can infiltrate again to recharge groundwater. Therefore, the surface water and groundwater systems are connected.