The hydrologic cycle, or water cycle, describes the movement of water to and from the atmosphere, surface, and subsurface of Earth. Water is found in all three phases: gas, liquid, and solid. Water (H2O) is a chemical compound formed from the elements hydrogen (H) and oxygen (O). Water exists as a gas in the atmosphere and as a liquid or solid (ice and snow) at the surface and underground on Earth. Most water is salty and found primarily in the oceans (over 96.5%), and about 1% is found as salty groundwater. Of the 2.5% of global water that is fresh water, more than half is frozen in polar ice caps, glacial ice, permanent snow, and permafrost. The remaining fresh water available for all life, from microbes to plants to humans and their activities on land, makes up less than 1% of all water on Earth. Most of that freshwater is groundwater, with the rest dispersed among lakes, streams and rivers, wetlands, living organisms, and the atmosphere.
Energy from the sun drives the flux of water from one location, or source, to another. The sun warms the surface of Earth and the oceans, causing water to evaporate into the atmosphere. The sun also drives photosynthesis, causing plants to open pores in their leaves that transpire water vapor into the atmosphere. Water has a lower density than air, so it rises and cools as it does so. Cooling water vapor condenses to form clouds that move with weather patterns.
The formation of precipitation (rain or snow) requires a small particle around which the precipitation forms. These particles can be mineral or bacterial in origin. When rain or snow forms around a bacterium, the bacterium is carried back to the ground with that precipitation. Many of these bacteria are pathogenic in origin, including Bacillus cereus, one causative agent of food poisoning. Precipitation hitting the ground can infiltrate soils or runoff and flow downhill into streams that eventually flow into lakes or oceans. Water that infiltrates soils becomes part of the groundwater. Groundwater is stored in soils and flows through them when they are saturated. Some soil water may seep into deep groundwater and into an aquifer, a rock layer permeable to water that acts as a reservoir. Other groundwater flows through soil and permeable rock until it breaches the surface and becomes runoff that enters water sources, such as streams.
Water in the atmosphere makes less than half of one-tenth of 1% of fresh water. Though small in volume, atmospheric water is the major driver of weather. For example, when water vaporizes, it extracts heat from its surroundings, cooling the surface of the ocean, the ground, or the plant leaf from which it arose. As water cools, it loses heat (a form of energy) to its surroundings. By vaporizing and condensing, water moves massive amounts of heat energy around the globe. Water vapor is also a potent greenhouse gas, modulating Earth's temperature. Greenhouse gases absorb radiant heat from Earth's surface and reflect it back to the surface.