Precipitation and the Water Cycle
Earth’s water is stored in ice and snow, lakes and rivers, the atmosphere and the oceans. How much do you know about how water is cycled around our planet and the crucial role it plays in our climate?
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One of the five fundamental processes in the water cycle, evaporation involves the conversion of water from a liquid to a gas (or "vapor"). The main cause of evaporation is heat from the sun warming the Earth's surface. The water cycle is a never-ending process of water being circulated around the planet from clouds to land, to the ocean, and back to the clouds.
Earth is often referred to as the "Water Planet" because you can see water in all three forms as you gaze at Earth from space. As we search for life elsewhere in the cosmos, we look for places that have liquid water, as it seems to be the primary requirement for life as we know it. About 70 percent of Earth is covered by water, and most of that water (97 percent) is found in our vast oceans.
All living things depend on water; knowing just how much moisture is in soil is vital for understanding how our planet functions. Soil moisture is important for crop yields, crop productivity and a host of ecosystems.
Although land makes up about 30 percent of the Earth's surface, most global evaporation occurs over the ocean, and much of that water falls back into the oceans as precipitation. Only about 22 percent is transported over land to fall as rain or snow.
Rain gauges measure the amount of rainfall at a particular location and are used by gardeners, farmers and scientists. Although countries like the U.S. have an extensive rain gauge (and ground radar) network, the coverage is much spottier in developing nations and almost non-existent over the oceans. This is why space satellites, which can measure precipitation all over the world, are important.
Water can take many paths as it travels around the planet. For example, water is in the atmosphere for just 8 to ten days, but it can stay in the soil for 1 to 2 months and as seasonal snow for 2 to 6 months. Water can remain locked into a glacier as ice for 20 to 100 years, in the oceans for 3,000 years or stored away as deep groundwater for 10,000 years. An epic journey!
The sun regulates how much water evaporates, condenses, falls as precipitation, is absorbed by the ground and runs off or flows over the land. About 29 percent of the sun's incoming energy is reflected directly back into space (by the atmosphere and brightly-colored ground surfaces), and another 23 percent is absorbed by our atmosphere. That leaves 48 percent to be absorbed by the Earth's land surface and water.
Although it's commonly thought that raindrops take the form of a teardrop, they are actually shaped more like a hamburger bun as they fall. Up in the clouds, water vapor condenses into near-spherical raindrops because of surface tension. But as they fall, larger drops are affected by the air friction pushing against them and flatten out on the bottom.
Warm air can hold more moisture than cool air. This is because at higher temperatures, more liquid water molecules evaporate, which means there is more water vapor in the air. Warmer, wetter air can have other knock-on effects on storms and extreme weather events like floods. Extra heat in the atmosphere and oceans (related to global warming) fuels storms, potentially causing them to become stronger and more intense.
The Antarctic Circumpolar Current is the only ocean current that flows all the way around the globe uninterrupted by land. Also known as the West Wind Drift, it is the largest and strongest current system on Earth and flows clockwise (from west to east) around Antarctica. It connects the Atlantic, Pacific and Indian Ocean basins.