Most of the moisture in the atmosphere—about 90 percent—comes from the oceans. Water is constantly recycled from the ocean into the air and back through a process called the water cycle. At any one time, the oceans contain about 97 percent of the earth’s water; the atmosphere contains only about 0.001 percent. Landmasses and ice hold the remainder. Still, if that seemingly tiny amount of atmospheric water vapor suddenly turned into rain, it would cover the entire Earth with an inch of water.
About 121,000 cubic miles of water evaporate from the earth’s surface each year, with around 86 percent of that coming from the oceans. The evaporation occurs due to the Sun’s heating of the sea surface. Warm air can hold a lot of moisture (think of steam), so some of the ocean surface converts to water vapor and is drawn up into the air.
Water can absorb a lot of heat before it begins to heat up itself. That’s why water makes such a good coolant for automobile radiators and why oceans prevent abrupt seasonal changes. Instead, winter comes on gradually as oceans slowly release their stored heat into the atmosphere, and summer takes a while to set in as the sea begins to reabsorb heat.
Evaporation occurs anywhere there is water, from lakes and rivers to storm drains and birdbaths. Plants even give off water through a process called transpiration, as they ooze small droplets of moisture from the undersides of their leaves. All of this warm water vapor begins to rise, joining billions of other water molecules in a dizzying ascent into the troposphere.
Eventually the vapor reaches cooler layers and condenses around small particles of dust, pollen, or pollution. As the condensation process continues, the droplets become too big for the wind to support and they begin a plunge toward the surface. Not all the precipitation reaches the ground, however; some of it evaporates directly back into the atmosphere on its way down. What’s left finally reaches the ground in the form of rain, snow, hail, or sleet, sometimes ruining picnics or closing schools in the process.
If the precipitation falls in the ocean, the cycle is ready to begin again right away, and that’s exactly what happens to the majority of raindrops and snowflakes. After all, oceans cover more than 70 percent of the earth’s surface, making them a big target. When it rains or snows over land, however, the cycle takes a little more time to complete.
Most water reaching the land surface runs off into ditches and streams where it finds its way back into lakes or the ocean. But some water seeps into the ground, percolating down until it is either trapped or it encounters a horizontal flow deep under the surface. The seeping water goes with the flow until it encounters a large underground reservoir known as an aquifer. Most aquifers eventually drain off into streams, which carry the water to rivers and canals and back to the sea. Then, of course, the whole cycle begins anew.