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Climate and the Water Cycle

Scientists seek to understand how water vapor, precipitation, and land-surface hydrology interact across scales to define the hydrological cycle, and thus improve large-scale prediction models. Water is intrinsic to the climate system, circulating continuously between Earth and the atmosphere. The sun puts this cycle in motion by evaporating an enormous quantity of water. Although most comes from the oceans, water in lakes, rivers, and soil also evaporates, and additional water vapor comes from factories and motor vehicles.

Once in the atmosphere, water vapor is transported by winds over short or long distances. It eventually rises into cooler regions, condenses into clouds, and falls as precipitation. In time, the water evaporates again, continuing the cycle between Earth and sky.

Scientists refer to this process as the hydrologic cycle. The cycle is enormously complex, and much research is being conducted into what happens to water as it changes phases from solid to liquid to gas in the atmosphere and on the ground.

Scientists are still discovering exactly what drives the behavior of clouds and why water vapor that condenses as cloud droplets sometimes falls as rain. A cloud droplet is far too small to fall to Earth as a raindrop; in fact it takes about 1 million average-sized cloud droplets to make a single average-sized raindrop. The droplets can become raindrops in warmer weather by colliding with each other and coalescing. In cold weather or at higher latitudes, the droplets may form raindrops by becoming attached to ice nuclei.

Predicting the rate that raindrops form or even whether a cloud will produce any precipitation remains one of the most difficult challenges in meteorology. NCAR spearheaded a massive field experiment in 2002, the International H20 Project (IHOP), to map water vapor in three dimensions. This type of research may lead to a better understanding of water behavior in the atmosphere and, eventually, improved forecasts of storms.

Another scientific challenge is tracking rain, hail, or snow once it falls. Although most precipitation falls into the ocean and eventually evaporates again, close to a quarter falls on the continents. There, it may leave the hydrologic cycle for some time if it seeps into groundwater or is absorbed into the soil, although much of it eventually is washed back into the oceans.

NCAR scientists have estimated the freshwater discharge into the oceans based on the world’s major rivers. They found that peak discharge into the Arctic and Pacific Oceans occurs in June, compared to May for the Atlantic Ocean and August for the Indian Ocean. Since the mix of fresh- and saltwater is an important component of ocean circulation, such research may lead to insights into global climate patterns.

The hydrologic cycle has other profound effects on our climate. Water vapor is an important greenhouse gas that traps solar radiation in the atmosphere and warms Earth. However, when water vapor condenses into clouds, the effects become anything but straightforward. Low-forming stratocumulus clouds block sunlight and cool the planet, whereas high-forming cirrus clouds have a warming effect because they keep sunlight from radiating into space. At NCAR, scientists are using powerful computer models to sort out these myriad and sometimes conflicting impacts.

Floods and droughts are enormous challenges for societies across the world. Flooding kills more Americans in a typical year than tornadoes, hurricanes, or lightning. Scientists are unsure exactly what triggers droughts, although some evidence points to large-scale ocean changes, such as El Niño events. A key effort at NCAR is bridging the gap between researchers and public policy leaders around the world to help societies better prepare for droughts and reduce the risk of devastating crop failures.