Water circulates continuously through Earth’s crust, oceans, and atmosphere. The physical movement of water in all its physical states, including a complex series of phase changes below, on, and above Earth’s surface is called the water cycle. It’s a process that has been cycling on our planet for billions of years!

Two things stand out when following the water cycle: the formation of freshwater from salt water and the transfer of energy using water.

About 97.5 percent of Earth’s water is salt water found in oceans, seas, bays, saltwater lakes, and some groundwater. One of the distinguishing differences between the water from these saline sources and the water found in freshwater lakes, rivers, streams, and even ice and snow is the amount of salt each contains. If you were to analyze the physical makeup of ocean water, you would find that about 3.5 percent of the weight of the water is salt. In freshwater less than 0.1 percent of the weight is salt. Even freshwater usually contains a small amount of salt.

 

Freshwater is formed by the evaporation of water from saltwater bodies. As evaporation occurs, the salt is left behind in the ocean or sea. Freshwater, in the form of water vapor, is transported into the atmosphere.

Solar radiation striking the ocean warms the surface water. Individual water particles gain energy and evaporate. Along with other warmed air particles, they spread out, forming an air mass of low density. This warmed, low-density air mass rises. As it gains altitude, the air cools and contracts. Eventually, the rising air cools to a temperature at which its water vapor changes state to ice crystals and liquid water, which then become visible as clouds. The water remains aloft until it falls back to Earth as precipitation.

Most precipitation falls over Earth’s oceans, simply because oceans cover about 70% of our planet’s surface. Precipitation that falls over land masses is our primary source of freshwater. Freshwater can flow along the surface into streams and rivers, or beneath the surface as groundwater. Eventually, most water, moving on a downward slope with the aid of gravity, reaches the oceans. There, the water cycle repeats as solar energy once again warms and evaporates surface water.

In fact, there are many ways that water can circulate through the water cycle by means of other sub-cycles. Water may return directly to the ocean after evaporating. Freshwater may also evaporate and return to the atmosphere without reaching the ocean. Each process is important to how weather develops and changes.

With all of this water cycling around, a lot of energy is being transferred throughout the atmosphere. The water cycle drives both local weather and climatological patterns everywhere on Earth. There are regions of the planet where energy is absorbed more readily and places where it can be released more readily too. Forecasters spend a great deal of time trying to predict the time and location of these phase changes and the precipitation events that can result. We call these precipitation events storms; and the more energy that is released, the bigger the storm. Perhaps some may even form monster storms.

Most of the freshwater on Earth’s surface is not in the liquid state. It’s frozen and stored, mostly within the ice cap atop Antarctica. Like its liquid counterpart, this water is also a part of the water cycle. Unlike a quickly evaporating spill of water, however, water stored in Antarctic ice may need thousands or even millions of years to be recycled.

Scientists like Robbie Hood want to understand how water, in the form of liquid water and ice, behaves in hurricanes and other storms. The water cycle can tell her where the water is before the hurricane develops and how it might influence the development and maximum strength of the storm. She needs to know how much water the atmosphere can absorb and what it is most likely to do as the storm develops.

Right now, many of the instruments Robbie uses are mounted in specially outfitted airplanes, like the ER-2, that fly high above the weather events she monitors. In addition, some of these same instruments are on satellites like NASA’s Aqua. As Robbie refines and improves the interpretation of the data collected from the planes, this also improves the analysis she can do with the satellite data. This will allow her and other researchers to monitor the atmosphere continuously by satellite and gather data well beyond the locations where they are performing actual research flights.