Hydrologic Cycle - A Multi Phased Journey

The hydrologic cycle describes the pilgrimage of water as water molecules make their way from the Earth's surface to the atmosphere, and back again. This gigantic system, powered by energy from the sun, is a continuous exchange of moisture between the oceans, the atmosphere, and the land.

In the hydrologic cycle, individual water molecules travel between the oceans, water vapor in the atmosphere, water and ice on the land, and underground water.

Studies have revealed that the oceans, seas, and other bodies of water (lakes, rivers, streams) provide nearly 90% of the moisture in our atmosphere. Liquid water leaves these sources as a result of evaporation, the process by which water changes from a liquid to a gas.

In addition, a very small portion of water vapor enters the atmosphere through sublimation, the process by which water changes from a solid (ice or snow) to a gas. (The gradual shrinking of snow banks, even though the temperature remains below the freezing point, results from sublimation.) The remaining 10% of the moisture found in the atmosphere is released by plants through transpiration. Plants take in water through their root systems to deliver nutrients to their leaves, then release it through small pores, called stomates, found on the undersides of their leaves. Together, evaporation, sublimation, and transpiration, plus volcanic emissions, account for all the water vapor in the atmosphere. While evaporation from the oceans is the primary vehicle for driving the surface-to-atmosphere portion of the hydrologic cycle, transpiration is also significant. For example, a cornfield 1 acre in size can transpire as much as 4000 gallons of water every day.

ROUGHLY ONE THIRD OF THE EARTH'S ANNUAL PRECIPITATION THAT FALLS ON THE CONTINENTS RUNS OFF INTO THE OCEANS

After the water enters the lower atmosphere, rising air currents carry it upward, often high into the atmosphere, where the air cools and loses its capacity to support water vapor. As a result, the excess water vapor condenses (i.e., changes from a gas to a liquid) to form cloud droplets, which can eventually grow and produce precipitation (including rain, snow, sleet, freezing rain, and hail), the primary mechanism for transporting water from the atmosphere back to the Earth's surface.

Plants return water to the atmosphere through transpiration. In this process, water evaporates from pores in the plant's leaves, after being drawn, along with nutrients, from the root system through the plant.

Related Links:

What You Should Know

The Water Cycle

The Water Table

The Water Cycle and Climate Change

Cloud Fraction

The Aqua Mission

The Aqua Spacecraft

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