How the Water Cycle Transports Energy & Matter

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lesson, you will be able to explain what the water cycle is, and how the transfer of both matter and energy is involved. A short quiz will follow.

What is the Water Cycle?

The water cycle is the process by which water moves around the Earth, between rivers, oceans, the atmosphere and land. In general, water is always moving. When it rains in the mountains, that water finds its way into streams. There it heads downhill, and streams combine to form larger rivers. Eventually the water drains out into the sea. Water in the sea evaporates, forming a gas called water vapor. This water vapor rises over time. As it gets higher, the temperature gets colder and it condenses into clouds. These clouds can move over the land and rain water and snow back down onto the mountain again. This process continues over and over, and that's why it's a cycle: the water cycle.

There are other subtleties and inputs in the water cycle we can consider. Water doesn't just evaporate from the oceans after all; it also evaporates from those same rivers, streams and lakes. Water even transpirates out of plants, forming even more water vapor. Transpiration is the evaporation of water from plant leaves, which is possible because water can find its way through the leaf pores.

Water also doesn't just come back when it rains, because there's also fog and dew, which leaves droplets on plants and cold surfaces. Water can even move through the ground, instead of over it in the form of rivers. And even volcanoes create steam, which ultimately will feed clouds. So there are a lot of subtleties, but the basic concept is that water continues in this cycle, never really being lost.

How the Water Cycle Transports Energy & Matter

Water is matter, just like anything else. So the water cycle clearly transports matter. Whether water is in the form of a liquid, a gas (water vapor), or a solid (snow), it's still matter. But it turns out that the water cycle also transports energy.

The entire water cycle is driven by energy. It's energy from the Sun that causes the water to evaporate in the first place. This gives the molecules their kinetic energy (or movement energy), allowing them to rise up into the air. As they rise up, they slow down and that kinetic energy turns into gravitational potential energy (or height energy). Whenever an object is raised up in the air, it gains this kind of energy, and water vapor is no exception.

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