Looking at the Sea The Water Cycle

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Millions of years ago there were no oceans on the planet. The surface of the Earth was so hot that water simply boiled away. But volcanoes poured huge amounts of steam into the atmosphere and as the Earth cooled down the steam turned to water vapor that condensed as droplets and began to fall as rain. This downpour lasted for many thousands of years filling great hollows in the land and thus forming the world's first seas.

Diagram of the water cycle.

Today, the oceans are always losing and gaining water in a never-ending process called the water, or hydrologic, cycle. They lose water when the sun and wind lift tiny particles of moisture from the ocean surface. These invisible particles of water vapor mix with air. If the air cools, vapor particles join up as water droplets that form clouds. Clouds shed rain or snow and most of it, in fact 77 percent of all precipitation, falls directly back over the sea. Rain water and melted snow that fall on land run into rivers flowing back into the sea. Thus, the oceans never dry up.

Within the last few decades, scientists have developed a fairly clear understanding of the role the ocean and the hydrologic cycle play in weather formation. One interesting result has been the explanation of the effects of El Niño. The warm surface water temperatures associated with El Niño lead to alterations in the movement of air masses and the development of irregular ocean currents. Changes in the normal formation of weather conditions are the result, and the entire world is eventually affected.


Building Models of the Water Cycle

This simple model represents the cycle of water from the ocean to the atmosphere and down again. You'll need two half gallon jars, a rock, masking tape and food coloring.

  1. Pour about 1.5 inches of water into one of the half gallon jars. Add a few drops of food coloring. This water represents the ocean.
  2. Put the rock in the middle of the jar. Some of the rock should stick up out of the water. This rock represents land.
  3. Invert the second jar and place it over the first jar. Tape the two jars together.
  4. Place the model in a sunny windowsill and observe.

The colored water in the bottom of the jar is heated by the sun. Some of this water receives enough energy to evaporate into water vapor (particles of pure water too small to be seen). The water vapor rises up in the warm air. When the water vapor comes close to the cooler sides of the jar it cools and condenses onto the jar. As more water vapor condenses onto the jar, droplets form and eventually grow big enough to precipitate down to the bottom. The droplets that condense out onto the sides of the jar are not colored like the water in the bottom of the jar. The larger food coloring particles are left behind just as salt and pollutants are left behind when water evaporates from oceans. You can build another simple model of the water cycle using a zip lock baggie.

This activity adapted from the
Museum of Science
Weather Kit.

Science Learning Network | email: sln@mos.org | © 1998 The Museum of Science