How the Earth Works

Water and Fire

The basics of the water cycle.

The sun has a huge effect on our water. It warms the oceans around the tropics, and its absence cools the water around the poles. Because of this, ocean currents move large amounts of warm and cold water, drastically affecting the weather and climate around the world. The sun also drives the water cycle, which moves about 18,757 cubic miles (495,000 cubic kilometers) of water vapor through the atmosphere every year [ref].

If you've ever gotten out of a swimming pool on a hot day and realized a few minutes later that you were dry again, you have firsthand experience with evaporation. If you've seen water form on the side of a cold drink, you've seen condensation in action. These are primary components of the water cycle, also called the hydrologic cycle, which exchanges moisture between bodies of water and land masses. The water cycle is responsible for clouds and rain as well as our supply of drinking water.


Here's what happens:

  1. The sun shines on the surface of oceans and lakes, exciting molecules of water. The more the sun excites the molecules, the faster they move, or evaporate.
  2. The molecules rise through the atmosphere as water vapor. Plants add to this water vapor through transpiration, a byproduct of photosynthesis, which also depends on the sun. In some locations, water sublimates, or changes directly from ice to vapor.
  3. All of this water vapor rises into the atmosphere. The higher it rises, the cooler it gets. The molecules of water slow down and stick together, or condense, as they cool. This forms clouds. Depending on how high and thick they are, clouds can either warm or cool the surface of the planet under them.
  4. Droplets continue to combine inside the clouds. When they get big and heavy enough, they fall as precipitation. (Pollution in clouds can decrease the amount of rainfall by requiring droplets to be bigger and heavier before they can fall.)
  5. Precipitation falls as rain, snow, sleet or hail, depending on the temperature and other conditions. Over land, it falls onto the ground and into rivers and lakes. Some of the water seeps into the soil, nourishing plants and joining the groundwater. Much of it flows into rivers and lakes, which eventually run into the ocean.

Without the sun to start the process of evaporation, the water cycle wouldn't exist. We wouldn't have clouds, rain or weather. The water on the planet would be stagnant. It would also be solid, since without the sun to warm it, the Earth would be entirely frozen.

The sun powers the processes that control our climate and the content of our atmosphere. Without it, we wouldn't have oxygen or liquid water on our planet. We wouldn't have weather or seasons. But the sun's immense source of power also has some drawbacks. Next, we'll look at some of phenomena that protect Earth from the power of the sun.