How come we never run out of water

Wednesday, November 01, 2006
After four and a half million years you'd think the water would be all used up! It's not, though, because of one special process:
The hydrologic cycle, more commonly called "the Water Cycle."

This Hydrologic Cycle recycles the earth's valuable water supply. In other words, the water keeps getting reused over and over. Just think, the next glass of water you drink could have been part of a dinosaur's bath in the Mesozoic Era one hundred million years ago. Water in that glass of water could have been a liquid, a solid, and a gas countless times over thanks to the water cycle.

The sun is the energy that powers this remarkable process. It's energy in the form of light, and heat causes water to EVAPORATE from oceans, rivers, lakes and even puddles. "Evaporate" means it turns the water from a liquid to a gas, or "vapor." Warm air currents rising from the earth's surface lift this water vapor up into the atmosphere.

When the air currents reach the cooler layers of the atmosphere, the water vapor condenses around and clings on to fine particles in the air. This step is called CONDENSATION. When enough vapor attaches itself to tiny pieces of dust, pollen or pollutants, it forms a cloud. Clouds do not last forever. Old clouds constantly re-evaporate and new ones form, creating ever-changing patterns in the sky.

As the air gets more and more moist, the droplets that form the clouds grow larger and larger. Eventually they will get so big that the swirling atmospheric winds can no longer hold them up. The droplets then fall from the sky as PRECIPITATION. Precipitation can be in the form of rain, snow, sleet or hail depending on other atmospheric conditions such as temperature.

Once the precipitation reaches the ground, several things can happen to it. First, it might be re-evaporated. For instance, we've all seen the mist rising off hot roads after a summer shower. If it isn't re-evaporated, much of the water will become RUN -OFF that goes into streams and rivers as it flows back to the ocean.

Some of the precipitation will be absorbed into the ground. This is called INFILTRATION. Once in the ground, the water can join the earth's GROUND WATER supply. This is one of the world's largest storehouses of water. The water could also be absorbed from the ground by the roots of plants.  

Another form of evaporation that contributes to the water cycle is TRANSPIRATION. Here, water given off through the pores of plants and animals joins the atmosphere as a vapor. Check, for instance, your own breath on a cold day. What you are seeing is water vapor being given off by your body.  

With transpiration and evaporation, the cycle begins again: EVAPORATION, CONDENSATION, PRECIPITATION, RUN-OFF, INFILTRATION, and TRANSPIRATION. Each time a molecule of water goes through the cycle it is cleaned, or purified, so it can be used by plants and animals again tomorrow, next year, and hopefully forever.

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