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You may be familiar with how water is always cycling around, through, and
above the Earth, continually changing from liquid water to water vapor to ice.
One way to envision the water cycle is to follow
a drip of water around as it moves on its way. I could really begin this
story anywhere along the cycle, but I think the ocean is the best place to
start, since that is where most of Earth's water is.
If the drip wanted to stay in the ocean then it shouldn't have been
sunbathing on the surface of the sea. The heat from the sun found the
drip, warmed it, and evaporated it into water vapor. It rose (as tiny
"dripettes") into the air and continued rising until strong
winds aloft grabbed it and took it hundreds of miles until it was over
land. There, warm updrafts coming from the heated land surface took
the dripettes (now water vapor) up even higher, where the air is quite cold.
When the vapor got cold it changed back into it a liquid (the process is condensation). If it was cold enough, it would have turned into tiny ice crystals, such as those that make up cirrus clouds. The vapor condenses on tiny particles of dust, smoke, and salt crystals to become part of a cloud. After a while our drip combined with other drips to form a bigger drop and fell to the earth as precipitation. Earth's gravity helped to pull it down to the surface. Once it starts falling there are many places for water drops to go. Maybe it would land on a leaf in a tree, in which case it would probably evaporate and begin its process of heading for the clouds again. If it misses a leaf there are still plenty of places to go.
The drop could land on a patch of dry dirt in a flat field. In this
case it might sink into the ground to begin its journey down into an
underground aquifer as ground water. The
drop will continue moving (mainly downhill) as ground water, but the
journey might end up taking tens of thousands of years until it finds
its way back out of the ground
But our drop may be a land-lover. Plenty of precipitation ends up
staying on the earth's surface to become a component of
surface
water. If the drop lands in an urban area it might hit your house's
roof, go down the gutter and your driveway to the curb. If a dog
or squirrel doesn't lap it up it will run down the curb into a
storm sewer
If no one interferes, the trip will be fast (speaking in "drip time") back to the ocean, or at least to a lake where evaporation could again take over. But, with 250+ million people here needing water for most everything, there is a good chance that our drop will get picked up and used before it gets back to the sea.
2nd graders view of the water cycle |
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Go to: Earth's water
USGS home page The URL for this page is http://ga.water.usgs.gov/edu/followdrip.html Comments? Contact hperlman@usgs.gov Last Modified: Feb 03, 2000 |
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