Geology Reference
In-Depth Information
The sun's heat fuels the evaporation of water from the ocean. In vapor form,
the molecules hang out in the atmosphere together and collect additional water
molecules, becoming clouds. Whenever you see clouds shape-shifting across the
sky or blotting out the sun, you're actually seeing condensed water vapor that is
thick enough to be visible. When the water vapor in the atmosphere is very close
to earth's surface, you call it fog.
Eventually the amount of water vapor in the air becomes too heavy, either because mo-
lecules are added or because the air temperature cools down. (Cold air holds less water
than warm air.) When the water vapor becomes too heavy, water droplets fall to the
earth's surface as rain or snow. Once on the surface, all this water has the same goal: to
travel back to the ocean. Water can take a number of different paths to the ocean de-
pending on where (and when) it falls.
Traveling across a continent
When clouds release rain over the continents, the water begins a long journey back to
the ocean. Most water does not immediately enter a river and flow directly to the sea. In-
stead, it travels over ground or underground, and sometimes it becomes trapped as
snow, ice, or lake water for many years before it continues to the ocean.
Here are the different paths water can take after falling from the clouds as rain (or
snow):
Surface runoff: Most of the water that falls to the surface becomes surface runoff
and makes its way to the nearest lake, stream, or ocean by traveling across the
surface.
Groundwater: Some of the water that hits the surface is absorbed into the ground
and moves through underground rocks and sediment as groundwater. This water
can be accessed when you dig a well. I describe groundwater in detail in the final
section of this chapter.
Snowpacks or ice sheets: In some areas, water falling as snow remains part of the
snowpack (the snow that builds up and remains on mountaintops for many
months of the year) until the next warm season, when it melts and becomes sur-
face runoff and groundwater. Snow that falls on ice, such as at the South Pole or in
Greenland, becomes part of the ice sheet (thick layers of ice across a large area of
continent) and stays there for thousands of years.
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