Geography Reference
In-Depth Information
In addition to the stages described in the next section, the water cycle has two overriding character-
istics that are good to keep in mind. Here they are:
It really is a cycle.
The water really does go round and round just like the diagram suggests.
“Cycle,” therefore, very appropriately describes what's going on.
The cycle is a closed system.
By closed system, I mean that nothing gets added to it or
subtractedfromit.Forallintentsandpurposes,theamountofwateronEarthisfixed—closed
to change. Thus, if a lake or reservoir dries up, the water is not really “gone.” Instead, it has
relocated elsewhere within the system.
The stages of the water cycle
Here are the principal components of the water cycle:
Solar energy:
Technically, sunshine isn't part of the water cycle for the simple reason that
sunlight isn't water. But it's the sun that sets the cycle in motion. The sun is the pump that gets
things moving.
Evapotranspiration:
Whew! That's a mouthful. And it's a combination of two words:
evaporation and transpiration. With regards to the first word, some water evaporates when it
receives solar energy. That is, it changes from a liquid state to a gaseous state: vapor. With
salty sea-water, only the water evaporates, and not the salt. So salty seawater becomes fresh-
water vapor in the atmosphere.
With regards to the second word, transpiration, you perspire, plants transpire. Plants give off
moisture when the sun heats them up — only it's called transpiration. Plant sweat is a key in-
put into the vapor in the atmosphere, especially in lush, tropical areas.
Condensation:
Water vapor is so small that it's invisible. But when lots of individual bits
of vapor cluster and combine, they become visible miniature droplets of water. This is con-
densation, and if it happens by the gazillions, you get a cloud. As to exactly what happens to
produce condensation, well, I'm holding that until Chapter 9.
Precipitation:
If condensation continues, then the droplets continue to get bigger and put
on weight until they are too heavy to remain suspended in the atmosphere. They then fall to
Earth as precipitation. This phenomenon may take various forms, including snow, sleet, hail,
and most commonly, of course, rain.
Run-off
