energy needed to create atmospheric disturbances such as thunderstorms and hur-
ricanes. Of tremendous importance is water vapor’s major role in causing the
greenhouse effect.
Homer’sIliad, written in the eighth century BCE, provided the first recorded
notion of the connected nature of water over the planet. It remained for modern
science to measure the various components with instruments. Although all of
non-ocean water is about 3 percent of the world’s supply, the hydrologic cycle
moves huge quantities of water over time. Annually, the cycle has been estimated
to have over 400,000cubic kilometersof water entering the atmosphere. The
hydrologic cycle is not to be viewed as a smooth transference of water but works
in “fits and starts.” One week the Great Plains of North America might receive
flooding rains from the Gulf of Mexico and the next several weeks can be very
dry with the steering patterns of the upper troposphere blocking the flow of low-
level moisture. So, too, the hydrologic cycle has geographic variations of its com-
ponents. For instance, the evaporation may be prodigious from a tropical ocean
surface, whereas coldtemperaturesand ice in polar waters make the hydrologic
cycle much more sluggish.
At any slice of time, 97 percent of Earth’s water is in the ocean. The ocean is
large compared to the total surface of the planet and also quite deep. The deep
ocean water is away from the direct influences of the atmosphere. It is in the ocean
for thousands of years at a time because of its very slow circulation; in spatially
limited upwelling areas these bottom waters come to the top. Near the surface
the waters are mixed by wind and heated by solar radiation so these waters have
a shorter average residence time.
Solar energy and energy in the wind evaporate liquid water from the surface and
add it to the air as water vapor. Additionally, there is a transformation called sub-
limation that changes ice directly to water vapor even at air temperatures well
below freezing. Worldwide, this latter process generates less water vapor than
evaporation because of less total energy locally available in colder places and the
fact that most of Earth’s ocean is unfrozen. The vapor added to the atmosphere
leaves behind the impurities of liquid water and ice so that eventual precipitation
will be fresh water.
Water vapor is circulated as part of the wind and sometimes is condensed and
deposited onto hygroscopic nuclei to formclouds. Cloud formation can occur very
far from the oceanic moisture source. It is thought that water vapor has an average
atmospheric residence time of about nine days. The vast majority of clouds are
non-precipitating, but in selected places with rising air, the hydrologic cycle is fur-
thered through precipitation processes.
As precipitation reaches Earth’s surface, about 77 percent of it falls back into
the ocean. The other 23 percent makes life on the land possible. Precipitation
Hydrologic Cycle 181