Humidity and Rainfall - Agroecology: The Ecology of Sustainable Food Systems - page 74

as a lake, gulf, or ocean. The rain associated with summer
thunderclouds is an example of convective rainfall. High
winds, and even tornadoes, can accompany these storms,
as can lightning and localized fires. In many regions, such
as the American Midwest, agroecosystems are dependent
on this type of rainfall, at least at certain times of the year.
Traditional Hopi agriculture in the southwest of the U.S. is
completely dependent on convective rainfall with the tor-
rent, which often accompanies these storms being chan-
neled down, washes from the mountains and then spreads
out over planted fields at the mouths of the canyons.

Transport

Condensation

Precipitation

Transpiration

Evaporation

Percolation and Runoff

O ROGRAPHIC R AINFALL

FIGURE 6.1 The hydrological cycle.

Orographic rainfall occurs when a moisture-laden air
mass meets a mountain range that forces it up into the cooler
layers of the atmosphere. Such precipitation occurs on the
western flanks of California's Sierra Nevada — as rain in
the foothills and as snow at the higher elevations. This pre-
cipitation is an important replenisher of streams and aquifers,
which later become sources of irrigation water downstream
in drier locations. Agriculture in a region such as the Great
Central Valley of California would not be possible without
orographic precipitation in nearby mountains.

water, and other wet surfaces. Evaporation of water from
inside the bodies of plants also occurs on the surface of
leaves. This kind of evaporation, called transpiration, is part
of the mechanism, by which plants draw water from the
soil into their roots (Chapter 3). Evaporation from all these
sources is collectively termed “evapotranspiration.”
When the absolute amount of water vapor in the air is
sufficient to approach or exceed 100% relative humidity,
condensation begins to occur. Small water droplets form
and aggregate to create clouds. Precipitation occurs when
droplets of water in clouds become heavy enough to fall.
This usually happens when the moisture-containing air rises
(by being forced up a mountain by winds or rising on cur-
rents of warm air) and begins to cool. As the air cools, its
ability to hold moisture in vapor form or as very small cloud
droplets begins to decrease, resulting in more condensation
and aggregation of droplets. This cooling and condensing
process is called adiabatic cooling. The precipitation formed
by adiabatic cooling falls to earth, enters watersheds or the
ocean, and eventually returns to the atmosphere.

C YCLONIC R AINFALL

This type of rainfall is associated with areas of low atmo-
spheric pressure that form over the ocean. Warm, moisture-
laden air rises, creating a low-pressure area. As this air
rises, it cools, forms precipitation, and then falls back
toward the ocean surface where it can collect more mois-
ture. In addition, the air currents of this self-perpetuating
system begin to revolve counter-clockwise around the
low-pressure area, and the entire system begins to move.
The revolving air currents form the characteristic cyclonic
storms and frontal systems, which we can see on weather
maps. When one of these cyclonic systems moves ashore,
the moisture-laden air masses may be forced up against
mountain masses, creating rainfall with both orographic
and cyclonic causes (Figure 6.2).

TYPES OF RAINFALL

The precipitation part of the hydrological cycle is highly
variable. Masses of moisture-laden air are constantly being
moved over the earth's surface by the complex movements
of the atmosphere. Rainfall (and other forms of precipita-
tion) occurs locally in different ways depending on latitude,
season, temperature, topography, and the movement of the
air masses. In general, however, rainfall can be classified
into three types depending on the mechanism that produces
the adiabatic cooling of the moist air mass.

C ONVECTIVE R AINFALL

Convective rainfall occurs when high levels of solar gain
heat the air close to the ground, causing it to rise rapidly,
cool, and condense the moisture it contains. Often the rising
air draws moisture-laden air from some distant source such

FIGURE 6.2 A cyclonic storm system over the eastern Pacific.