Temperature - Agroecology: The Ecology of Sustainable Food Systems - page 62

M ARITIME VS . C ONTINENTAL I NFLUENCE T OPOGRAPHIC V ARIATION

Large bodies of water, especially the oceans, greatly
affect the temperature of adjacent landmasses. Because
water reflects a larger proportion of insolation in relation
to land and loses heat readily through surface evapora-
tion, it has a high specific heat, and readily mixes layers
vertically; the temperature of large bodies of water is
slower to change than that of landmasses. Land heats up
more during the summer because all the absorbed heat
stays in the surface horizon and the atmosphere close to
that surface, and it cools to a lower temperature during
the winter because of reradiation and heat loss. Water
masses are, therefore, moderators of broad fluctuation in
temperature, tending to lower temperatures in the sum-
mer and to raise temperatures in the winter. This water-
or marine-mediated effect on temperature is called a
maritime influence , in contrast to the more widely fluc-
tuating variations in temperature encountered at a dis-
tance from water under a continental influence . Maritime
influences help create the unique Mediterranean climates
of such places as coastal California and Chile, where
nearby upwelling cold currents accentuate the moderat-
ing influences during the dry summer season.

Slope orientation and topography introduce variation in
temperature as well, especially at the local level. For
example, slopes that face toward the sun as a result of the
inclination of the earth on its axis experience more solar
gain, especially in the winter months. Hence, an equator-
facing slope is significantly warmer than a pole-facing
slope — all other factors being equal — and offers unique
microclimates for crop management.
Valleys surrounded by mountain slopes create unique
microclimates as well. In many parts of the world, air that
moves downslope due to winds or pressure differences
can rapidly expand and heat up as it descends — a process
known as catabatic warming. (The wind associated with
this phenomenon will be discussed in Chapter 7.) As the
air is warmed, its ability to hold moisture in vapor form
(relative humidity) goes up, increasing the evaporative
potential of the warmer air.
Valleys are subject to nighttime microclimate varia-
tion as well. On the higher elevation slopes above a
valley, reradiation occurs more rapidly; since the cooled
air that results is heavier than the warmer air below, the
cooler air begins to flow downslope — a phenomenon

FIGURE 5.3 Lettuce grown year-round in a temperate maritime climate. Cooling summer fog and the warming effect of the
nearby ocean in the winter permit year-round vegetable and fruit production on the central coast of California.