Precipitation and runoff also follow a zonal
pattern, generally decreasing with latitude. Near
the equator the trade winds converge toward the
intertropical convergence zone (ITC). The
trade winds moving toward the equator pick up
moisture over the oceans and, when lifted in the
ITC, yield abundant precipitation.
On top of this, differential heating at different
latitudes causes transfer of heat from lower to
higher latitudes, partly through circulation of the
atmosphere. The result is a series of belts of
ascending and subsiding air masses (Fig. 4.4).
Subsiding air masses of the subtropical highpressure cells, roughly centered on the tropics
of Cancer and Capricorn, have adequate heat
but a shortage of moisture. These zones are too
dry for tree growth.
4.1.2 Continental Position
The interaction of land and sea modifies the situation. This division of the Earth’s surface
between land and sea, each with quite different
thermal characteristics, results in distinctive
differences between marine-influenced and
inner-continental climates. For a hypothetical
continent of uniform elevation, temperature distribution would look like that shown in Fig. 4.5.
In addition, the distribution of land and sea
forms complicated but predictable, precipitation
patterns—less precipitation over margins of
continents bathed by cold water. The dry zone,
controlled by the subtropical high-pressure cells,
is shifted to the west side of the continents,
adjacent to these cold currents (Fig. 4.6). These
dry zones are too dry for trees and consist of
deserts and grasslands. This creates a distribution
of dry climatic zones that strongly affect ecosystem distribution.
Combined Latitude (Thermal) and Moisture
Considerations By combining the thermally
defined zones with the moisture zones, we can
delineate four ecoclimatic zones or ecoregions:
humid tropical, humid temperate, polar, and dry
Fig. 4.1 Zones determined by thermal limits for plant growth. From Strahler (1965), p. 103; redraw with permission
28
4 Continental Types and Their Controls
pattern, generally decreasing with latitude. Near
the equator the trade winds converge toward the
intertropical convergence zone (ITC). The
trade winds moving toward the equator pick up
moisture over the oceans and, when lifted in the
ITC, yield abundant precipitation.
On top of this, differential heating at different
latitudes causes transfer of heat from lower to
higher latitudes, partly through circulation of the
atmosphere. The result is a series of belts of
ascending and subsiding air masses (Fig. 4.4).
Subsiding air masses of the subtropical highpressure cells, roughly centered on the tropics
of Cancer and Capricorn, have adequate heat
but a shortage of moisture. These zones are too
dry for tree growth.
4.1.2 Continental Position
The interaction of land and sea modifies the situation. This division of the Earth’s surface
between land and sea, each with quite different
thermal characteristics, results in distinctive
differences between marine-influenced and
inner-continental climates. For a hypothetical
continent of uniform elevation, temperature distribution would look like that shown in Fig. 4.5.
In addition, the distribution of land and sea
forms complicated but predictable, precipitation
patterns—less precipitation over margins of
continents bathed by cold water. The dry zone,
controlled by the subtropical high-pressure cells,
is shifted to the west side of the continents,
adjacent to these cold currents (Fig. 4.6). These
dry zones are too dry for trees and consist of
deserts and grasslands. This creates a distribution
of dry climatic zones that strongly affect ecosystem distribution.
Combined Latitude (Thermal) and Moisture
Considerations By combining the thermally
defined zones with the moisture zones, we can
delineate four ecoclimatic zones or ecoregions:
humid tropical, humid temperate, polar, and dry
Fig. 4.1 Zones determined by thermal limits for plant growth. From Strahler (1965), p. 103; redraw with permission
28
4 Continental Types and Their Controls