The Dry Ecoregions
7
7.1
300 Dry Domain
The essential feature of a dry climate is that
annual losses of water through evaporation at
the Earth’s surface exceed annual water gains
from precipitation. Due to the resulting water
deficiency, no permanent streams originate in
dry-climate zones. Because evaporation, which
depends chiefly on temperature, varies greatly
from one part of the Earth to another, no specific
value for precipitation can be used as the boundary for all dry climates. For example, 610 mm of
annual precipitation produces a humid climate
and forest cover in cool northwestern Europe,
but the same amount in the hot tropics produces
semiarid conditions.
The tropical dry climates occupy the air
masses in the subtropical high-pressure cells centered over the tropics of Cancer and Capricorn,
both north and south of the equator, in the zone
between 20
and 30
. This subsiding air mass is
stable and dry. Dry land regions can be found on
the western sides of all the continents. The dry
lands also extend inland from the western sides of
the continents, bending poleward in each hemisphere. The general regularity of the global pattern of dry climates is a reflection of the regular
arrangement of certain climatic and water
features. Fig. 4.6 (p. 31) shows that cold water
baths parts of the west coasts of all the continents.
Because evaporation is much less from cold water
than warm water, the rainfall is much less here
than continental margins bathed by warm water.
The presence of dry lands along the east coast of
South America in Patagonia is associated with a
wide expanse of cold water of the South Atlantic
Ocean (Falkland Island Current). In North America the dry lands cannot extend so far toward the
east as they do in Asia because of the movement
of moist maritime air from the Gulf of Mexico up
to the Mississippi Valley, and the lack of such air
moving across the Himalayas, in Asia.
We commonly recognize two divisions of dry
climates: the arid desert (BW) and the semiarid
steppe (BS). Generally, the steppe is a transitional belt surrounding the desert and separating
it from the humid climates beyond. The boundary between arid and semiarid climates is arbitrary, but commonly defined as one-half of the
amount of precipitation separating steppe from
humid climates. These climates are displayed in
Fig. 7.1 and mapped in Fig. 7.2.
Of all the climatic groups, dry climates are the
most extensive; they occupy a fourth or more of
the Earth’s land surface (see Fig. 4.7, p. 32).
In these climates, many plants and animals
have adapted to live with minimal rain, drying
winds, and high temperatures.
7.2
310 Tropical/Subtropical
Steppe Division
Tropical steppes occur along the less arid
margins of the tropical deserts on both the north
R.G. Bailey, Ecoregions, DOI 10.1007/978-1-4939-0524-9_7, # Springer Science+Media, LLC 2014
69
7
7.1
300 Dry Domain
The essential feature of a dry climate is that
annual losses of water through evaporation at
the Earth’s surface exceed annual water gains
from precipitation. Due to the resulting water
deficiency, no permanent streams originate in
dry-climate zones. Because evaporation, which
depends chiefly on temperature, varies greatly
from one part of the Earth to another, no specific
value for precipitation can be used as the boundary for all dry climates. For example, 610 mm of
annual precipitation produces a humid climate
and forest cover in cool northwestern Europe,
but the same amount in the hot tropics produces
semiarid conditions.
The tropical dry climates occupy the air
masses in the subtropical high-pressure cells centered over the tropics of Cancer and Capricorn,
both north and south of the equator, in the zone
between 20
and 30
. This subsiding air mass is
stable and dry. Dry land regions can be found on
the western sides of all the continents. The dry
lands also extend inland from the western sides of
the continents, bending poleward in each hemisphere. The general regularity of the global pattern of dry climates is a reflection of the regular
arrangement of certain climatic and water
features. Fig. 4.6 (p. 31) shows that cold water
baths parts of the west coasts of all the continents.
Because evaporation is much less from cold water
than warm water, the rainfall is much less here
than continental margins bathed by warm water.
The presence of dry lands along the east coast of
South America in Patagonia is associated with a
wide expanse of cold water of the South Atlantic
Ocean (Falkland Island Current). In North America the dry lands cannot extend so far toward the
east as they do in Asia because of the movement
of moist maritime air from the Gulf of Mexico up
to the Mississippi Valley, and the lack of such air
moving across the Himalayas, in Asia.
We commonly recognize two divisions of dry
climates: the arid desert (BW) and the semiarid
steppe (BS). Generally, the steppe is a transitional belt surrounding the desert and separating
it from the humid climates beyond. The boundary between arid and semiarid climates is arbitrary, but commonly defined as one-half of the
amount of precipitation separating steppe from
humid climates. These climates are displayed in
Fig. 7.1 and mapped in Fig. 7.2.
Of all the climatic groups, dry climates are the
most extensive; they occupy a fourth or more of
the Earth’s land surface (see Fig. 4.7, p. 32).
In these climates, many plants and animals
have adapted to live with minimal rain, drying
winds, and high temperatures.
7.2
310 Tropical/Subtropical
Steppe Division
Tropical steppes occur along the less arid
margins of the tropical deserts on both the north
R.G. Bailey, Ecoregions, DOI 10.1007/978-1-4939-0524-9_7, # Springer Science+Media, LLC 2014
69