Geoscience Reference
In-Depth Information
Low-Latitude Dry Climates
The Köppen system recog-
nizes two arid climate regions in the low latitudes of Earth.
The most extreme of this pair of climate zones is the hot low-
latitude desert climate (
BWh
). This climate zone is found in the
center and eastern sides of the STH Pressure Systems, spanning
15° to 30° latitude in both hemispheres, and is characterized by
mean annual precipitation
<
38 cm (15 in.) and average annual
temperature
>
18°C (64.4°F). These conditions fundamentally
occur due to subsiding air in the center of the STH that pro-
motes extensive adiabatic warming and very little rainfall. The
ground is further heated because the skies are usually clear
and solar radiation strikes the surface directly due to the high
Sun angle. On the ground, the hot low-latitude desert climate
(
BWh
) is the classic tropical desert that you have seen pictured
in many places (Figure 9.5a). The vegetation in these regions
is extremely sparse and drought-tolerant, with waxy and spiny
leaves designed to reduce water loss and trunks able to store
water.
The second arid climate zone in the low latitudes is the hot
low-latitude steppe climate (
BSh
) region. This climate zone is
very closely related to the hot low-latitude desert climate (
BWh
)
and results from similar air mass patterns. Because these re-
gions are located slightly poleward of the hot low-latitude des-
ert climate (see Figure 9.2 again), they have a greater annual
temperature range and an annual precipitation that ranges be-
tween 38 cm and 76 cm (15 in. and 30 in.). The greater tem-
perature range results from the occasional incursion of cP air
masses, related to strong midlatitude cyclones, in the winter. As
a result, the periodic influence of these storm systems causes
some precipitation to fall during the winter months. Vegetation
in this climate region looks very similar to what occurs in the
Mojave Desert (Figure 9.5b) in the southwestern part of the
United States.
KEY CONCEPTS TO REMEMBER ABOUT
TROPICAL CLIMATES
1.
Three major tropical climates are recognized: (a) tropi-
cal rainforest climate (
Af
); (b) tropical monsoon climate
(
Am
); and (c) tropical savanna climate (
Aw
).
2.
The tropical rainforest climate (
Af
) has a very narrow
temperature range due to persistent high Sun angle.
Annual precipitation is consistently high throughout the
year due to the influence of the ITCZ.
3.
The tropical monsoon climate (
Am
) has a distinct wet
and dry season that is related to seasonal landward and
seaward low of air that occurs at low latitudes. This
climate region has a slightly more seasonal temperature
range than the tropical rainforest climate (
Af
).
4.
The tropical savanna climate (
Aw
) lies poleward of the
tropical rainforest climate (
Af
). It has distinct wet and
dry seasons that are related to the seasonal dominance
of the ITCZ and STH, respectively.
Arid and Semi-Arid (
B
) Climates
Arid and semi-arid
B
climates are regions with both hot and
cold temperatures that are relatively dry, with annual precipita-
tion
<
76 cm (30 in.). These climate regions are poleward of the
A
climates and are the most widespread on Earth, with
B
cli-
mates occurring virtually on every continent (Figure 9.2). From
a geographical perspective, the arid climates can be first subdi-
vided on the basis of latitude into low-latitude arid climates and
middle-latitude arid climates. The low-latitude arid climates
tend to be the hottest and driest and are closely associated with
the subsiding air of the subtropical high-pressure belt between
20° and 30° N and S latitude. To see this distribution, examine
Figure 9.2 and note the extensive deserts in Australia, southern
and northern Africa, and Asia. In contrast, middle-latitude arid
climates are less dry, semi-arid climates that are related both
to the STH and to rain shadow effects in continental interiors.
Midlatitude Dry Climates
In addition to the pair of arid
climates that occur in the low latitudes, two dry climates are
also recognized in Earth's middle latitudes. The most exten-
sive of these middle-latitude dry zones is the cold midlatitude
steppe climate (
BSk
). This climate region is most often found
Tropical Savanna Climate (
Aw
)
To better visualize how the tropical humid climate functions, go
to the
Geo Media Library
and select
Tropical Savanna Climate
(Aw)
. This animation presents an oblique view (an elevated view
from the side) of Timbo, New Guinea, from the Atlantic Ocean.
In this fashion, you can see how the migration of pressure sys-
tems influences the climate of the region. You will also be able
to manipulate the migration of the ITCZ, which will allow you to
better see the importance of this pressure system on precipita-
tion in any specific geographic region. Once you complete this
simulation, be sure to answer the questions at the end to test
your understanding of this concept.
