approaches held that areal differentiation was overly descriptive, lacked an inte-
grative perspective, and limited geography to a narrow philosophical foundation
that ignored the development of emerging statistical and mathematical methodol-
ogies. Nevertheless, areal differentiation continues to inform the scholarship of
many geographers who study the distribution and interaction of human systems,
and the perspective remains a cornerstone of geographical analysis and theory.Atmosphere
Geographers are quite interested in the atmosphere because it interacts with
continental and oceanic surfaces, is variable by latitude, and variable over time.
The spatial characteristic of the atmosphere impart important possibilities and
constraints to the human mosaic. Where is it too cold to grow crops or where must
irrigation water be used? What sorts of materials (e.g., trees or grasses) do indige-
nous peoples incorporate in their constructions? Surely, the atmosphere is the most
vital resource of our planet.
Of all the planets surrounding the sun,Earthis the only one that has an atmos-
phere containing enough oxygen and the right temperatures to support plentiful
life. Our atmosphere acts as a blanket shielding us from the harsh radiative envi-
ronment of space while vigorously circulating to prevent Earth from becoming
impossibly hot or cold in any latitude. Crucially, the circulation also brings
with it the moisture of thehydrologic cycleneeded to maintain life on continental
surfaces.
Usually, we think of “atmosphere” as synonymous with “air” but that is true
only within a very few kilometers of sea level where air has the energy and mois-
ture characteristics capable of supporting life. In truth, the atmosphere varies con-
siderably with altitude. The atmosphere is usually taken to extend from the surface
to about 10,000 km. However, the top of our atmosphere is not a definite boundary
and is considered atmosphere merely because it is not as totally devoid of mass as
is space. Most of the atmosphere is near sea level and this is because of the sharp
drop in gravity with altitude above sea level. Along the ascent to the top of Mt.
Everest (Sagamartha) at 8.8 km above sea level, atmospheric pressure drops to
somewhat over 300 mb, a third of the average pressure at sea level; this is a condi-
tion that cannot sustain human life. Above these altitudes the pressures are even
lower and by the altitude of 50 km, the 1 mb of pressure compares favorably to
pressures produced in laboratory vacuum chambers. Thus, the life-sustaining
properties of our atmosphere exist only within a very few kilometers of sea level.18 Atmosphere