Geoscience Reference
In-Depth Information
climate variables is as the
zonal mean
, in which the variable is averaged over
all longitudes and displayed in the latitude/height plane.
The international system of units (SI) is used as reviewed in Appendix A.
2.1 THE ATMOSPHERE
We begin our description of the atmosphere with air pressure. Pressure is de-
fined as “force per unit area” and is expressed in SI units of pascals (abbrevi-
ated Pa). Pressure is simply the weight of the overlying mass,
m
, per unit area:
2
kg ms
$
mg
area
newton
_
i
kg
p
/
Pa
~
,
(2.1)
&
m
2
m
2
ms
$
2
where
g
is the acceleration due to gravity.
Figure 2.1
shows the global distribu-
tion of surface pressure in units of hectapascals (hPa), where 100 Pa 1 hPa.
This figure is not helpful for learning about the atmosphere, however, because
surface topography dominates the distribution. Surface pressure is lowest over
the highest mountains, and high and uniform over the oceans, because the over-
lying air column is thinner (less massive) at higher elevations. Consequently,
surface pressures in the Himalayan Mountains and over Antarctica drop below
600 hPa but are close to 1000 hPa everywhere over the world's oceans.
Figure 2.1
demonstrates the close connection between pressure and eleva-
tion. Pressure is often used as a vertical coordinate in describing the atmo-
sphere, replacing elevation,
z
. The average relationship between
p
and
z
in
the earth's atmosphere, typical of large space and time scales, is in
Figure 2.2.
Note that
p
is not a linear function of
z
, that is,
p zb
!
+ , where
a
and
b
are
constants. Instead, pressure decreases exponentially with height.
60°N
30°N
Equator
30°S
60°S
0°
30°E
60°E
90°E 120°E 150°E 180° 150°W 120°W 90°W 60°W 30°W 0°
Figure 2.1 The annually averaged surface pressure climatology. Contour interval is 50 hPa.
