Geoscience Reference
In-Depth Information
mercury. By exerting pressure downward on the
surface of mercury in the cistern, the atmosphere
is able to support a mercury column in the
tube of about 760mm (29.9in or approximately
1013mb). The weight of air on a surface at sea level
is about 10,000kg per square meter.
Pressures are standardized in three ways. The
readings from a mercury barometer are adjusted
to correspond to those for a standard temperature
of 0°C (to allow for the thermal expansion of
mercury); they are referred to a standard gravity
value of 9.81ms
-2
at 45° latitude (to allow for the
slight latitudinal variation in
g
from 9.78ms
-2
at
the equator to 9.83ms
-2
at the poles); and they are
calculated for mean sea level to eliminate the effect
of station elevation. This third correction is the
most significant, because near sea-level pressure
decreases with height by about 1mb per 8m. A
fictitious temperature between the station and sea
level has to be assumed and in mountain areas this
commonly causes bias in the calculated mean sea-
level pressure (see Note 4).
The mean sea-level pressure (
p
0
) can be
estimated from the total mass of the atmosphere
(
M
, the mean acceleration due to gravity (
g
0
) and
the mean earth radius (
R
)):
i.e., the rate of change of pressure (
p
) with height
(
z
) is dependent on gravity (
g
) multiplied by the
air density (
). With increasing height, the drop
in air density causes a decline in this rate of
pressure decrease. The temperature of the air also
affects this rate, which is greater for cold dense air
(see Chapter 7A.1). The relationship between
pressure and height is so significant that meteo-
rologists often express elevations in millibars:
1000mb represents sea level, 500mb about 5500m
and 300mb about 9000m. A conversion nomo-
gram for an idealized (standard) atmosphere is
given in Appendix 2.
ρ
2
Vapor pressure
At any given temperature, there is a limit to the
density of water vapor in the air, with a con-
sequent upper limit to the vapor pressure, termed
the
saturation vapor pressure
(
e
s
).
Figure 2.14A
illustrates how
e
s
increases with temperature (the
Clausius-Clapeyron relationship), reaching a
maximum of 1013mb (1 atmosphere) at boiling
point. Attempts to introduce more vapor into
the air when the vapor pressure is at saturation
produce condensation of an equivalent amount
of vapor.
Figure 2.14B
shows that whereas the
saturation vapor pressure has a single value at any
temperature above freezing point, below 0°C the
saturation vapor pressure above an ice surface is
lower than that above a supercooled water surface.
The significance of this will be discussed in
Chapter 5D.1.
Vapor pressure (
e
) varies with latitude and
season from about 0.2mb over northern Siberia in
January to over 30mb in the tropics in July, but
this is not reflected in the pattern of surface
pressure. Pressure decreases at the surface when
some of the overlying air is displaced horizon-
tally, and in fact the air in high pressure areas
is generally dry owing to dynamic factors,
particularly vertical air motion (see Chapter
7A.1), whereas air in low pressure areas is usually
moist.
R
E
2
)
where the denominator is the surface area of a
spherical earth. Substituting appropriate values
into this expression:
M
= 5.14
P
0
=
g
0
(
M
/4
π
×
1018kg,
g
0
=
9.8ms
-2
,
R
E
= 6.36
10
6
m, we find
p
0
= 10
5
kg ms
-2
= 105Nm
-2
, or 10
5
Pascals. Hence the mean sea-
level pressure is approximately 10
5
Pa or 1000mb.
The global mean value is 1013.25mb. On average,
nitrogen contributes about 760mb, oxygen 240mb
and water vapor 10mb. In other words, each gas
exerts a partial pressure independent of the others.
Atmospheric pressure, depending as it does on
the weight of the overlying atmosphere, decreases
logarithmically with height. This relationship is
expressed by the
hydrostatic equation
:
×
-- = -
g
ρ
z
