Geoscience Reference
In-Depth Information
Philippines injected twenty megatons of SO
2
into
the stratosphere. However, only about twelve eruptions
have produced measurable dust veils in the past 120
years. They occurred mainly between 1883 and 1912,
and 1982 and 1992. In contrast, the contribution of man-
made particles (particularly sulphates and soil) has been
progressively increasing, and now accounts for about
30 per cent of the total.
The overall effect of aerosols on the lower atmos-
phere is uncertain; urban pollutants generally warm the
atmosphere through absorption and reduce solar radia-
tion reaching the surface (see Chapter 3C). Aerosols
may lower the planetary albedo above a high-albedo
desert or snow surface but increase it over an ocean
surface. Thus the global role of tropospheric aerosols is
difficult to evaluate, although many authorities now
consider it to be one of cooling. Volcanic eruptions,
which inject dust and sulphur dioxide high into the
stratosphere, are known to cause a small deficit in
surface heating with a global effect of -0.1° to -0.2°C,
but the effect is short-lived, lasting only a year or so
after the event (see Box 13.3). In addition, unless the
eruption is in low latitudes, the dust and sulphate
aerosols remain in one hemisphere and do not cross
the equator.
remainder. The gas laws may be combined to give the
following relationship:
PV
=
RmT
where
m
= mass of air, and
R
= a gas constant for dry air
(287 J kg
-1
K
-1
) (see Note 3). If
m
and
T
are held fixed,
we obtain Boyle's Law; if
m
and
P
are held fixed, we
obtain Charles's Law. Since it is convenient to use
density, ρ (= mass/volume), rather than volume when
studying the atmosphere, we can rewrite the equation
in the form known as the equation of state:
P
=
R
ρ
T
Thus, at any given pressure, an increase in temperature
causes a decrease in density, and vice versa.
1 Total pressure
Air is highly compressible, such that its lower layers
are much more dense than those above. Fifty per cent
of the total mass of air is found below 5 km (see Figure
2.13), and the average density decreases from about
1.2 kg m
-3
at the surface to 0.7 kg m
-3
at 5000 m
(approximately 16,000 ft), close to the extreme limit of
human habitation.
Pressure is measured as a force per unit area. A force
of 10
5
newtons acting on 1 m
2
corresponds to the Pascal
(Pa) which is the Système International (SI) unit of
pressure. Meteorologists still commonly use the millibar
(mb) unit; 1 millibar = 10
2
Pa (or 1 hPa; h = hecto)
(see Appendix 2). Pressure readings are made with a
mercury barometer, which in effect measures the height
of the column of mercury that the atmosphere is able to
support in a vertical glass tube. The closed upper end of
the tube has a vacuum space and its open lower end is
immersed in a cistern of 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 760 mm (29.9 in or approximately
1013 mb). The weight of air on a surface at sea-level is
about 10,000 kg per square metre.
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 9.81 ms
-2
at 45°
latitude (to allow for the slight latitudinal variation in
g
from 9.78 ms
-2
at the equator to 9.83 ms
-2
at the poles);
B MASS OF THE ATMOSPHERE
Atmospheric gases obey a few simple laws in response
to changes in pressure and temperature. The first,
Boyle's Law, states that, at a constant temperature, the
volume (
V
) of a mass of gas varies inversely as its
pressure (
P
), i.e.
k
1
P
= --
V
(
k
1
is a constant). The second, Charles's Law, states
that, at a constant pressure, volume varies directly with
absolute temperature (
T
) measured in degrees Kelvin
(see Note 2):
V
=
k
2
T
These laws imply that the three qualities of pressure,
temperature and volume are completely interdependent,
such that any change in one of them will cause a
compensating change to occur in one, or both, of the
