Geoscience Reference
In-Depth Information
Table 6.2. Summary of characteristics of Northern Hemisphere surface low- and high-pressure systems
Surface low-pressure systems
Surface high-pressure systems
Characteristic
Semipermanent
Thermal
Semipermanent
Thermal
45-65 N
25-45 N
25-45 N
45-65 N
Latitude range
Surface pressure gradients
Strong
Variable
Weak
Variable
Surface wind speeds
Fast
Fast/variable
Slow
Slow/variable
Surface wind directions
Converging,
counterclockwise
Converging,
counterclockwise
Diverging,
clockwise
Diverging,
clockwise
Ve r tical air motions
Upward
Upward
Downward
Downward
Cloud cover
Cloudy
Cloudy or cloud free
Cloud free, sunny
Cloud free
Surface solar radiation
Low
Low or high
High
High
Storm formation?
Yes
Sometimes
No
No
Effect on air pollution
Reduces
Reduces
Enhances
Enhances
form in low-pressure systems, they block sunlight that
would otherwise drive photochemical reactions, reduc-
ing pollution further. If clouds do not form, signifi-
cant solar radiation, including UV radiation, reaches
the surface, heating the surface and driving photochem-
ical reactions.
Surface high-pressure systems are characterized by
relatively slow surface winds, sinking air, cloudfree
skies, and high penetrations of solar radiation to the
surface. In such pressure systems, air sinks, confining
near-surface pollution. Slow near-surface winds associ-
ated with high-pressure systems also prevent horizontal
dispersion of pollutants, and the cloudfree skies caused
by the pressure systems maximize the sunlight avail-
able to drive photochemical smog formation. In sum,
the major effects of pressure systems on pollution are
through vertical pollutant transfer, horizontal pollutant
transfer, and cloud cover. Each of these effects is dis-
cussed in turn.
air to become negatively buoyant and sink or stagnate.
Both cases illustrate free convection. To understand
better how free convection in thermal pressure sys-
tems and forced convection in semipermanent pres-
sure systems affect pollutant dispersion, it is neces-
sary to discuss adiabatic processes and atmospheric
stability.
6.6.1.1. Adiabatic and Environmental Lapse Rates
Whether air rises or sinks buoyantly in a thermal pres-
sure system depends on atmospheric stability, which
depends on adiabatic and environmental lapse rates.
These terms are discussed next.
Imagine a balloon filled with air. The air pressure
inside the balloon exactly equals the air pressure out-
side the balloon; otherwise, the balloon would continue
to expand or contract. Also imagine that no energy
(e.g., solar or thermal-IR energy or latent heat energy
created by condensation of water vapor) can enter or
leave the balloon, but that the balloon's membrane is
flexible enough for it to expand and contract due to
changes in air pressure outside the balloon. Suppose
now that the balloon rises. Because air pressure always
decreases with increasing altitude, the balloon must rise
into decreasing air pressure. For the air pressure inside
the balloon to decrease to the air pressure outside the
balloon, the balloon must now expand, increasing in
volume. This type of expansion, caused by a change in
air pressure alone, is called an adiabatic expansion .
Solar heating and latent heat release are diabatic heat-
ing processes and do not contribute to an adiabatic
expansion.
6.6.1. Vertical Pollutant Transport
Pressure systems affect vertical air motions and, there-
fore, pollutant dispersion by forced and free convec-
tion (Section 3.2.2). In a semipermanent low-pressure
system, for example, near-surface winds converge and
rise, dispersing near-surface pollutants upward. In a
semipermanent high-pressure system, winds aloft con-
verge and sink, confining near-surface pollutants. Both
cases illustrate forced convection. In thermal low-
pressure systems, surface warming causes near-surface
air to become buoyant and rise. In thermal high-
pressure systems, surface cooling causes near-surface
 
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