Environmental Engineering Reference
In-Depth Information
tropics, at the poleward edge of the tropical
Hadley cell, and in mid-latitudes at the polar
front (see Figure 2.12). Both of these jets
generally flow from west to east. In addition, an
Arctic jet, associated with the long polar night,
has been identified (Hare and Thomas 1979),
and an intermittent, but recognizable, easterly
jet is a feature of the upper atmospheric
circulation in equatorial regions (Barry and
Chorley 1992).
The northern hemisphere polar front jet
stream, the one most commonly encountered as
headwinds or tail winds during trans-continental
or trans-oceanic flights, is the best known of all
the jet streams. It circles the earth in midlatitudes,
following a meandering track from west to east
at speeds averaging perhaps 100 km per hour,
but with a maximum recorded speed of almost
500 km per hour (Eagleman 1985). During the
winter, it follows a more southerly track, close
to 35°N, and has an average velocity of 130 km
per hour, whereas in the summer it is located
closer to 50°N, and its velocity decreases to about
65 km per hour.
The influence of the polar front jet extends to
the lower atmosphere, through its control over
the various systems which produce the surface
weather conditions. For example, the difference
between a mild winter and a cold one, in the
interior of North America, is often determined
by the location of the polar front jet stream. A
more southerly track allows the cold, polar air
on the north side of the jet to penetrate into
lower latitudes, whereas a more northerly track
allows the continent to remain bathed in the
warmer, southern air. The jet also exerts its
influence on moisture regimes—through its
control over the tracks followed by mid-latitude
low pressure systems—and it has been
implicated in the tornado outbreaks which
occur in North America every spring. North-
south thermal contrasts are strong, at that time,
and the jet is therefore particularly vigorous
(Eagleman 1985).
The importance of the jet stream and the
associated upper westerlies, from an
environmental point of view, lies in their ability
to transport pollutants over great distances
through the upper atmosphere. Smoke, volcanic
debris and acid particles are all spread by such
transportation, and, as a result, the problems
they represent are global in scale. When above-
ground atomic tests were being carried out in
the USSR and China, during the 1950s and early
1960s, radioactive fallout was carried over
northern Canada in the jet stream (Hare 1973).
A similar mechanism spread the products of the
Chernobyl nuclear accident. Any future nuclear
war would cause great quantities of debris to be
thrown into the upper atmosphere, where the jet
streams would ensure a hemispheric
distribution, and contribute to the rapid onset of
nuclear winter.
The effects of surface conditions and seasonal
variations
Modern representations of the general circulation
of the atmosphere take into account the non-
uniform nature of the earth's surface, with its
mixture of land and water, and include
consideration of seasonal variations in energy
flow (see Figure 2.13).
Land and water respond differently to the
same energy inputs, because of differences in their
physical properties. Land tends to heat up and
cool down more rapidly than water, and
temperatures over land exhibit a greater range,
diurnally and seasonally, than those over water.
These temperature differences in turn have an
impact on atmospheric pressure, particularly in
the northern hemisphere with its juxtaposition
of oceans and major land masses. During the
northern summer, for example, higher
temperatures promote lower pressure over the
continents, whereas the adjacent seas are cooler,
and pressure remains high in the cells which
represent the sub-tropical high pressure belt over
the North Atlantic and Pacific oceans. By altering
the regional airflow, such pressure differences
cause disruption of the theoretical global
circulation patterns.
The permanent or semi-permanent features of
the circulation also vary in extent and intensity,
