Environmental Engineering Reference
In-Depth Information
moist and a dry air stream are adjacent we can see a sudden edge to the cloud within the
moist air.
amplitude. The surface anticyclones may intensify within the usual depression tracks,
diverting the cyclones from their normal routes and giving rise to exceptional patterns of
weather.
Blocking anticyclones
, as they are called, are most frequent over north-west
Europe and the north Pacific. Blocking in the Atlantic was responsible for the droughts of
the late 1980s and 1995 and the severe winter of 1978-9 over north-west Europe.
Unfortunately we do not yet know enough about the causes of blocking anticyclones to
predict their future development.
Anticyclones are normally associated with dry weather and light winds. Clear skies or
extensive cloud and very warm or very cold conditions may occur. Which we get
depends upon the time of the year, the degree of moistness, the source of the air and the
location and intensity of the anticyclone. In Europe, in summer, anticyclones usually
bring hot, dry weather if centred over the Mediterranean or central Europe, but in winter
cold weather is more usual, especially if the anticyclone is centred over Scandinavia and
dry, cold continental air is drawn from the east.
DEPRESSIONS
Wind speed rises, pressure falls and the clouds get thicker: a common sequence of events
in the mid-latitudes heralding the approach of a depression. The depression or
extratropical cyclone, or
low
, as it is also known, brings with it conditions very different
from those associated with anticyclones. Air pressure is relatively low and the air
circulating around the low is rising. Depressions usually move relatively quickly, in the
northern hemisphere normally towards the north-east. They are smaller in size than an
anticyclone, but within them air is rising more quickly. Pressure and temperature
gradients are much steeper, so that horizontal winds are strong. In essence, they are the
main mobile systems of the middle latitudes and they are responsible for the
characteristic climates of those regions. Much of the precipitation there comes from this
source. Depressions are the pressure systems responsible for the sudden swings in
temperature from hot to cold or vice versa as air masses change.
For over seventy years the Bergen model of depression formation has dominated our
views (Figure 7.4). However, subsequent work, especially that involving the upper
atmosphere, has revealed significant deficiencies in the model. For example, it is now
clear that depression formation does not need a polar front but rather a zone of strong
temperature gradient known as a baroclinic zone. The process of cyclogenesis (or
depression formation) actually intensifies the thermal gradients to produce the fronts. In
many parts of the world warm fronts, an important part of the Bergen model, are weak or
limited in extent. Finally, the classic 'catching-up' occlusion process is difficult to
identify and many studies have shown that ideal occluded frontal structures are rarely
observed in their entirety. Unfortunately no clear conceptual model of depresson
evolution has replaced the Bergen one, possibly because depression development is
highly variable, depending upon surface conditions, topography and upper atmospheric
flows.
