Geoscience Reference
In-Depth Information
Original
lapse
rate
Large-scale
subsidence
Descending air
warms at DALR
Level 1 before
subsidence
New lapse rate
after subsidence
Level 2 after
subsidence
Little subsidence
possible near surface
Temperature
Air streams
KEY CONCEPTS
There are other ways of classifying air than by its source regions as is done in air mass analysis. One way is to
identify air movement in terms of air streams. These represent the streamlines of the mean resultant wind in a
particular area and so when mapped spatially they are more representative of the dynamics of flow rather than the
static picture presented by the air mass concept. In many ways this gives a more realistic picture of what happens
within the atmosphere. They identify areas of divergence that are the basis of the air mass source region and the
confluence zone between air streams of different origin (
Figure 7.4
).
A study of air streams found nineteen source regions in the northern hemisphere, but not all of them were present
in all months. Four regions dominated in terms of their area of influence. Inevitably these are associated with the
subtropical anticyclones that also form the basis of air mass source regions. However, the study of air streams also
involves their subsequent movement.
Climatologists have found that the extent of the various air streams is often sufficiently stable throughout the year
for the boundaries between air streams to mark the average position of fronts. These airstream meeting zones are
termed confluences. One distinguishing feature of air streams can be their very different moisture content. Within
an air steam the temperature and moisture content can be relatively uniform. Where a moist and a dry air stream
are adjacent we can see a sudden edge to the cloud within the moist air.
of the air, 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.
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.
Extra-tropical cyclones
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
Depressions are the pressure systems
