Geoscience Reference
In-Depth Information
7
CHAPTER SEVEN
systems
Not a year goes by without weather events somewhere in
the world causing damage or loss of life. Floods, gales,
blizzards, tornadoes, hurricanes and even heatwaves can
create problems and generate much economic stress over
the areas affected. To be prepared, it is vital to understand
the weather, and be able to predict with accuracy,
preferably well in advance, events such as these. It is
important too that we understand not only the vagaries
of day-to-day weather conditions but also longer-term
trends. How useful it would be for farmers to know what
the weather over the next few weeks or even the whole
growing season will be like; they could plan which crop
would be most suitable and alter sowing, ploughing or
harvesting far more successfully. How useful it would be
to have a clear idea of the weather in the year ahead so
that cereal harvests could be predicted, plans for winter
frost and snow could be made and measures could be
taken to deal with drought. Any such detailed under-
standing is a long way away. It may come as we gather
more knowledge about the medium-term processes
operating within the atmosphere, and about the myriad
factors that influence those processes. However, most
scientists doubt whether it will ever be possible to predict
in any detail the long-term movements of a chaotic and
turbulent 'fluid' such as our atmosphere.
The key to understanding and predicting short-term
weather changes, say up to one week ahead, lies in
understanding what we call weather-forming systems. If
we look at a satellite photograph showing half the globe
it is clear that the distribution of clouds is not random
showing certain patterns which make it possible to
identify their origin. Many areas are devoid of cloud
altogether and surface features can be seen. Comparing
this photograph with a map of surface pressure, we would
see that the large spirals of cloud are associated with
cyclones in the middle latitudes and the main cloud-free
areas with the large anticyclones of the subtropics.
Between these areas the cloud patterns are less clear. Over
the south Atlantic Ocean the trade winds have produced
some interesting forms, and over the cold Benguela
current off south-west Africa there are extensive layers of
low cloud. Viewing this instantaneous picture, we can see
the way in which different areas of the atmosphere
interact, and by using the surface pressure information we
can relate these cloud patterns to the weather systems
which produce them.
AIR MASSES
An air mass is a large, uniform body of air with no major
horizontal gradients of temperature, wind or humidity. In
the anticyclonic areas of the world, where air movement
is gentle, the air is in contact with the ground and
gradually acquires the thermal and moisture properties
of that surface. We find that the air then has relatively
uniform distributions of temperature and humidity over
large areas - for example, Siberia in winter. Whether or
not the air will fully reach equilibrium with the surface
characteristics will depend upon how long it remains over
the source region.
The character of an air mass is dependent upon
conditions in the area in which it forms. Because of this
