Environmental Engineering Reference
In-Depth Information
Chapter
7
Weather-forming 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 snows could be made and measures could be taken to deal with drought.
Any such detailed understanding 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, many 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 (Plate 7.1). In some areas clouds are abundant, sometimes 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
