Environmental Engineering Reference
In-Depth Information
If we follow depressions over a period of several days we find that many, though by
no means all, conform to the general pattern of the Bergen model. Initially a small wave
develops along the front or baroclinic zone, separating polar and tropical air masses
(Figure 7.4a). In some cases no further development takes place and the wave gradually
dies out. More often the wave begins to amplify and a small low-pressure centre forms.
Gradually air pressure within this centre falls, the winds strengthen and the area
Figure 7.4
Cloud distribution and pressure changes during
the evolution of a mid-latitude cyclone.
Figure 7.5
Interaction between surface and upper
atmospheric flow near an upper trough.
of low pressure expands (Figure 7.4b). Eventually the system starts to fill and the
depression gradually disappears (Figure 7.4c).
What we see at the surface is only part of the story, however, for the depression also
extends up into the atmosphere. The low-pressure centre represents a complex column of
rising air - one which is often visible on satellite photographs as a characteristic spiral
(Plate 7.2). To understand the depression more fully, we need to ascend to the top of the
column, to the upper atmosphere, where we find the waves in the upper westerlies. The
flow around the ridges and troughs is not always in equilibrium with the pressure
gradients. Where air moves out of a trough it accelerates; as it approaches a trough it
