Environmental Engineering Reference
In-Depth Information
forces we can now explain equilibrium horizontal flows of air. They are initiated by
pressure differences, then modified by the effects of Earth's rotation and friction.
Where flows occur across the isobars in the direction of lower pressure, there will be a
transfer of air towards the low-pressure centre, leading to convergence or a net
accumulation of air. Where flow is away from a high-pressure centre, there will be a
divergence of air away from the surface anticyclone, leading to a net outflow of air.
Convergence and divergence can also be found as a result of speed variations within a
uniform air flow (Figure 6.11a) as well as in ridges and troughs in the upper atmospheric
flows (see Figure 6.15). If convergence or divergence is maintained for any time, a
transfer of mass of air will result and the original pressure gradient will be changed.
Convergence will produce an accumulation of air, increase surface pressure and so
decrease the pressure gradient and hence the convergence which produced the original air
flow. The system will stop. To maintain surface convergence (or divergence), vertical
movement is required. In general, if air is converging at the surface, it must rise, while if
it is diverging it is usually associated with subsiding air. Because of these vertical
movements resulting from horizontal flows, surface convergence often produces cloud
sheets and precipitation, whilst surface divergence is associated with clear skies and dry
weather. In the middle troposphere there is a level at about 600 hPa at which the
horizontal convergence and divergence are effectively zero (Figure 6.11b). This link
between horizontal and vertical flows in the atmosphere through
Figure 6.11
The development of divergence and convergence
in (a) horizontal and (b) vertical movements in the
atmosphere.
