Environmental Engineering Reference
In-Depth Information
Unfortunately their diversity makes it difficult to generalize about their weather
properties.
WEATHER
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FORMING SYSTEMS OF THE TROPICS
EASTERLY WAVES
The weather of the trade-wind zone normally shows little variety. It is characterized by
small convectional clouds drifting across the sky in response to the prevailing winds
(Plate 7.4) and is dominated by the Trade Wind Inversion (Figure 7.12). Showers may
develop in the afternoon, and they are likely to be heavier and more frequent in the
summer season, but otherwise the weather remains remarkably constant throughout the
year.
Occasionally disturbances arise to upset this quiet regime. On a dramatic scale there is
the tropical cyclone, which is discussed in the next section, but on a smaller scale there is
the easterly wave. As its name implies, this represents weather-forming systems related to
wave-like structures in the easterly flow of air. They reach their maximum intensity at
about the 700 hPa level.
Figure 7.12
Structure of the Trade Wind Inversion.
The wave does not necessarily move at the same speed as the easterly flow, and it may
even exceed the average wind speed. Preceding the wave, convectional cloud dies down,
owing to surface divergence and subsidence of the air, while the wind backs towards the
north-east (Figure 7.13). As the main axis of the wave approaches, convergence becomes
dominant, causing ascent of the air, cloud formation and precipitation just ahead of the
