Stability or Instability of Atmospheric Layering

In areas of low pressure, where there is instable layering, there is an upward current with corresponding formation of clouds and precipitation. High-pressure areas on the other hand are characterized by sinking air, which causes clouds to disperse. Often high-pressure areas also have an inversion (the normal decrease of temperature with altitude changes to an increase of temperature) at the upper limit of the ground layer (roughly 1.5 km), which again inhibits the formation of clouds. This is a typical phenomenon in the trade-wind zone (Figure 1). These two contrasting types of air pressure conditions result in

Figure 1 Mean annual sum of precipitation (1901-2000).

Mean summer precipitation 1901/2000

0 100 500 1000 1250 1750 mm

Figure 2 Mean sum of precipitation, June-August (1901-2000).

Figure 2 Mean sum of precipitation, June-August (1901-2000).

Mean winter precipitation 1901/2000

0 100 500 1000 1250 1750 mm

Figure 3 Mean sum of precipitation, December-February (1901-2000).

Figure 4 Mean annual sum of precipitation on 53° N from the Netherlands up to the Ural mountains.

Figure 4 Mean annual sum of precipitation on 53° N from the Netherlands up to the Ural mountains.

contrasting precipitation sums, relatively high precipitation in the middle latitudes where areas of low pressure are carried by westerly air streams and little or no precipitation in the subtropical high-pressure belt.

Convection and thus cloud formation can be hindered; layering is extremely stable (inversion) due to the ground surface being colder than the air above it. This phenomenon is found particularly at coasts adjacent to cold ocean currents (e.g., Southwest Africa — Benguela current! Namibian desert; Chile — Humboldt current! Atacama desert; West Australian current! dry areas of the Australian interior, which reach to the coast).

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