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temperature (°C)

The implications of the 'dry' and 'saturated' adiabatic lapse rate for the stability of various columns of air with various different temperature-height profiles is shown in Figure 2.10(b). Over most of the oceans, particularly in winter when the sea-surface is warmer than the overlying air, the variation of temperature with height in the atmosphere, and the water content of the air, are such that conditions are unstable, air rises, and convection occurs. Convection is further promoted by turbulence resulting from strong winds blowing over the sea-surface. When turbulence is a more effective cause of upward movement of air than the buoyancy forces causing instability, convection is said to be forced. The cumulus clouds characteristic of oceanic regions within the Trade Wind belts are a result of such forced convection (Figure 2.11(a)).

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temperature fC}

Figure 2.12 Typical variation of temperature Figure 2.13 The ITCZ over the tropical Pacific, clearly marked out by cumulonimbus clouds, with height in the Trade Wind zone, at about 5° (This is an infra-red image, obtained from the GEOS satellite. The area shown is between of latitude, showing the temperature inversion ~40° N and -40° S.) (here at an altitude of ~2 km).

Figure 2.11 (a) Cumulus clouds over the ocean in the Trade Wind belt, (b) Cumulonimbus and, at lower levels, cumulus clouds, In the Intertropical Convergence Zone over the Java Sea. Like cumulus, cumulonimbus form where moist air rises and cools, so that the water vapour it contains condenses to form droplets. However, cumulonimbus generally extend to much greater heights than cumulus, and their upper parts consist of ice crystals.

As illustrated in Figure 2.2(b). upward development of the Trade Wind cumulus clouds is inhibited by the subsidence of warm air from above. This leads to an increase of temperature with height, or a temperature inversion (Figure 2.12). Rising air encountering a temperature inversion is no longer warmer than its surroundings and ceases its ascent. The warmer air therefore acts as a 'ceiling' as far as upward convection is concerned.

Along the Intertropical Convergence Zone, the 'ceiling' is the tropopause, so the vigorous convection in the ITCZ can extend much higher than that associated with cumulus formation (Figure 2.2(b)). The towering cumulonimbus that result (Figure 2.11(b)) allow the ITCZ to be easily seen on images obtained via satellites (Figure 2.13). Convection in the ITCZ is the main way that heat becomes distributed throughout the troposphere in low latitudes.

Figure 2.11 (a) Cumulus clouds over the ocean in the Trade Wind belt, (b) Cumulonimbus and, at lower levels, cumulus clouds, In the Intertropical Convergence Zone over the Java Sea. Like cumulus, cumulonimbus form where moist air rises and cools, so that the water vapour it contains condenses to form droplets. However, cumulonimbus generally extend to much greater heights than cumulus, and their upper parts consist of ice crystals.

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temperature fC}

Figure 2.12 Typical variation of temperature Figure 2.13 The ITCZ over the tropical Pacific, clearly marked out by cumulonimbus clouds, with height in the Trade Wind zone, at about 5° (This is an infra-red image, obtained from the GEOS satellite. The area shown is between of latitude, showing the temperature inversion ~40° N and -40° S.) (here at an altitude of ~2 km).

QUESTION 2.1 The labels nn the right-hand side oí Figure 2.2(a) indicate characteristic conditions at the Fiarth's surface. By reference to Figures and 2.tO. explain briefly w h_\ < I) subpolar lows arc associated with clouds and rain, and (2) the semipermanent high pressure regions of the subtropics are associated with dry/arid conditions.

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