Chapter

Question 2.1 The answers to this question relate to the contrasting conditions associated with rising and sinking air. In the case of (l) subpolar lows, air spirals in cyclonically and rises (Figure 2.5(a)). The rising air expands and cools adiabatically, causing moisture in the air to condense, releasing heat to the atmosphere, and further encouraging convection. Condensation from the rising air produces clouds and rain.

In the case of (2) the subtropical high pressure regions, air is sinking (cf. Figure 2.5(b)). Sinking air becomes compressed and warms adiabatically, causing water droplets in it to evaporate. Thus subtropical highs are typically dry. with little cloud; land areas affected by subtropical highs tend to be arid, and include desert regions, such as the Sahara and Sahel.

Question 2.2 Easterly waves and tropical cyclones are generated in the vicinity of the equatorial low pressure zone/Intertropical Convergence Zone (zone of highest sea-surface temperature). Figure 2.3 shows that the ITCZ is generally displaced northwards with respect to the geographical Equator; in the Atlantic and eastern Pacific it generally does not lie more than 5° south of the Equator, even in the southern summer.

Question 2.3 (a) True. Moist air convects more readily than dry air, because when it rises, condensation releases latent heat of evaporation which partly offsets adiabatic cooling.

(b) False. Rising air cools adiabatically and cannot contain as much water vapour as air at ground/sea-level, so that clouds and precipitation result. The ITCZ is a global-scale example of rising air leading to high precipitation; the depressions of mid-latitudes are smaller-scale examples. Sinking air tends to be dry, as a result of warming adiabatically.

(d) False. As discussed in the text in connection with monsoonal circulations, the flow of air over Eurasia is cyclonic (inwards and upwards) in the northern summer (Figure 2.3(a)); and anticyclonic (downwards and outwards) in the northern winter (Figure 2.3(b)).

(e) False. Figure 2.2(b) shows that in polar regions the tropopause (the top of the troposphere) is about 9-10 km above the surface of the Earth. It is about 15 km above the surface of the Earth in mid-latitudes.

(f) True. Undulations in the polar jet stream mean that the British Isles are sometimes beneath poleward-trending 'limbs' of the jet stream, and sometimes beneath equatorward-trending 'limbs', and so are sometimes affected by trains of lows/depressions (Figure 2.8). sometimes by highs.

Question 2.4 (a) Most of the clouds visible in ihe Hurricane "Andrew' satellite image are at the top of the troposphere, where air which has spiralled cyclonically in and up. diverges and spirals outwards from the 'eve' anticyclomcally (clockwise in the Northern Hemisphere) (cf. Figure 2.16). The inward spiralling cloud bands below are obscured Animated satellite images of tropical cyclones, as sometimes seen on weather or news programmes, clearly show the outward ant icy clonic flow, (b) The 'fuel' thai drives a tropical cyclone could be said to be water vapour, because it is the change in stale from gas (water vapour) to liquid (cloud droplets) that provides the latent heat which enables the upward cyclonic movement of air around the central region of the cyclone to continue.

Question 2.5 See Figure A1. Note that the subtropical high pressure regions are dark in Figure 2.2ft because ihey are regions where dry air sinks, in contrast to the ITCZ. which is a region where moist air rises.

cumulonimbus clouds in ITC2

Figure A) Answer to Question 2 5

Question 2.6 The surface wind pattern shown in Figure 2.3(c) is for a day in the northern summer. It is quite hard to see the arrows on the flow lines, but there are three obvious clues which do not rely on your being able to see the wind direction. They are:

(1) in the eastern Pacific and Atlantic, the ITCZ is well north of the Equator (though not clearly defined on the western side of either ocean);

(2) a tropical cyclone is developing north-east of Taiwan (i.e. in the Northern Hemsiphere); and

(3) wind speeds are high, i.e. conditions are very stormy, in the Southern Ocean - compare the cyclonic subpolar storms with those in Figure 2.7(b).

In addition (though perhaps hard to see), there are strong anticyclones in the North Pacific and North Atlantic, a situation which corresponds to the typical pattern for the northern summer (cf. Figure 2.3(a)). In fact, the winds shown are those for 1 August 1999, and the tropical cyclone in the western North Pacific is typhoon 'Olga', en route to landfall in Korea.

Question 2.7 Examples given in this Chapter are the large-scale undulations of the jet stream in the upper westerlies (i.e. Rossby waves, Section 2.2.1) and the easterly waves in the Trade Wind belts (Section 2.3.1). You may also have thought of the waves that often occur in the polar front, leading to the formation of depressions and cyclonic storms (Figure 2.8).

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