These lows occur almost exclusively in summer, resulting from intense daytime heating of continental areas. Figure 7.1C illustrates their vertical structure. The most impressive examples are the summer low-pressure cells over Arabia, the northern part of the Indian subcontinent and Arizona. The Iberian Peninsula is another region commonly affected by such lows. They occur over southwestern Spain on 40 to 60 per cent of days in July and August. Typically, their intensity is only 2 to 4 mb and they extend to about 750 mb, less than in other subtropical areas. The weather accompanying them is usually hot and dry, but if sufficient moisture is present the instability caused by heating may lead to showers
Source: After Wick (1973).
and thunderstorms. Thermal lows normally disappear at night when the heat source is cut off, but in fact those of India and Arizona persist.
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