THE SEASONS OF THE TROPICAL MONSOON CLIMATE

In countries like India, Pakistan and Bangladesh, which have a true Tropical Monsoon Climate, three distinct seasons are distinguishable. 1. The cool, dry season (October to February). Temperatures are low, 24 'C (76 'F), in Bombay and only 10 'C (50 'F) in Punjab, with heavy sinking air. Frosts may occur at night in the colder north. The centre of high pressure is over the Punjab. Outblowing dry winds, the North East Monsoon, bring little or no rain at all to the sub-continent of Indo Pakistan. However, a small amount of rain falls in Punjab from cyclonic sources and this is vital for the survival of winter cereals. Where the North East Monsoon blows over the bay of Bangal it acquires moisture and thus brings rain to the south eastern tip of the peninsula at this time of the year. For instance, in Madras 1270 mm (50 inches) of rain falls during October and November, accounting for half its annual rainfall. 2. The hot dry season (March to mid-June). Bombay has a mean May temperature of 30 'C (86 'F), which is considered moderate, for many parts of India are even hotter. The heat is so great that schools and offices are closed. The stifling heat and the low relative humidity make outdoor life almost unbearable. Day temperatures of 35 'C (95 'F) are usual in central India and the mean temperature in Sind may be as high as 43 'C (110 'F). Coastal districts are a little relieved by sea breezes. There is practically no rain anywhere. By May, the temperature is so high that an intense low pressure zone is set up in north west India. Duststorms are frequent, followed by long awaited rainstorms that break by the middle of June. The transitional period between 'no rain' and 'plenty rain' is over. 3. The rainy season (mid-June to September). With the burst of the South West Monsoon in mid-June, torrential downpours sweep across the country to the delight of everybody. Almost all the rain for the year falls within this rainy season. For example, in Bombay 505 mm (19.9 inches) are recorded in June, 610 mm (24 inches) in July, 368 mm (14.5 inches) in August and a further 269 mm (10.6 inches) in September. As much as 95 percent of the annual rainfall is concentrated within four months. This pattern of concentrated heavy rainfall in summer is a characteristic of the Tropical Monsoon Climate. The torrential downpours have an immediate impact on the local temperature. They lower the temperature considerably. The mean temperature for Bombay is 30 'C (86 'F) in May but only 27 'C (81 'F) in July. In the north the drop is even greater as much as 7 'C (13 'F). Some of the windward stations on the Himalayan foothills have very heavy rainfall, though this is partly orographic. Cherrapunji has an average annual rainfall of 10795 mm (425 inches) and a record of 22987 mm (905 inches) in 1861.

CLIMATIC CONDITIONS IN TROPICAL MONSOON LANDS

The basic cause of monsoon climates is the difference in the rate of heating and cooling of land and sea. In the summer, when the sun is overhead at the Tropic of Cancer, the great land masses of the northern hemisphere are heated. Central Asia, backed by the lofty Himalayan ranges, is more than 9 'C (15 'F) hotter than its normal temperature, and a region of intense low pressure is set up. The seas, which warm up much slower, remain comparatively cool. At the same time, the southern hemisphere experiences winter and a region of high pressure is set up in the continental interior of Australia. Winds blow outwards as the South-east Monsoon, to Java and after crossing the equator are drawn towards the continental low pressure area reaching Indo Pakistan as the South West Monsoon. In the winter, conditions are reversed. The sun is overhead at the Tropic of Capricorn, central Asia is extremely cold, resulting in rapid cooling of the land.

THE TROPICAL MONSOON AND TROPICAL MARINE LANDS

DISTRIBUTION
The tropical monsoon lands are with on-shore wet monsoons in the summer and off-shore dry monsoons in the winter. They are best developed in the sub-continent of India, Pakistan and Bangladesh, Burma, Thailand, Laos, the Khmer Republic, parts of Vietnam and South China and Northern Australia. Outside this zone, the climate is modified by the influence of the on-shore Trade Winds all the year round, and has a more evenly distributed rainfall. Such a climate, better termed the Tropical Marine Climate, is experienced in Central America, West Indies, North-eastern Australia, the Philippines, parts of East Africa, Madagascar, the Guinea Coast and Eastern Brazil.