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The summer rains
Monsoon (Arabic mauism,”season”) is a wind that changes direction with the change of seasons. The monsoon prevails mainly in the Indian Ocean. It blows from the southwest, generally from April to October, and from the opposite direction, the northeast, from October to April.
The southwest, or summer, monsoon is usually accompanied by heavy rain in areas of India and the East Indies, constituting the dominant climate event of the area. The appearance of this wind pattern over geological time has been linked, through sedimentary evidence, to the uplift of the Himalayas and the Tibetan Plateau as the Indian subcontinent began to collide with the Asian crustal plate about 20 million years ago. The northern land mass was high enough by about six million years ago to cause air rising from the southern land mass to be replaced by the monsoon, establishing this wind pattern. Monsoons, in weaker form, also occur in other parts of the world.
Monsoons are like strong sea breeze; the rains begin when the summer sun heats up tropical continents much faster than the oceans around. Warm air rising overland draws in cool, moist air from the sea, and rain-bearing winds gradually pushed further inland. In the northern hemisphere for example, for about six months, showers of torrential rain sweep north over the country, right upto the Himalayas, until in October the southwest wind dies down and the rains slacken. A monsoon’s onset is hard to predict, and sometimes it fails to bring any rain to the hot, drought-stricken lands that year. Then crops fail, posing great danger of famine.
Some of the major monsoon winds that occur in Asia are discussed below:
Southwest Monsoon
The hot, dry lands of Asia draw warm air, laden with moisture, from the Indian Ocean during the early summer.
Northeast Monsoon
The cold, dry winter air spreads out from central Asia, bringing chilly, dusty conditions to the lands around.
Southwest and Northeast Monsoon
Monsoons affect large areas of the tropics and the sub-tropics from northeast Australia to the Caribbean. Asian monsoons are the most marked because of the vast area it covers.
Although our country is in the monsoon region, it is quite arid; with the exception of the southern slopes of the Himalayas and the sub-mountainous tracts, which have a rainfall from 76 to 127cm. Balochistan is the driest part of the country with an average rainfall of 21cm. On the southern ranges of the Himalayas, 127cm of precipitation takes place, while under the lee of these mountains (Gilgit and Baltistan) rainfall is hardly 16cm. Rainfall also occurs from western cyclonic disturbances originating in the Mediterranean.
It is appreciable in the western mountains and the immediate fore lying area; here the rainfall average ranges from 27 to 76 cm. The contribution of these western disturbances to rainfall over the plains is about 4cm. A large part of the precipitation in the northern mountain system is in the form of snow, which feeds the rivers. The all-pervasive aridity over most of Pakistan, the predominant influence on the life and habitat of the people, coupled with the climatic rhythm, characteristic of a monsoon climate, are conducive to homogeneity of the land.
Moonsoon’s effect on Pakistan
During summer, pressure becomes low over Asia where there is intense surface heating, but pressures remains relatively high over the cool seas to the south. Most moisture laden equatorial air moves north due to this pressure gradient as well as earth’s diminishing rotational velocity as we move from the equator to the North Pole. The moving air mass with an anti-clockwise rotation reach the subcontinent as the southwest monsoon. Air moves north towards the shortest route, away from the equator and the Carioles force exerted due to rotation of earth from west to east causes these winds to move south westerly over South India and almost westerly over Pakistan.
The permanent low-pressure zone over Pakistan is located over Balochistan and its surrounding areas during summer season. Mountains force the monsoon upward causing even more rain. Large clouds pile up against high ground as the monsoon blows inland. The monsoon thus brings some of the strongest torrential rains. Monsoon rain can be so intense that floods are frequent. Asian monsoons may also be triggered off when westerly jet streams in the upper air swinging north over the Himalayas.
Its intensity and major floods
Monsoon Season is the main rainy season in the country marked by heavy downpours and frequent floods. Inter Tropical Zone of Convergence (ITZC) moves northwards to reach its north most position south of the Himalayan Range, which generates the typical southeasterly wind commonly known as “Monsoon”. The southeast air currents meet the southwesterly system described earlier along the east and northeastern regions of Pakistan. This phenomenon affects the upper catchments of the eastern rivers of Pakistan including Sutlej, Ravi, Chenab and Jhelum Rivers.
Apart from the southeast and southwest monsoon currents, very unique weather systems called the monsoon depressions also arrive in Pakistan from the Bay of Bengal moving across India generally along the west/northwest direction. A number of such monsoon depressions are able to reach Pakistan and of these a few turn north/north-eastwards to enter the upper catchments of Sutlej. Ravi Chenab and Jhelum Rivers.
Chances of monsoon depressions affecting the upper catchments of eastern rivers increase with the advance of monsoon season, specially from middle of August till the end of September. One reason for this is the intensification of the high pressure over Tibetan Plateau, which due to its elevation remains cold as compared to the surrounding atmosphere. This creates a high-pressure core with clockwise movement of air around it. Tibetan high is thus an added factor to facilitate the northeast turning of the monsoon depressions. The westerly waves continue to move from west to east along the northern latitudes even during summer season while strong monsoon winds (south-easterlies) are moving from east to west. These phenomena are responsible for heavy monsoon rains in Pakistan causing extremely high floods in rivers and general flooding in Pakistan.
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