The Indian subcontinent is thus isolated from the rest of Asia. Winds blow outwards from the land over which air is subsiding; the air dries out and rainfall is largely prevented. Over India the subtropical high pressure cells give rise to dry winds blowing down the Ganges Valley and then changing to a north-easterly direction across the Bay of Bengal.
Above 3000m the circulation is quite different. A strong westerly flow, including the jet stream, flows across the continent and bifurcates around the Tibetan mountain mass in this season. The southern branch is much stronger and a permanent feature from November to April because of the strong thermal gradient above northern India. This upper flow has the effect of intensifying the surface anti-cyclone, and it also brings a sting of surface cyclonic disturbances into the north-west from the Middle East and Mediterranean areas.
The general pattern of clear skied and warn dry weather is interrupted by some cloud and rain, which becomes snow on the Himalayan sloped and even by cold spells as continental polar air spills into the Indus lowlands. In the extreme north-west these winter are generally greater than those received in summer.
As the airflow passes over the Bay of Bengal it picks up moisture and becomes involved in weak tropical disturbances which bring rain to south-eastern India and the east coasts of Sri Lanka. Thus although most of India is dominated by a winter-drought, it may be interrupted by welcome light rains in the extreme north and south and even elsewhere if the northern cyclone draw in maritime tropical (MT) air from the Indian Ocean.
Spring becomes extremely hot and dry. The sub-tropical jet circulation begins to weaken relative to the northern branch, and eventually moves to a position entirely north of the Tibetan mountains mass as the summer monsoon breaks.
India is still dominated in spring by a subsiding and outward- blowing anti-cyclonic circulation, and the clear skies allow maximum and increasing insulation, High temperatures, a heavy heat haze and drought are the characteristic features of the weather, especially in the central areas. The equatorial low pressure trough begins to move especially in the central areas.
It begins to move northwards, bringing rain to the south and the increased heat intensified some of the equally disturbances in the north. The weakening of the jet allows convection and thunderstorms due to explosive cumulo-nimbus growth in the afternoon. In the northwest, such conditions give rise to dust storms forming in the absence of atmospheric moisture.
The summer monsoon breaks suddenly but progressively from the south. Burma receives heavy rains from May onwards, but they are delayed until June over India. This change to wet conditions is thought to be associated with the upper air changes; the westerly jet moves north of the Himalayan Tibetan Mountains due to the formation of a thermal high pressure zone over the Tibetan plateau which shifts the zone of maximum contrast to this northerly position. Easterly winds are established around the southern margin of this feature due to the northward movement of the jet, which has impeded convection though the winter and spring.
A spring convergence zone is established in summer over India beneath the easterly winds aloft and moist winds are drawn in at the surface from the expanses of the Indian Ocean. Thus, air reaches India, which has had long passages over the water and is extremely moist (although some observations have suggested that it is relatively dry until it reaches the Arabian Sea).
The south-westerly flow of air brings a favorable set of circumstances for the production of rain, but atmospheric disturbances are necessary to trigger the major storms, since the basic temperature stratification of the masses does not favour upward movement.
These disturbances include the south-westerly surges in which speeds rise due to convergence near the equator and lead to instability and heavy rain as the flow reaches the mountains Western Ghats facing the Arabian Sea.
The Bay of Bengal area is associated with the formation of monsoon depressions, assisted by the easterly waves moving into the area from the Pacific. These move westwards and northwards over the land approximately once every 6 days, and the intensified surface heating there leads to widespread instability and heavy rain. In addition some of the cyclonic still move into the northern areas, and sudden, up rushes of air are responsible for heavy showers on the west coast.
The summer, from mid-June to September, is a season of heavy rains over much of the subcontinent; most places receive 80 percent of their annual total of rain. Even at this season there are considerable spells of fine, sunny weather and the amounts vary from place and from years to year.
Large areas are semi-arid. Some of these are in the rain shadows of the Western Ghats and some in the north are influenced by a wedge-like inversion which allows the south-westerly flow to penetrate on the surface but largely prevents vertical convective activity.
In some years the westerly circulation aloft is re-established during the summer season and rainfall totals are extremely low. This is disastrous to the peasant farmers and famine follows. It is now clear that the rainfall is associated closely with a series of disturbances and these in turn with events in the upper troposphere, rather than an inexplorable seasonal reversal of airflow.
In the autumn there is a transition from the wet season to the dry, as the equatorial trough moves back southwards across India. It brings rain to the south of the peninsula (which has little rain is summer). Easterly winds from the Pacific replace some of the southwesterly airflow and enable serve, hurricane type storms to reach the Bengal coast.
Two-thirds of this type of disturbances arrives between September and November. The most notable of these in recent years struck the Ganges delta in November 1970, causing a disastrous tidal wave to sweep across the low-lying area. Finally the westerly jet stream reappears and the disturbances entering from the west increase in number. The winter drought returns.
The tropical Indian monsoon, which used to thought of as very distinct from the other climates of the world, can be seen a rather case of the summer maximum rainfall type. The unusual features are related once again to the factors of relief and land-sea distribution, rather than to local atmospheric heating effects. Other tropical areas show relatively slight changes in the atmospheric situation from those encountered in India.