The Southwest Monsoon Season

The southwest monsoon is the single most important weather system for India — it delivers the bulk of the country's annual rainfall and shapes the agricultural calendar. Understanding it means following the chain of cause and effect: the intense May heat over the northwestern plains, the deepening low pressure that results, and the way this low pressure pulls the moisture-laden trade winds of the Southern Hemisphere across the equator toward India.

How the Monsoon Is Set in Motion

By May, the rapid rise in temperature over the northwestern plains intensifies the low pressure conditions already present there. By early June, this low pressure becomes powerful enough to attract the trade winds of the Southern Hemisphere coming from the Indian Ocean. These southeast trade winds cross the equator and enter the Bay of Bengal and the Arabian Sea, where they are caught up in the air circulation over India. As they pass over the equatorial warm currents, they pick up abundant moisture.

After crossing the equator, these winds follow a southwesterly direction — which is exactly why they are called the southwest monsoons.

The Burst of the Monsoon

The rain in the southwest monsoon season begins rather abruptly. The first rain brings down the temperature substantially. This sudden onset of moisture-laden winds, associated with violent thunder and lightning, is often termed the "break" or "burst" of the monsoons.

The monsoon may burst in the first week of June in the coastal areas of Kerala, Karnataka, Goa and Maharashtra, while in the interior parts of the country it may be delayed to the first week of July. The day temperature registers a decline of 5°C to 8°C between mid-June and mid-July.

As these winds approach the land, their southwesterly direction is modified by the relief and by the thermal low pressure over northwest India. The monsoon approaches the landmass in two branches:

• The Arabian Sea branch
• The Bay of Bengal branch

The Arabian Sea Branch

The monsoon winds originating over the Arabian Sea split into three branches.

First branch: One branch is obstructed by the Western Ghats. These winds climb the slopes of the Western Ghats from 900 to 1200 metres, become cool, and as a result the windward side of the Sahyadris and the Western Coastal Plain receive very heavy rainfall ranging between 250 cm and 400 cm. After crossing the Western Ghats, these winds descend and get heated up, which reduces their humidity. As a result, they cause little rainfall east of the Western Ghats. This region of low rainfall is known as the rain-shadow area.

Second branch: Another branch strikes the coast north of Mumbai. Moving along the Narmada and Tapi river valleys, these winds cause rainfall in extensive areas of central India. The Chotanagpur plateau gets about 15 cm of rainfall from this part of the branch. Thereafter, they enter the Ganga plains and mingle with the Bay of Bengal branch.

Third branch: A third branch strikes the Saurashtra Peninsula and Kachchh. It then passes over west Rajasthan and along the Aravalis, causing only scanty rainfall. In Punjab and Haryana, it too joins the Bay of Bengal branch.

These two branches, reinforced by each other, cause rains in the western Himalayas.

The Bay of Bengal Branch

The Bay of Bengal branch strikes the coast of Myanmar and part of southeast Bangladesh. But the Arakan Hills along the coast of Myanmar deflect a big portion of this branch toward the Indian subcontinent. The monsoon therefore enters West Bengal and Bangladesh from the south and southeast, rather than from the southwesterly direction.

From here, this branch splits into two under the influence of the Himalayas and the thermal low in northwest India: …