The monsoon is likely to begin withdrawing from the mainland from October 6, said the India Meteorological Department (IMD).
Let us learn all terminologies related to Monsoon.
What is Monsoon?
- Indian monsoon, the most prominent of the world’s monsoon systems, which primarily affects India and its surrounding water bodies.
- It blows from the northeast during cooler months and reverses direction to blow from the southwest during the warmest months of the year.
Onset of Monsoon
- This process brings large amounts of rainfall to the region during June and July.
- As the high-sun season (that is, the Northern Hemisphere summer) moves northward during April, India becomes particularly prone to rapid heating because the highlands to the north protect it from any incursions of cold air.
- There are three distinct areas of relative upper tropospheric warmth—namely, (1) above the southern Bay of Bengal, (2) above the Plateau of Tibet, and (3) across the trunks of the various peninsulas that are relatively dry during this time.
- These three areas combine to form a vast heat-source region.
- In contrast, a heat sink appears over the southern Indian Ocean as the relatively cloud-free air cools by emitting long-wavelength radiation.
- Monsoon winds at the surface blow from heat sink to heat source.
Peak period
- The position of the easterly jet controls the location of monsoonal rains, which occur ahead and to the left of the strongest winds and also behind them and to the right.
- The surface flow, however, is a strong, south-westerly, humid, and unstable wind that brings humidifies of more than 80 percent and heavy squally showers that are the “burst” of the monsoon.
- The overall pattern of the advance follows a frontal alignment, but local episodes may differ considerably.
Key areas
- Most spectacular clouds and rain occur against the Western Ghats in India, where the early monsoonal airstream piles up against the steep slopes, then recedes, and piles up again to a greater height.
- Each time it pushes thicker clouds upward until wind and clouds roll over the barrier and, after a few brief spells of absorption by the dry inland air, cascade toward the interior.
- Various factors, especially topography, combine to make up a complex regional pattern.
Break in Monsoon
- During the south-west monsoon period after having rains for a few days, if rain fails to occur for one or more weeks, it is known as break in the monsoon.
- These dry spells are quite common during the rainy season.
- In northern India rains are likely to fail if the rain-bearing storms are not very frequent along the monsoon trough or the ITCZ over this region.
- Over the west coast the dry spells are associated with days when winds blow parallel to the coast.
Withdrawal of Monsoon
- By August the intensity and duration of sunshine have decreased, temperatures begin to fall, and the surge of south-westerly air diminishes spasmodically almost to a standstill in the northwest.
- In September, dry, cool, northerly air begins to circle the west side of the highlands and spread over north-western India.
- The easterly jet weakens, and the upper tropospheric easterlies move much farther south.
- Because the moist southwesterlies at lower levels are much weaker and variable, they are soon pushed back.
- The rainfall becomes extremely variable over most of the region, but showers are still frequent in the south-eastern areas and over the Bay of Bengal.
- By early October, variable winds are very frequent everywhere.
Winter rains
- At the end of the month, the entire Indian region is covered by northerly air and the winter monsoon takes shape.
- The surface flow is deflected by the Coriolis force and becomes a north-easterly flow.
- Tropical depressions and cyclones are important contributing factors.
- Most of India thus begins a sunny, dry, and dusty season.
- Conversely, the western slopes of the Karakoram Range and Himalayas are then reached by the midlatitude frontal depressions that come from the Atlantic and the Mediterranean.
- The winter rains they receive, moderate as they are, place them clearly outside the monsoonal realm.
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