KOLKATA, July 11 — The monsoon is not a single weather event but a vast, seasonal wind reversal, and right now it is in full swing over eastern India. For anyone in Kolkata, the overcast skies and heavy air are not a surprise — they are the signature of an active monsoon phase, driven by mechanisms that stretch from the Bay of Bengal to the shifting bands of tropical weather near the equator. The science behind it is straightforward but powerful.
The South Asian monsoon is driven by seasonal wind reversals, which bring heavy rainfall to eastern India from June to September. Kolkata sits in the Ganges Delta, and the southwest monsoon typically arrives here around June 10, retreating by early October.
July is the peak month, with average rainfall of about 330 mm.
How the winds work
The monsoon is influenced by the Intertropical Convergence Zone, or ITCZ, and the Bay of Bengal branch of the monsoon winds. The ITCZ is a belt of low pressure where the trade winds from the northern and southern hemispheres meet. As it shifts northward during the northern summer, it pulls moist air from the Bay of Bengal over the Indian subcontinent.
That is the basic engine: warm ocean, converging winds, rising air, and rain. The timing of the rain is not random.
The forecast for July 12 shows a 53% chance of rain around 2 PM. That aligns with the diurnal cycle of monsoon convection, which often peaks in the afternoon due to solar heating. As the sun warms the land, the air rises, and the moisture-laden air from the Bay of Bengal is drawn in and lifted, forming clouds and rain.
It is a daily rhythm, not a constant downpour.
Stagnant and humid
The light winds of 8 km/h forecast for the area are characteristic of the monsoon trough’s influence. The trough is an elongated area of low pressure that extends across northern India. When it is active, it can produce stagnant, humid conditions.
The air feels heavy because it is full of moisture, and without strong winds to stir it, the humidity just sits. That is what makes the monsoon feel oppressive even when it is not raining hard.
For residents and visitors in Kolkata, this means the typical active monsoon phase: grey skies, high humidity, and the likelihood of afternoon showers. The monsoon is not a single storm but a season, and right now it is at its peak.
