Heavy rainfall continued across parts of Delhi-NCR rain-affected regions on Thursday as an active low-pressure area and the seasonal monsoon trough strengthened monsoon conditions over the region. The India Meteorological Department (IMD) had issued alerts for heavy rain in parts of Delhi-NCR, while meteorologists said East Delhi, Noida, Ghaziabad and Faridabad experienced the heaviest rainfall from Wednesday night through Thursday morning. West and central Delhi, along with Gurugram, received comparatively lighter showers.
The widespread rainfall led to waterlogging and traffic disruptions across the National Capital Region. Boats were seen navigating flooded streets in Ghaziabad’s Vasundhara, high-rise buildings in Noida were engulfed by dense clouds, and long traffic snarls were reported across Gurugram and New Delhi.
According to private forecaster Skymet Weather, the strengthening monsoon conditions are linked to two key weather systems.
“The monsoon has finally gained strength in Delhi-NCR… due to the influence of an active low-pressure area and the monsoon trough, there is a possibility of intermittent rain continuing until July 10,” said private forecaster Skymet Weather in an X post.
What Is a Low-Pressure Area?
A low-pressure area is a region of the atmosphere where surface air pressure falls below that of the surrounding areas. As a result, air from relatively higher-pressure regions moves inward to fill the gap.
In the Northern Hemisphere, the incoming air spirals inward in an anti-clockwise direction, converging towards the centre of the system before rising upward. As the air ascends, it cools. Since cooler air holds less moisture than warmer air, the water vapour condenses into cloud droplets, eventually leading to rainfall.
The India Meteorological Department explains a low-pressure area as being:
“associated with a whirling motion of air, convergence and upward motion of air. In the low, usually clouds and rainfall are present”.
A low-pressure area can extend across hundreds of kilometres and persist for several hours or days. When the system intensifies and wind speeds exceed 17 knots, the IMD upgrades its classification first to a “well-marked” low and later to a depression, indicating stronger convergence and typically heavier rainfall.
According to the IMD’s 9:15 am bulletin on Wednesday, a well-marked low-pressure area was located over northeast Madhya Pradesh and adjoining southeast Uttar Pradesh. The seasonal monsoon trough was passing through this system from southwest Rajasthan to Bangladesh. The weather system has since moved close enough to influence rainfall across Delhi-NCR.
How the Monsoon Trough Is Influencing Delhi-NCR Rain
The monsoon trough is an elongated belt of low pressure that typically extends during the southwest monsoon from a heat-induced low over Pakistan to the head of the Bay of Bengal, broadly following the Gangetic plains.
Meteorologists often describe it as the backbone of the monsoon because smaller weather systems, including low-pressure areas and monsoon depressions, develop and move westward along it.
The IMD describes the monsoon trough as:
“one of the semi-permanent features of monsoon circulation”
and notes that its shape
“may be a characteristic of”
the east-west alignment of the Himalayas and the north-south orientation of Meghalaya’s Khasi-Jaintia Hills.
The position of the trough is not fixed and can shift significantly within a day. Its location determines where the heaviest rainfall occurs.
When the trough moves south of its usual position, rainfall becomes concentrated over central and northern India, including Delhi-NCR. When it shifts north towards the Himalayan foothills, rainfall decreases across the plains in what forecasters refer to as a “break” in the monsoon, while the Himalayan foothills and the Brahmaputra basin receive heavier rainfall.
The IMD states:
“The northward migration of this trough leads to break monsoon conditions over major parts of India and heavy rains along foothills of Himalayas and sometimes floods in Brahmaputra river,”
According to Skymet Weather, Thursday’s rainfall occurred because the monsoon trough was favourably positioned over Delhi while being reinforced by the well-marked low-pressure area moving along it.
The IMD describes these weather systems as:
“The monsoon lows and depressions are the principal rain bearing systems of the southwest monsoon period over India”.
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