Why rain returned with force to Delhi-NCR, the science behind the downpour

Delhi-NCR woke up to another day of relentless rain on Tuesday as a powerful combination of four weather systems triggered widespread showers, waterlogging and severe traffic disruption across the region.

The India Meteorological Department (IMD) steadily upgraded its weather alert through the day, moving from yellow to orange before issuing a red alert at 12.25 pm, signalling the highest level of warning.

Several parts of the national capital, including ITO, Nizamuddin, South Extension and stretches of the Delhi-Meerut Expressway, remained inundated, while Noida, Ghaziabad and Gurugram also witnessed very heavy rainfall.

The intense spell marks a significant turnaround after a sluggish start to this year's southwest monsoon. India had ended June with a rainfall deficit of nearly 40 per cent.

Although showers during July reduced the nationwide shortfall to around 16 per cent, several regions continued to receive below-normal rainfall.

Delhi, too, had experienced a relatively dry July until this week. According to Skymet Weather, Safdarjung had received 160.3 mm of rainfall against the monthly normal of 195.8 mm before Tuesday, with only 27.5 mm recorded over the previous 15 days.

Meteorologists attribute the sudden revival to the simultaneous influence of four atmospheric systems.

At the centre of the development is the monsoon trough, the elongated low-pressure zone that acts as the primary rain-bearing corridor during the southwest monsoon.

According to the IMD, the trough is currently positioned south of Delhi, extending through Bikaner, Gwalior, Satna and Daltonganj before connecting to a deep depression that crossed the West Bengal and north Odisha coast early on Tuesday.

The depression is particularly significant because it is the first Bay of Bengal depression of the season to move inland, a development that independent meteorologist Navdeep Dahiya highlighted as a major driver of the renewed rainfall activity.

Supporting the system are two upper-air cyclonic circulations. One is located over southwest Rajasthan at the western end of the monsoon trough, while the other lies over northern parts of central Uttar Pradesh, closer to Delhi-NCR.

Together with the depression and the monsoon trough, these four systems are reinforcing one another, allowing warm, moisture-laden air to rise, cool and condense into widespread rain-bearing clouds.

The position of the monsoon trough plays a crucial role in determining where heavy rainfall occurs. When it shifts south of its normal location, the belt of intense rainfall extends across central and northern India, bringing regions such as Delhi-NCR directly under active monsoon conditions.

When it moves north towards the Himalayan foothills, rainfall over the plains usually weakens, leading to what forecasters describe as a ‘break’ in the monsoon.

The cyclonic circulation over Rajasthan and the deep depression over Odisha have further intensified the ongoing spell by enhancing moisture convergence and upward air movement, resulting in heavier rainfall than the trough alone would typically produce.

The revival has also brought relief to the agricultural sector. Summer crop sowing, which had lagged significantly earlier in the season, has gathered pace following improved rainfall.

According to data from the Union Ministry of Agriculture and Farmers' Welfare cited by Reuters, farmers had sown crops across 78.7 million hectares as of July 24, compared with 82.6 million hectares during the corresponding period last year.

Rice cultivation, which depends heavily on timely monsoon rains, covered 23.4 million hectares, only marginally below last year's level.

'We were waiting for rain before transplanting the seedlings. After we received good rainfall this month, we started paddy transplanting,' Odisha farmer Krishna Beuraa from Konark told Reuters.

For Delhi-NCR, the IMD has forecast heavy to very heavy rainfall at isolated places on July 28 and 29, with fairly widespread rainfall expected to continue until August 3.

Looking beyond the current spell, weather experts say the course of the remaining monsoon season will largely depend on two broader climate factors:

El Niño in the Pacific Ocean, which has historically weakened Indian monsoons, and the Indian Ocean Dipole, whose temperature variations can either strengthen or suppress rainfall over the subcontinent.

Until then, the alignment of these four weather systems is expected to keep northern India under an active monsoon phase.