The abnormally heavy rainfall across northwest India in recent days needs little explanation for those experiencing it. The science behind these deluges, however, reveals a complex interaction of multiple weather phenomena: low-pressure systems, western disturbances, and cyclonic circulations working in dangerous harmony. A look at what these systems are, and how they intensify rain.
Low-pressure systems and cyclonic circulations
Cyclonic circulations create cyclone-like circular air movement (anti-clockwise in the northern hemisphere) with relatively slow wind speeds. They form around low-pressure regions, which pull air inward as it moves from high to low pressure areas.
These low-pressure zones force air and moisture upward, causing rainfall as the air cools. Four such systems affected India during August’s second half alone.
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During monsoon season, these low-pressure regions often stretch into elongated shapes at lower atmospheric levels (1.4-1.6 km height or at a pressure of 850 millibar), forming what meteorologists call a monsoon trough.
This monsoon trough can be seen in the anti-clockwise movement of air in an elongated region running from the Bay of Bengal to northwestern states on August 25 (See Map 1).
Western disturbances: The catalyst
To be sure, rainfall was not intense everywhere along the monsoon trough around August 25 (see Map 2). Northwestern states experienced particularly severe downpours due to a western disturbance active in the region.
These storms originate in the west, typically the Mediterranean, and primarily affect northern India. Like other storm systems, western disturbances feature low-pressure and cyclonic circulations (see dashed red line on Map 3), but operate higher in the atmosphere and usually remain further north during peak monsoon months.
And typically, they do not make an appearance in the months of July or August – peak periods when the monsoon system is active (see chart). For example, according to a long-term dataset of these storms created by Kieran Hunt, a climate researcher at University of Reading, there were just 1.42 western disturbances on average in August during the 1971-2020 period, with some years recording none. In contrast, five such storms affected India in August 2025.
What drove August’s unusual activity?
A robust explanation will require a detailed study. But some observations offer vital clues. High-level winds at around 12km altitude remained stable around Iran during the August 25-27 period. This can be seen in 200mb winds (see Map 3) moving clockwise around the region — the opposite of cyclonic movement. This likely pushed western disturbances southward toward India.
“Whilst the monsoon supplies moisture required for heavy rainfall, a western disturbance can act like a catalyst, translating available moisture into widespread and heavy precipitation,” explained Akshay Deoras, research scientist at the University of Reading. “Such interactions are usually seen more commonly before monsoon onset or after withdrawal compared to August.”
Deoras identified atmospheric blocking over mid-latitudes as a likely factor, where high-level wind patterns become stagnant and disrupt western disturbance paths. Such blocking occurred between August 25-27 north of Iran, potentially redirecting a western disturbance toward northern India and triggering intense rainfall over Jammu and Kashmir and surrounding regions.
The interaction mechanisms
Western disturbances intensify monsoon rain through two main mechanisms, according to a 2021 paper by Hunt and colleagues. In one interaction, one system can supply moisture to the other—as occurred during the devastating 2013 Uttarakhand floods. In the other, western disturbances either physically merge with monsoon low-pressure systems from the Bay of Bengal or enhance overall conditions for heavy rain production.
The August 25-27 event exemplified the latter, with western disturbances enhancing rather than directly merging with monsoon conditions.
