What explains the ongoing deluge in north-west India?

NEW DELHI: The abnormally heavy rainfall across northwest India in recent days needs little explanation for those experiencing it.

- Abhishek Jha

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 (anticlockwise 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.

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 anticlockwise 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