New Delhi, Sept. 3 -- 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. 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. Fo...