Definition
Extra-tropical low-pressure systems that originate over the Mediterranean region and travel east with the subtropical westerly jet stream, bringing winter rain and snow to north-west India and the western Himalaya, and occasionally severe storms and floods in other seasons.
Background and evolution
Most of India's rain comes from the summer monsoon, but north-western India and the western Himalaya also depend on a winter source of moisture. In the cold months, the subtropical westerly jet stream shifts south and flows across Iran, Afghanistan and Pakistan towards northern India, sometimes splitting around the Tibetan plateau. Low-pressure systems that form over the Mediterranean Sea and pick up moisture from the Caspian Sea, the Black Sea and the Persian Gulf travel along this jet. As they reach the Himalaya, the moist air is forced upward, cooling and releasing precipitation.
Indian meteorologists named these systems western disturbances because they arrive from the west, unlike the monsoon depressions that form over the Bay of Bengal. A typical winter sees several of them, each bringing a spell of clouds, rain in the plains, snow in the hills and a dip in temperatures, followed by cold, clear weather and sometimes fog. This page explains them as a climatic mechanism and links them to agriculture and hazards.
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