Western Disturbance in India, Pakistan, Bangladesh and Nepal is an extratropical storm originating in the Mediterranean, that brings sudden winter rain and snow to the northwestern parts of the Indian subcontinent. This is a non-monsoonal precipitation driven by the Westerlies. The moisture in these storms usually originates over the Mediterranean Sea and the Atlantic Ocean. Extratropical storms are a global phenomena with moisture usually carried in the upper atmosphere (unlike tropical storms where it is carried in the lower atmosphere). In the case of the subcontinent, moisture is sometimes shed as rain when the storm system encounters the Himalayas.
Western Disturbances are beneficialto the development of the Rabi crop in the northern subcontinent, which includes the locally important staple wheat.
Western Disturbance causes winter and pre monsoon season rainfall across northwest India. Winter months Rainfall has great importance in agriculture, particularly for the rabi crops. Wheat among them is one of the most important crops, which helps to meet India’s food security. During the season, normally 4-5 western disturbances in a month can be seen over northwest India. Some of the western disturbances bring well-distributed and good rainfall, while some pass with negligible rain or sometimes no rain. The Western disturbance affects day-to-day weather of northwest India especially during winter season. It is usually associated with cloudy sky, higher night temperatures, unusual rain etc. Over the Indo-Gangetic plains, it brings cold wave conditions and occasionally dense fog and cold day conditions. These conditions remain stable until it is disturbed by another Western Disturbances.
Western Disturbance before South-West monsoon
When the Western Disturbance moves across northwest India before the onset of monsoon, a temporary advancement of monsoon current appears over the states including Punjab, Uttar Pradesh, Jammu and Kashmir etc. When it passes across north India, it helps to increase monsoon activity over Punjab and Uttar Pradesh.
The numbers of western disturbances start declining after winter to pre-monsoon season. During hot weather months of April and May, they move across north India. Monsoon current generally progresses from east to west in northern Himalayan region of India. But western disturbances follows reverse trend i.e. they move from west to east across north India with consequent rise in pressure carrying cold pool of air. So it helps to activate monsoon in certain parts of northwest India.
During winter,there is an inflow of low pressure depressions called western disturbances in north-west India.These westerly depressions originate in India and enter India after crossing Pakistan,Iran and Iraq and Afghanistan.They intensify over the north west India and move eastwards causing rain in Punjab and Haryana and snowfall in Himalayan belt. During summer,the whole of east coast of India comes under tropical depressions(low pressure systems originating at the head of the Bay of Bengal).They cause heavy loss to life and property.
Sources:http://en.wikipedia.org/wiki/Western_Disturbance
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