Western Disturbance

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A Western Disturbance over the northern Indian subcontinent on 23 November 2012

Western Disturbance is the term used in India, Pakistan, Bangladesh and Nepal to describe an extratropical storm originating in the Mediterranean, that brings sudden winter rain and snow to the northwestern parts of the Indian subcontinent.[1][2] This is a non-monsoonal precipitation pattern driven by the Westerlies. The moisture in these storms usually originates over the Mediterranean Sea and the Atlantic Ocean. Extratropical storms are a global, rather than a localized, 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 important to the development of the Rabi crop in the northern subcontinent, which includes the locally important staple wheat.[3][4]

Importance

Strong Western Disturbances in winter like the one pictured bring moderate to heavy rain in low-lying areas and heavy snow to mountainous areas of the Indian Subcontinent

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.

References

  1. ↑ J. S. Lall, A. D. Moddie (1981), The Himalaya, aspects of change, India International Centre, "... The onset of a western disturbance is heralded by dense thunder clouds accompanied by heavy rain or snow. The weather conditions during winter are generally influenced by the distribution pattern of pressure in Central and West Asia. In winter, due to the shifting of the pressure belts, the westerlies blow from 20°N to 50°N latitude and Northern India comes under the influence of the winds and cyclones rising from the Mediterranean Sea. In the wake of the disturbance comes cold air often accompanied by strong winds ..." 
  2. ↑ D. K. Majumdar. Irrigation water management: principles and practice. PHI Learning Pvt. Ltd., 2004. ISBN 978-81-203-1729-1. "... Rains during winter mont in north India are due to occurrence of low pressure system called western disturbances. This moves from the west to the east causing snow fall in the high ranges in the Himalayas and rains in sub-mountainous tracts and adjoining areas ..." 
  3. ↑ Social Science (A Textbook in Geography Part II) Class X. FK Publications. "... These states are: Punjab, Haryana, Himachal Pradesh, Jammu & Kashmir, Uttarakhand and Uttar Pradesh. These states receive some rainfall (precipitation) during the rabi season, brought about by western disturbances, which originate in the Mediterranean region. These showers are very beneficial for rabi crops ..." 
  4. ↑ J.E. Parkinson, R. H. Whitehouse. Proceedings. Printed at the Mufid-i-'Am Press, 1927. "... In northern India rainfall occurs again during the year in connection with the Western Disturbances which are of great agricultural importance to north-west India as they occur chiefly during the Rabi ..." 

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