Western Disturbance (WD) is the term used to describe an extratropical storm that brings sudden winter rain and snow to the northwestern parts of the Indian subcontinent.
- · 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.
Even before the onset of monsoon over east Uttar Pradesh, temporary advancement of monsoon current over Punjab, west Uttar Pradesh, Jammu and Kashmir takes place when a WD moves across northwest India. Also, the passage of a WD across north India increases monsoon activity over Punjab and Uttar Pradesh.
It was found that if the frequencies of formation and movement of depression over the Bay of Bengal in a particular year increase, then the frequencies of WDs in the same year decrease.
Satellite study of WD revealed the secondaries of extra-tropical depressions move northeastward from the eastern Mediterranean and are confined in the latitudinal belt 25°N to 35°N. The frequency of WDs abruptly decreases from winter to the pre-monsoon season. Even in the hot weather period of April and May, WDs move across north India.
In the Himalayan region of India, monsoon current progresses from east to west. But the WDs move across north India from west to east, with consequent rise in pressure and cold pool of air in the rear.
Though WDs activate monsoon in certain areas of NW India, it is not clear whether the visit of pre-monsoon WDs across north India has any impact on the progress of forthcoming monsoon current towards NW India and its activity.
Western Disturbances are important to the development of the Rabi crop in the northern subcontinent, which includes the locally important staple wheat.