A western disturbance (WD) is defined as an eastward-moving extra-tropical upper air trough in the subtropical westerlies, often extending down to the lower atmospheric level of the north Indian latitude during the winter months. Sometimes, these are observed as closed cyclonic circulations at the sea-level. This definition was first put forward by Pisharoty and Desai1 for WDs in general and subsequently accepted by Rao and Srinivasan2 for use in India Meteorological Department (IMD). The study by Mooley3 revealed that 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. Chakravorty and Basu4 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.
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