What are Western disturbances? How do they affect the climate of India? -

Western disturbances are the temperate storms (weak cyclones) originating in Mediterranean regions. 

Effect of western disturbances on the climate of India:

While passing over the Mediterranean Sea, Caspian sea, and other water bodies, these storms gather moisture and bring winter rain in western regions of India including Punjab, Haryana, Delhi among others. In higher reaches of Jammu and Kashmir, they cause snow.  

Western Cyclonic Disturbances are the temperate cyclones. They are low-pressure areas embedded in the Westerlies, the planetary winds that blow from west to east between 30 and 60° latitude which originate over Mediterranean Sea and Western Asia, and move into India, along with the westerly flow of jet streams.
With southward shift of polar front in winters, the tracks of temperate cyclone pass causing western disturbances across the northern portions of Indian sub-continent. The cyclones originating in west Asia, the Mediterranean region and even in the Atlantic ocean, reach Indian ocean in course of their eastward passage.

Since the cyclones reach India from west, they are usually referred as western disturbances. Most of these disturbances are in mature stage and hence by the time they reach western coast of India, they weaken.

Reaching Rajasthan,Haryana and Punjab , these disturbances slow down and stagnate due to nearly closed nature of high hills,

With the feeded moisture from arabian sea, these disturbances may intensify and may move farther south.

By causing cyclonic rains over the plans and snowfall in the mountains they distrub he quite weather conditions of north and north west India.

Winter rainfall caused by them locally known as ' Mahawat' is of immense importance for the cultivation of Rabi crops.