During winters in the north western part of India, rainfall is mainly caused by which of the following weather systems?

Introduction / Context:
India experiences different types of rainfall at various times of the year due to changing wind patterns and pressure systems. While most of the country receives its main rainfall from the southwest monsoon during summer, the north western states such as Punjab, Haryana, and parts of Rajasthan and Uttar Pradesh also receive some winter rainfall. This question asks which specific weather system is responsible for that winter rain. Recognising the role of western disturbances is crucial for understanding the climate of North India and its impact on rabi crops like wheat and mustard.

Given Data / Assumptions:
– The region mentioned is the north western part of India.
– The season referred to is winter, not the main monsoon period.
– The options include western disturbances, cyclonic depressions, southwest monsoon, and retreating monsoon.
– We assume standard climatology taught in Indian geography, where different systems are linked with different seasons and regions.

Concept / Approach:
Western disturbances are extratropical weather systems that originate over the Mediterranean region and West Asia. They move eastwards and reach North India during the winter months. As they travel over land and sea, they pick up moisture and cause cloud formation and precipitation when they encounter the Himalayan region and the plains of North West India. This leads to light to moderate rainfall and sometimes snowfall in the Himalayas. Cyclonic depressions over the Bay of Bengal are primarily associated with the southwest monsoon or with post monsoon cyclones affecting eastern and southern coasts, not with typical winter rainfall in North West India. The southwest monsoon brings summer rain from June to September. The retreating or northeast monsoon mainly affects the southeastern coast, especially Tamil Nadu, in the post monsoon season. Therefore, western disturbances are the correct answer for winter rainfall in the north western states.

Step-by-Step Solution:
Step 1: Focus on the region specified: north western India, including states like Punjab and Haryana. Step 2: Note that the season mentioned is winter, when the normal southwest monsoon is absent. Step 3: Recall from Indian climatology that western disturbances travel from the west and northwest, bringing winter rain and snow. Step 4: Recognise that cyclonic depressions over the Bay of Bengal and the retreating monsoon mainly affect eastern, central, and southern parts of India. Step 5: Conclude that western disturbances are responsible for winter rainfall in the north western part of India.

Verification / Alternative check:
Indian geography textbooks describe in detail how western disturbances move across Pakistan and northern India during winter, causing precipitation in the form of rain and snow. They emphasise that this rainfall, though modest, is very important for rabi crops in Punjab, Haryana, and western Uttar Pradesh. Climate diagrams for these regions show small rainfall peaks in winter months that coincide with Western disturbance activity. In contrast, the main monsoon rainfall peaks occur in the summer due to the southwest monsoon. This evidence consistently supports western disturbances as the cause of winter rainfall in north western India.

Why Other Options Are Wrong:
Cyclonic depressions over the Bay of Bengal: These systems mainly bring heavy rain to eastern coastal areas and sometimes central India during the southwest monsoon or post monsoon cyclone season, not typical winter rain in the north west.
Southwest monsoon winds: The southwest monsoon operates from June to September and is responsible for the main rainy season, not winter rainfall in North West India.
Retreating northeast monsoon: This affects primarily the southeastern coast of India, especially Tamil Nadu and parts of Andhra Pradesh, during October and November, not the north western states in winter.

Common Pitfalls:
Students sometimes forget that the question is about winter rainfall and choose the southwest monsoon simply because it is the best known rain bearing system for India. Others confuse western disturbances with cyclones over the Bay of Bengal because both involve low pressure systems. To avoid such mistakes, link western disturbances specifically with winter rain and snow in the north and northwest, and link the southwest monsoon with the main summer rainy season. Keeping these associations clear will help you answer similar climate questions correctly.

Final Answer:
Rainfall during winters in the north western part of India is mainly caused by Western disturbances.