Context: A cooler-than-usual summer over large geographical areas of the country has contributed to keeping all-India average temperatures within the normal range in May 2025. The primary reason is the frequent passage of streams of western disturbances in the lower latitudes.
Relevance of the Topic: Prelims: Key facts about Western Disturbances.
Unusual weather conditions
As per the India Meteorological Department (IMD):
- All-India weekly average maximum temperature was 3-5 degrees Celsius below normal over West, Central, and North India. Normal temperatures prevailed over remaining parts of the country.
- Core Heatwave Zone (CHZ)- spanning Central, North, and Peninsular India between Gujarat and West Bengal, which is prone to heatwave conditions every year from March to June, has not as yet experienced significant heatwaves.
- May has been exceptionally wet over the South and Central Indian regions. Southern Peninsular India has benefited from intermittent spells of rain throughout the ongoing pre-monsoon season.
Reasons for cooler temperature in Summers:
- Western Disturbances: The primary reason for a wetter- and cooler-than-usual summer is the frequent passage of streams of western disturbances in the lower latitudes. Western disturbances are eastward-bound winds that originate in the Mediterranean Sea and cause rain or snow along their way.
- Continuous incursion of Moisture: There has also been a continuous incursion of moisture from the Bay of Bengal and Arabian Sea into the Indian mainland, with the subsequent wind interactions causing rainfall and thundershowers.
What are Western Disturbances?
- Western disturbances are extra-tropical cyclones that originate in the Mediterranean region and move eastwards towards the Indian subcontinent, affecting Northern India, northern Bangladesh, and south-eastern Nepal.
- The sub-tropical westerly jet streams help western disturbances to enter the Indian sub-continent and affect its weather. The winds while moving take up the moisture from the Mediterranean Sea, Black Sea and Caspian Sea.
- These moisture laden winds eventually reach the northwestern Himalayas and get blocked, as a result the moisture gets trapped, and precipitation is shared. This ultimately leads to:
- Snowfall in western Himalayas (J&K, Himachal Pradesh, Uttarakhand)
- Non-monsoonal rainfall over Indo-Gangetic plains (Punjab, Haryana, Delhi, Uttar Pradesh). These disturbances provide moisture to Rabi season crops
- On an average 4-6 disturbances (temperate cyclones) per month pass over northern India between November to April.