NOR’WESTER

Prelims: Current events of national importance | Geography

Why in News?

Severe Nor’wester storm caused damage and casualties in Odisha IMD issued orange and yellow warnings.

Nor'westers are intense pre-monsoon thunderstorms occurring in the late afternoon over East India (especially West Bengal & Assam) and Bangladesh from March to May.
It is also Known as Kalbaisakhi.
Type – Convective thunderstorm system.
Wind Pattern – Northwest → Southeast. Hence the name Nor’wester.
Mechanism Key Components
Thermal Instability- Intense solar heating of the land surface, specifically over the Chota Nagpur Plateau and adjoining areas, creates convective instability.
Moisture Influx - Warm, moist air is transported from the Bay of Bengal into the lower atmosphere over West Bengal and Bangladesh.
Atmospheric Interaction - Dry, cold winds from the north/northwest intersect with the warm, moist southerly air, creating high instability.
Vertical Lifting & Shear - Strong vertical wind shear allows for the formation of large Cumulonimbus clouds with heights reaching 18-20 km.
Trigger Mechanism - Localized mesoscale processes (like sea breeze convergence) trigger the sudden release of this instability, causing explosive convective development.

Positive Effects	Negative Effects
Provide relief from pre-monsoon heatwaves.	Cause damage to houses and infrastructure.
Bring short bursts of rainfall.	Lead to strong winds (50–100 km/h).
Support agriculture (jute, early paddy).	Result in crop damage.
Maintain soil moisture before monsoon.	Uproot trees & electric poles.
Help release atmospheric heat via convection.	Cause lightning-related deaths/injuries.
Indicate pre-monsoon transition phase.	Disrupt power supply & transport.

Associated Weather Warnings –
- Green – No warning
- Yellow Warning – Be aware
- Orange Warning – Be prepared
- Red Warning – Take Action

India is among the most lightning-prone regions during pre-monsoon season.

Reference

Nor’westers | Indian Express