Nor'westers

Also called: Kalbaisakhi, Bardoisila · Topic: Climate of India and the Monsoon · NCERT: Class 11, Ch 4 "Climate"

Meaning

Nor'westers are violent evening thunderstorms of the pre-monsoon season (March to May) in east and north-east India, mainly West Bengal and Assam. They usually move from north-west to south-east, which is how they got their name [2]. They matter because they bring hail, lightning, squalls and sometimes tornadoes, but their rain also helps tea, jute and rice grow before the monsoon arrives.

Explanation

Names and season

• Kalbaisakhi is the name used in Bengal. It means "calamity of the month of Baisakh".
• Baisakh is the Bengali month from mid-April to mid-May.

• The storms are most feared in this month.

• Bardoisila is the name used in Assam.

• Season: these severe thunderstorms happen from March to May [2].
• This is the hot weather season (pre-monsoon season), the period between winter and the south-west monsoon.

• The IMD (India Meteorological Department) treats March–May as its pre-monsoon thunderstorm season [2].

• Time of day: they usually strike in the evening, after the land has heated up all day.

How they form (cause → process → result)

• Cause 1: a hot land surface
• After 21 March, the sun's apparent northward march (it only seems to move north because the Earth is tilted) heats the land of north and east India strongly.

• A thermal low (an area of low pressure caused by heating) forms. It runs from the Thar Desert to the Chota Nagpur Plateau.

• Cause 2: moist sea air

• Moist air flows in from the Bay of Bengal.

• At the edges of the trough, this moist sea air meets hot, dry air from the land.

• Process: convection

• The very hot ground makes the air unstable (it rises easily).
• The moist air rises fast. This is convection: rising warm air cools, its water vapour condenses, and clouds grow.

• Tall cumulonimbus clouds (dark, towering thunderclouds) build up during the afternoon.

• Result: the storm breaks in the evening with strong winds, torrential rain and hail.

Path, growth and size

• Main path: mostly north-west to south-east over east and north-east India [2].
• Other path: some storms move south-west to north-east over Nepal, Bhutan and Bangladesh [2].
• Where they start: in Jharkhand, Odisha and sub-Himalayan West Bengal. They become stronger as they move over Bangladesh and north-east India [3].
• Mesoscale Convective Complexes (MCCs): sometimes many storm cells join into one large storm system that lasts a long time. These are called MCCs [3].

Hazards and benefits

• Hazards [2]:
• heavy showers and lightning;
• hail (balls of ice falling from the cloud);
• squalls (sudden, strong bursts of wind);
• downbursts (strong downward blasts of air from the cloud);

• sometimes tornadoes.

• Benefits: the pre-monsoon rain helps:

• tea in Assam;

• jute and rice in Bengal.

• Monitoring: the IMD tracks these storms every 30–60 minutes using INSAT satellite images and overlapping radar coverage [3].

Where it occurs

• India:
• West Bengal, where the storm is called Kalbaisakhi.
• Assam, where it is called Bardoisila.
• East and north-east India in general.

• Source areas: Jharkhand, Odisha and sub-Himalayan West Bengal [3].

• Neighbouring countries:

• Bangladesh, where the storms grow stronger [3].

• Nepal and Bhutan, where some storms move from south-west to north-east [2].

• Moisture source: the Bay of Bengal.

Don't confuse with

• Loo: a hot, dry and dusty wind that blows in the afternoons in May–June over the northern plains (Punjab to Bihar, strongest between Delhi and Patna). It brings no rain and causes heatstroke. Nor'westers are rain- and hail-bearing evening thunderstorms in the east.
• Mango showers: pre-monsoon showers in Kerala and coastal Karnataka that help mangoes ripen early. They are not in Bengal or Assam.
• Blossom (cherry/coffee) showers: showers in Kerala and nearby Karnataka that make coffee plants flower. Nor'westers help tea, jute and rice. Swapping the crops is a common trap.
• May dust storms: evening dust storms over Punjab, Haryana, eastern Rajasthan and Uttar Pradesh. They give only a light shower or a cool breeze. Nor'westers are violent thunderstorms with hail, squalls and sometimes tornadoes.

Prelims Hooks

• Kalbaisakhi = West Bengal; Bardoisila = Assam. Both are local names for nor'westers.
• Kalbaisakhi means "calamity of the month of Baisakh" (mid-April to mid-May).
• Nor'westers mostly move from north-west to south-east. Some move south-west to north-east over Nepal, Bhutan and Bangladesh [2].
• Crops helped: tea (Assam), jute and rice (Bengal). They do not help coffee (blossom showers) or mangoes (mango showers).
• They form in Jharkhand, Odisha and sub-Himalayan West Bengal and grow stronger over Bangladesh and north-east India [3].
• Hazards include hail, lightning, squalls, downbursts and tornadoes [2]. They are a pre-monsoon (March–May) feature, not a monsoon or winter one.

Mains Points

• Origin shows where two air masses meet (GS-I):
• moist Bay of Bengal air meets hot, dry land air at the edge of the summer thermal low;
• the strong heating causes convection, which builds cumulonimbus clouds and evening storms;

• this explains why the east gets stormy weather in summer while the north-west gets the dry Loo and dust storms.

• Both a risk and a resource (GS-I/III):

• their rain helps tea, jute and rice in Assam and Bengal before the monsoon;
• but hail, lightning, squalls, downbursts and tornadoes damage crops, homes and power lines [2];

• large MCCs add to the damage because they last longer [3].

• Disaster management (GS-III):

• IMD nowcasting (short-range forecasts for the next few hours) uses INSAT images and overlapping radar every 30–60 minutes [3];
• timely warnings help farmers protect crops and help people find shelter from lightning and hail in eastern and north-eastern India.

Related concepts

• Hot weather season
• Loo
• Mango showers
• Blossom showers

Read more

• The hot weather season and its local storms