A sudden, sharp burst of wind — typically 60–120 km/h — lasting minutes to an hour, usually from a thunderstorm downburst. Common in pre-monsoon Nor'westers.
phenomenaWhat is a Squall?
A squall is a sudden, brief but intense burst of wind. The World Meteorological Organization (WMO) defines a squall formally as a wind event meeting all of:
- Sudden increase in wind speed by at least 16 knots (30 km/h)
- Sustained speed of at least 22 knots (40 km/h) at the peak
- Lasting at least 1 minute
A squall is not a steady gust; it has a clear onset and ends within roughly 1–60 minutes. Squalls usually arrive from thunderstorm downbursts — when cold dense air, cooled by evaporation of falling rain, accelerates downward and spreads horizontally upon hitting the ground. The leading edge of this cold-air outflow is the squall.
In South Asia, squalls are most dangerous during the pre-monsoon season (March-May), when they appear within violent thunderstorms locally called “Kal Baisakhi” (Bengal/Bihar) or “Nor’westers” (English meteorological term).
How squalls form
A typical squall sequence:
- Cumulonimbus develops through afternoon convection. Updrafts of 50-100 km/h carry moisture upward.
- Rain begins in the storm core. As raindrops fall, they evaporate partially, cooling the surrounding air.
- Cold downdraft forms — denser than surrounding air, it accelerates downward at speeds of 40-100 km/h.
- Downburst hits the ground — the cold air can’t penetrate deeper, so it spreads horizontally outward at high speed.
- Squall line at the gust front — the leading edge of this outflow can move at 60-150 km/h for minutes, then weakens.
- Cooler, drier air follows — temperature can drop 5-10°C within minutes.
The strongest squalls come from organized severe thunderstorms with structured downdrafts (multi-cells, supercells). Bangladesh has hosted some of the world’s most violent squalls during Nor’wester season.
Squalls in South Asia
Pre-monsoon “Kal Baisakhi” or “Nor’westers”:
The pre-monsoon thunderstorm season produces the region’s most violent squalls. Conditions are perfect:
- Bay of Bengal moisture flowing in at low levels
- Surface temperatures 38-44°C creating extreme instability
- Strong upper-level westerlies adding wind shear
- Surface temperatures cooling rapidly with downdraft
Worst-affected regions:
- West Bengal, Bangladesh, Bihar, Odisha — severe Nor’westers
- Assam, Meghalaya, Tripura — fewer but intense
- Eastern UP, MP, Jharkhand — moderate to severe
Famous Nor’wester events:
- April 2018 Kolkata-Howrah Nor’wester: 100+ km/h squalls killed 40+ people in a single evening
- May 2020 Bangladesh-Bengal: pre-monsoon storms with hail and squalls
- April 2023 Kolkata: severe Nor’wester with airline diversions
Monsoon squalls:
Even during the active monsoon, organized thunderstorms produce squalls. Less intense than pre-monsoon (only 60-90 km/h typically) but more frequent — daily in many regions.
Coastal sea squalls:
Off the Indian east coast and Bangladesh, sea squalls capsize fishing boats. Cyclonic systems forming in the Bay of Bengal produce squalls 200+ km from their centers.
Squall hazards
A typical strong squall (80-120 km/h) can:
- Uproot trees — fully grown trees with shallow roots fall first
- Damage roofs — corrugated metal sheets, thatch
- Collapse fragile structures — billboards, shanties, mud houses
- Capsize boats — especially small fishing craft caught at sea
- Down power lines — extended outages common
- Cause vehicle accidents — sudden lane wandering
- Aircraft turbulence — pilots avoid squall lines by 20+ km
The 2018 Kolkata-Howrah Nor’wester showed how a single squall can produce:
- 40+ deaths from collapsed structures, falling trees, drowning
- 8,000+ trees down
- 100,000+ homes damaged
- Disruption of metro, rail, road for 24+ hours
Squall line vs squall
A squall line is an organized line of thunderstorms — often hundreds of km long — with continuous strong winds along the leading edge. South Asia’s pre-monsoon storms sometimes organize into squall lines covering multiple districts.
A bow echo (radar term) is a squall line that bows outward, indicating particularly strong downbursts in the bow apex.
How to identify an approaching squall
Warning signs:
- Dark, threatening clouds approaching from west or northwest
- Sudden temperature drop by several degrees Celsius
- Strong odor of rain (petrichor) as wind picks up moisture
- Distant thunder within 10 km
- Wind shift — calm followed by sudden gust
- Lightning in the approaching storm
- Mammatus clouds beneath the anvil — indicate severe turbulence
When you see these signs, take shelter within 5-15 minutes — the squall typically follows that quickly.
Squall safety
Outdoors:
- Move indoors immediately
- Stay away from windows
- Don’t shelter under tall trees (lightning + fall risk)
- Avoid open fields (lightning + wind)
- Get out of boats and away from water
Inside:
- Close all windows and doors
- Move to interior room or basement
- Secure loose objects before storm arrives
- Don’t use plumbing (lightning risk)
- Don’t stand near windows
Vehicles:
- Pull over if visibility drops
- Stay inside (vehicle metal cage protects from lightning)
- Avoid trees and power lines
- Watch for fallen branches on road
Frequently asked questions
What’s the difference between a squall and a gust? A gust is a brief peak in wind speed (3-20 seconds typically). A squall is a sustained period of strong wind lasting at least 1 minute. Squalls are more dangerous because the prolonged force can topple structures that withstand brief gusts.
Why are Nor’westers so violent in Bengal? The combination of (1) Bay of Bengal moisture, (2) Tibetan Plateau pre-monsoon heating, (3) cool, dry westerly winds aloft, and (4) surface heating to 40°C+ creates extreme convective instability — sometimes CAPE values above 4,000 J/kg, comparable to severe US Midwest tornado outbreaks.
Can squalls produce tornadoes? Sometimes, especially in supercell-type storms. Bangladesh has the highest tornado death rate outside the US, with several major tornado events embedded in pre-monsoon squall systems (1989 Daulatpur-Saturia tornado killed 1,300+).
How long does a typical squall last? Most squalls last 5-30 minutes. Organized squall lines can produce extended periods of strong wind as multiple cells pass over a location.
Where can I check storm warnings for my city? Mausam Online displays precipitation forecasts and severe-weather codes (95-99 = thunderstorm) on every city page. IMD’s Sachet app provides severe weather warnings. See Kolkata, Patna, Bhopal, Dhaka.