Extra-tropical (temperate) cyclones

Detail

What an extra-tropical cyclone is

• An extra-tropical cyclone is a low-pressure system of the middle and high latitudes (roughly 35°–65° N and S).
• A cyclone is a low-pressure centre. Winds blow inwards and spiral around it.
• "Extra-tropical" means it forms outside the tropics.

• It is also called a temperate cyclone, mid-latitude cyclone, frontal cyclone, wave cyclone or depression.

• Depression is the name used for temperate cyclones in north-western Europe and North America. In the Class 11 hazards glossary it means an area of low pressure.

• It forms along the polar front.
• A front is the boundary zone where two air masses of different temperature and humidity meet.

• The polar front is the boundary between cold, dry polar air and warm, moist tropical air. It lies in the westerly belt.

• Other names include the "Norwegian cyclone model" and the "polar front theory" (Fig. 9.9, Class 11).

• The model came from the Bergen school in Norway, founded by the meteorologist Vilhelm Bjerknes [4].
• The key paper on its structure was published by Jakob Bjerknes in 1919 [6]. The full model was set out around 1920 by Bjerknes and colleagues (NCERT).

How it forms — the life cycle (Northern Hemisphere)

Stage 1 — Stationary front

• At first the front does not move. It is a stationary front: neither air mass pushes the other.
• Warm air lies to the south and cold air to the north.
• The two air masses blow parallel to the front in opposite directions.

Stage 2 — Wave or "kink" (the start of the cyclone)

• Pressure falls along the front → warm air pushes north and cold air pushes south.
• The straight front bends into a wave.
• An anticlockwise circulation begins. In the Southern Hemisphere it is clockwise, because the Coriolis force (the turning effect of the Earth's spin on moving air) acts the other way there.
• What sets off the wave: an upper-level low carried in the jet stream passes over the front. This puts a "kink" in it, and the stationary front splits into a cold front and a warm front [3].
• The jet stream is a narrow band of very fast westerly wind about 9–14 km up.

Stage 3 — Open wave, with a warm sector

• A warm front forms ahead (to the east) and a cold front forms behind (to the west).
• A warm front is where warm air advances and rises gently over cold air.

• A cold front is where cold air advances and pushes under warm air.

• Between them lies a wedge of warm air called the warm sector. It has cold air ahead and cold air behind.

• The fronts become sharper. The temperature contrast and the change in wind direction become clearer [3].

Weather ahead of the warm front

• Warm air glides up slowly over the cold air along a gentle slope.
• Rising air cools.

• Its water vapour condenses into layered clouds.

• Cloud sequence seen by a person on the ground as the front comes closer: cirrus → cirrostratus → altostratus → nimbostratus.

• Result: long, steady, light-to-moderate rain or drizzle over a wide belt, sometimes for many hours.

Weather in the warm sector

• Skies are fairly clear or there is low cloud.
• It is warmer and more humid.
• The wind shifts.

Weather at the cold front

• Cold, heavy air pushes the warm air up steeply from behind.
• Tall cumulus and cumulonimbus clouds form.
• Result: short, heavy showers, thunderstorms and gusty winds.
• After the front passes, the temperature drops and the sky clears.

Stage 4 — Occlusion (mature stage)

• The cold front moves faster than the warm front. It catches up with and overtakes it [3].
• The warm-sector air is lifted off the ground.
• The result is an occluded front. On weather maps it is drawn in purple [3].

Stage 5 — Dissipation (the cyclone dies)

• The warm, moist air at the surface is the cyclone's energy source.
• Occlusion cuts that air off from the centre, so the low slowly fills up and dies [3].

Stage	Key feature	Weather
1. Stationary front	Warm air to the south, cold air to the north	Calm, with some cloud along the front
2. Wave	Pressure falls; anticlockwise flow begins	Cloud starts to form
3. Open wave	Warm front, cold front and warm sector	Steady rain ahead; showers at the cold front
4. Occlusion	Cold front overtakes the warm front	Widespread cloud and rain
5. Dissipation	Warm air cut off at the surface	Low weakens and fills

Character

• Size: very large, often 1,000–2,000 km or more across. This is bigger than most tropical cyclones.
• Where they form: over both land and sea. Tropical cyclones form only over warm seas.
• Movement: west to east, steered by the westerlies (prevailing winds of the mid-latitudes, blowing from west to east) and the polar front jet stream.
• Season: they drive weather across the mid-latitudes mainly from autumn to spring [3]. They are strongest in winter, when the pole-to-equator temperature contrast is largest.
• Weather: changeable weather lasting several days — cloud, rain, snow and shifts in wind and temperature as each front passes.
• Where they are seen: the North Atlantic and north-western Europe (the British Isles, Norway), North America, the North Pacific, and the Southern Ocean belt around Antarctica.
• Wind speed: usually lower than in a tropical cyclone. Frontal systems are present; a tropical cyclone has no fronts.

Tropical vs extra-tropical cyclones

Feature	Extra-tropical (temperate)	Tropical
Latitude	35°–65° N/S, on the polar front	About 5°–30° N/S
Origin	Land and sea	Only over warm seas (above about 27 °C)
Energy	Temperature contrast between air masses	Latent heat from condensation of water vapour
Fronts	Clear warm and cold fronts	No fronts
Size	Larger (1,000–2,000 km+)	Smaller
Movement	West → east (westerlies)	East → west (trade winds), then curving poleward
Wind	Moderate	Very violent
Eye	None	A calm eye at the centre
Duration	Several days of changeable weather	Weakens fast after landfall
Indian example	Western disturbances	Fani (2019), Amphan (2020), Biparjoy (2023)

Western disturbances — India's extra-tropical lows

• Western disturbances (WDs) are extra-tropical low-pressure systems.
• They begin over the Mediterranean Sea region.
• They travel east across Iran, Afghanistan and Pakistan, carried by the subtropical westerly jet stream, and reach north-west India mainly in winter (December–February).
• Why they matter: about 30% of annual precipitation over north-west India (Jammu & Kashmir, Ladakh, Himachal Pradesh, Uttarakhand) falls in winter, mostly from WDs (MoES, 2021) [5].
• Benefits:
• Winter rain helps rabi wheat in Punjab, Haryana and western Uttar Pradesh.

• Snow builds up the Himalayan glaciers and stores water as snow [5].

• Hazards:

• Heavy snow, rain or hail can cause landslides and avalanches and damage crops and buildings.
• WDs also bring dense fog that disrupts air, rail and road traffic [5].

Bomb cyclone (beyond NCERT)

• A bomb cyclone is a mid-latitude cyclone that strengthens explosively.
• Bombogenesis is the process: the central pressure falls by at least 24 hPa (millibars) in 24 hours (Sanders and Gyakum, 1980).
• The threshold is adjusted for latitude:
• 24 hPa in 24 hours at 60° latitude [2].

• Only about 17.8 hPa in 24 hours at the latitude of New York City (about 41° N) [2].

• Cause:

• A cold air mass meets a warm air mass, for example air over warm ocean water such as the Gulf Stream [2].

• An active jet stream above removes air from the top of the storm. This deepens the low further.

• Where: off the US east coast (the Nor'easters) and in the north-west Pacific near Japan, mostly in winter.

• Effects:
• Strong winds and power cuts.
• Blizzards (snowstorms with strong wind), with whiteouts.
• Rain falling on snow, which can cause river floods [2].

Prelims Hooks

• The Norwegian (polar front) cyclone model came from the Bergen school (Vilhelm and Jakob Bjerknes, 1919–1920), not from a tropical meteorology school.
• In the Northern Hemisphere, circulation around a temperate cyclone is anticlockwise; in the Southern Hemisphere it is clockwise.
• The order of the fronts from east to west: warm front → warm sector → cold front.
• The cloud sequence ahead of a warm front: cirrus → cirrostratus → altostratus → nimbostratus. A cold front brings cumulonimbus.
• Occluded front: formed when the faster cold front overtakes the warm front. It marks the cyclone's final, dying stage.
• Temperate cyclones form over land and sea and move west to east. Tropical cyclones form only over sea and move east to west.
• Western disturbances start over the Mediterranean, are carried by the subtropical westerly jet and supply about 30% of north-west India's annual precipitation in winter (MoES, 2021).
• Trap: Fani (2019), Amphan (2020) and Biparjoy (2023) were tropical cyclones, not extra-tropical ones. India's main extra-tropical systems are western disturbances.
• Bomb cyclone = a pressure fall of ≥24 hPa in 24 hours at 60° latitude. The threshold is lower nearer the equator (about 17.8 hPa near New York).
• A temperate cyclone has fronts but no eye. A tropical cyclone has an eye but no fronts.

Mains Points

• How fronts make weather (GS-I):
• A temperate cyclone gets its energy from the temperature contrast along the polar front, not from warm seas.
• The warm front gives long, gentle rain; the cold front gives short, violent showers.

• This explains the changeable, rainy weather of north-western Europe, where depressions arrive about every few days on the westerlies.

• Western disturbances in India's economy (GS-I/III):

• Benefits: WD rain and snow support the rabi wheat crop, Himalayan glaciers and the snow-fed flow of the Indus and Ganga river systems.
• Hazards: extreme WDs bring hailstorms, avalanches, landslides and winter fog.

• Policy needs: better WD forecasts from IMD and avalanche and landslide warnings in the western Himalaya.

• Bomb cyclones and disaster management (GS-III):

• Rapid strengthening within 24 hours leaves little warning time.

• Better satellite monitoring and numerical forecasting are needed, as with the rapid intensification of tropical cyclones in the Arabian Sea and Bay of Bengal.

• Comparing cyclone types (GS-I): origin, energy source, fronts, size and direction of movement differ between tropical and temperate cyclones. These differences decide the hazards each brings: storm surge for tropical cyclones; snow, cold waves and long rain spells for temperate ones.