Extratropical cyclone

Also called: Temperate cyclone, Mid-latitude cyclone, Wave cyclone · Topic: Air Masses, Fronts, Cyclones and Thunderstorms · NCERT: Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"; Class 11, Ch 10 "Water in the Atmosphere"

Meaning

An extra-tropical cyclone (also called a temperate cyclone, mid-latitude cyclone, frontal cyclone, wave cyclone or depression) is a low-pressure system of the middle and high latitudes (roughly 35°–65° N and S). It forms along the polar front and has a warm front, a cold front and a warm sector between them.

• It brings the changeable, rainy weather of the mid-latitudes, such as north-western Europe and North America.
• In India it comes as western disturbances, which bring winter rain and snow to the north-west.

Explanation

Basic terms

• A cyclone is a low-pressure centre. Winds blow inwards and spiral around it.
• "Extra-tropical" means it forms outside the tropics.
• 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.
• Depression is the name used for temperate cyclones in north-western Europe and North America.
• Source of energy: the temperature contrast between the two air masses. It does not come from warm seas.
• The Norwegian cyclone model, or polar front theory (Fig. 9.9, Class 11), explains how these cyclones form.
• 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).

Life cycle (Northern Hemisphere)

• Stage 1: Stationary front
• The front does not move. 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"

• An upper-level low carried in the jet stream (a narrow band of very fast westerly wind about 9–14 km up) passes over the front. This puts a "kink" in it, and the stationary front splits into a cold front and a warm front [3].
• Pressure falls along the front. Warm air pushes north and cold air pushes south, so the straight front bends into a wave.

• An anticlockwise circulation begins. In the Southern Hemisphere it is clockwise. This is because the Coriolis force (the turning effect of the Earth's spin on moving air) acts the other way there.

• Stage 3: Open wave, with a warm sector

• A warm front forms ahead (to the east). Here warm air advances and rises gently over cold air.
• A cold front forms behind (to the west). Here cold air advances and pushes under warm air.
• Between them lies a wedge of warm air, the warm sector, with cold air ahead of it and cold air behind it.

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

• Stage 4: Occlusion (mature stage)

• The cold front moves faster than the warm front, so it catches up with and overtakes it [3].
• The warm-sector air is lifted off the ground.

• This forms an occluded front, drawn in purple on weather maps [3].

• Stage 5: Dissipation

• 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].

Weather as the fronts pass

• Ahead of the warm front:
• Warm air glides slowly up a gentle slope over the cold air.
• The rising air cools, and its water vapour condenses into layered clouds.
• As the front comes closer, the clouds change in this order: cirrus → cirrostratus → altostratus → nimbostratus.

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

• In the warm sector:

• Skies are fairly clear or have low cloud.

• It is warmer and more humid, and the wind shifts.

• At the cold front:

• Cold, heavy air pushes the warm air up steeply.
• 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.

Main features

• Size: very large, often 1,000–2,000 km or more across. This is bigger than most tropical cyclones.
• Where it forms: over both land and sea.
• Movement: west to east. It is steered by the westerlies (the main winds of the mid-latitudes, which blow from west to east) and the polar front jet stream.
• Season: it drives mid-latitude weather mainly from autumn to spring [3]. It is strongest in winter, when the temperature difference between the pole and the equator is largest.
• Wind: usually gentler than in a tropical cyclone. It has fronts but no eye.
• Bomb cyclone (beyond NCERT): a mid-latitude cyclone that strengthens explosively. This process is called bombogenesis.
• The central pressure falls by at least 24 hPa in 24 hours (Sanders and Gyakum, 1980).
• The threshold changes with latitude: 24 hPa in 24 hours at 60°, but only about 17.8 hPa near New York City (about 41° N) [2].
• Cause: a cold air mass meets a warm air mass, for example air over the warm Gulf Stream [2]. An active jet stream above removes air from the top of the storm, so the low deepens further.

Where it occurs

• World:
• The North Atlantic and north-western Europe (the British Isles, Norway)
• North America
• The North Pacific

• The Southern Ocean belt around Antarctica

• Bomb cyclones: off the US east coast, where they are called Nor'easters, and in the north-west Pacific near Japan, mostly in winter.

• India: western disturbances (WDs)
• WDs begin over the Mediterranean Sea region.
• They travel east across Iran, Afghanistan and Pakistan, carried by the subtropical westerly jet stream.
• They reach north-west India mainly in winter (December–February).
• About 30% of annual precipitation over Jammu & Kashmir, Ladakh, Himachal Pradesh and Uttarakhand falls in winter, mostly from WDs (MoES, 2021) [5].
• Their winter rain helps the rabi wheat crop in Punjab, Haryana and western Uttar Pradesh. Their snow builds up the Himalayan glaciers [5].

Don't confuse with

• Tropical cyclone: forms only over warm seas (above about 27 °C) at about 5°–30° N/S. It runs on latent heat (heat released when water vapour condenses), moves east to west, has a calm eye and has no fronts. Fani (2019), Amphan (2020) and Biparjoy (2023) were tropical cyclones.
• Anticyclone: a high-pressure centre, the opposite of a cyclone. A cyclone is a low-pressure centre with winds spiralling inwards.
• Occluded front vs cold front: an occluded front forms only when the faster cold front overtakes the warm front. It marks the cyclone's dying stage, not its start.
• Depression (Class 11 hazards glossary): there it just means an area of low pressure. In north-western Europe and North America, "depression" is the name for a temperate cyclone.

Prelims Hooks

• The Norwegian (polar front) model came from the Bergen school (Vilhelm and Jakob Bjerknes, 1919–1920). It did not come from a tropical meteorology school.
• Order of fronts from east to west: warm front → warm sector → cold front. The cloud order ahead of the warm front is cirrus → cirrostratus → altostratus → nimbostratus. The cold front brings cumulonimbus.
• Direction of circulation: anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Temperate cyclones form over land and sea and move west to east.
• Trap: a temperate cyclone has fronts but no eye. A tropical cyclone has an eye but no fronts.
• Western disturbances: they start over the Mediterranean, are carried by the subtropical westerly jet and give about 30% of north-west India's annual precipitation in winter (MoES, 2021) [5].
• Bomb cyclone: the central pressure falls by ≥24 hPa in 24 hours at 60° latitude. The threshold is lower nearer the equator, about 17.8 hPa near New York [2].

Mains Points

• How fronts make weather (GS-I):
• The temperature contrast along the polar front powers the cyclone.
• 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 a new depression arrives on the westerlies every few days.

• Western disturbances and India (GS-I/III):

• Benefits: WD rain and snow support rabi wheat, the Himalayan glaciers and the snow-fed flow of the Indus and Ganga river systems [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].

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

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

• These storms strengthen very fast, within 24 hours, so there is little time to warn people.
• They bring strong winds, power cuts, blizzards (snowstorms with strong wind) and rain falling on snow that can cause river floods [2].
• Better satellite monitoring and computer weather forecasting (numerical forecasting) are needed. The same is true for the rapid intensification of tropical cyclones in the Arabian Sea and Bay of Bengal.

Related concepts

• Bomb cyclone

Read more

• Extra-tropical (temperate) cyclones