Structure, movement and landfall; tropical vs extra-tropical cyclones

Detail

A. Structure of a mature tropical cyclone (NCERT Fig. 9.10, after Rama Sastry)

A mature tropical cyclone has three main parts, listed from the centre outward: the eye, the eye wall and the rain bands.

Part	What it is	Air movement	Weather
Eye	Calm centre	Air sinks	Light winds, often clear sky, lowest pressure
Eye wall	Ring of tall cumulonimbus clouds around the eye	Moist air spirals upward to the tropopause	Strongest winds, heaviest rain
Rain bands	Spiral cloud bands reaching outward from the eye wall	Air rises in bands	Bursts of rain and gusty winds, before the centre arrives

1. Eye of the cyclone

• Eye of the cyclone: the calm centre of the storm. The air here sinks, the sky is often cloud-free, and pressure is at its lowest (Class 7, Fig. 3.14).
• Size: usually 20–50 km across. Satellite studies give a range of 10–65 km in diameter [2].
• Why it is clear: sinking air warms as it is squeezed (compressed), so clouds cannot form.
• Exam trap: NCERT's figure of "150–250 km" is the size of the whole circulating system. It is not the size of the eye.
• The deceptive lull:
• As the eye passes over a place, the wind suddenly drops and the sky may clear.
• People think the storm is over and come out of shelter.
• Then the other side of the eye wall arrives, and the winds return with full force from the opposite direction.

2. Eye wall

• Eye wall: a ring of towering cumulonimbus clouds (tall, dark thunderstorm clouds) around the eye.
• Why it has the strongest winds and heaviest rain (NCERT Ch 9, exercise 3(iii)):
• Warm, moist air spirals inward over the sea and rises steeply in the eye wall, up to the tropopause (the top of the lowest layer of the atmosphere).
• As this air rises, it cools and its water vapour condenses. This releases a great deal of latent heat (heat stored in water vapour and set free when it turns back into liquid water).

• This heat makes the air rise faster still. The pressure difference across the eye wall is very steep, so the winds here are the strongest, up to about 250 km/h or more, with torrential rain.

• Category 5 hurricanes, the top class on the US Saffir–Simpson scale, have sustained winds above 249 km/h [2].

3. Rain bands

• Rain bands: curved bands of cumulus and cumulonimbus clouds that spiral outward from the eye wall.
• They bring bursts of heavy rain and gusts well before the centre arrives. This is an early warning sign on the coast.
• Rain-band winds and rain are roughly one-half to two-thirds as strong as those in the eye wall [2].

B. Size and movement

• Size: over the Bay of Bengal, Arabian Sea and Indian Ocean, a cyclone is about 600–1,200 km across.
• Speed: cyclones move slowly, about 300–500 km a day.
• Where they form:
• Latitude: cyclones generally form between 8° and 20° latitude. They need to be at least about 8° away from the equator, where the Coriolis force is strong enough to set the winds spinning [2].
• Coriolis force: the apparent turning of moving air caused by the Earth's rotation. It turns winds to the right in the Northern Hemisphere and is zero at the equator.

• Sea temperature: the sea surface must usually be 26.5 °C or warmer [2].

• Normal track: tropical cyclones are carried east to west by the trade winds (steady easterly winds of the tropics).

C. Landfall

• Landfall: the place and time at which the centre (eye) of a cyclone crosses the coast.
• Why the storm weakens after landfall:
• Over land there is no warm sea, so the storm's moisture supply is cut and no more latent heat is released.
• The land surface is rough, so friction increases and slows the winds.

• The storm loses energy and "peters out" (slowly dies away).

• But the coast suffers the most: the coastal belt takes the worst winds, rain and storm surge at landfall.

• Recent Indian examples:
• Fani (2019) crossed the Odisha coast.
• Amphan (2020) was a Bay of Bengal storm that intensified rapidly (gained strength very fast) into a Super Cyclonic Storm. It was forecast to make landfall on 20 May 2020 as an Extremely Severe Cyclonic Storm on the coasts of West Bengal (India) and Bangladesh, near the Sundarbans, bringing storm surge and flooding [3].
• Biparjoy (2023) was an Arabian Sea storm that struck Kutch, Gujarat.

D. Storm surge (NCERT Ch 6, "Consequences of Tropical Cyclones")

• Storm surge: an abnormal rise in sea level along the coast during a cyclone.
• How it happens:
• Very strong winds blowing onto the shore push sea water towards the land.
• The very low pressure at the centre also lets the sea surface bulge upward.

• Together, these raise the sea level and flood low coasts, fields and settlements.

• How big: the sea can rise by up to 10 m, as a dome of water 80–160 km wide [2].

• Where it is worst: on shallow, funnel-shaped coasts.
• The head of the Bay of Bengal (West Bengal–Bangladesh coast) is shallow and narrows like a funnel, so the water piles up higher.

• The 1970 Bhola cyclone in East Pakistan (now Bangladesh) killed several hundred thousand people, mostly through storm surge.

• Effects: drowning, salt water spoiling farmland, damaged embankments, and loss of homes and fishing boats.

E. Recurvature of cyclones

• Recurvature of cyclones: a cyclone first moves westward and poleward with the trade winds. Then it turns north-east when it reaches the belt of the westerlies (winds blowing from west to east in the middle latitudes).
• NCERT point: cyclones that cross about 20° N generally recurve, and they are more destructive.
• Indian pattern: many Bay of Bengal storms recurve towards Odisha, West Bengal, Bangladesh and Myanmar.
• Result: the track looks like a parabola. Instead of steadily weakening westward, the storm swings back towards the north-eastern coasts.

F. Tropical vs extra-tropical cyclones

• Extra-tropical cyclone: a low-pressure system of the middle and high latitudes (roughly 35°–65°). It forms along a front, the boundary where a warm air mass meets a cold air mass.
• Tropical cyclone: an intense low-pressure system that forms over warm tropical seas and has no fronts.

Feature	Extra-tropical cyclone	Tropical cyclone
Fronts	Clear frontal system (warm and cold fronts)	None
Area covered	Larger	Smaller (about 600–1,200 km in Indian seas)
Origin	Over land or sea	Only over warm seas (≥ 26.5 °C [2]); dies over land
Winds	Weaker	Much stronger (eye wall up to about 250 km/h), more destructive
Movement	West to east, with the westerlies	East to west, with the trades, then recurves north-east
Energy source	Temperature contrast between air masses	Latent heat of condensation [2]
Eye	Absent	Present (calm centre)
Latitudes	Middle and high latitudes	About 8°–20° N and S [2]
Season	Mostly winter (strong temperature contrast)	Late summer and autumn; in India, pre-monsoon (April–June) and post-monsoon (October–December)
Pressure pattern	Elliptical isobars	Nearly circular isobars, closely spaced
Example region	North Atlantic, Europe; Western Disturbances in north-west India	Bay of Bengal, Arabian Sea (Fani, Amphan, Biparjoy)

Prelims Hooks

• The eye is the calm part with the lowest pressure and sinking air. The strongest winds are in the eye wall, not in the eye.
• The eye is usually 20–50 km across. The "150–250 km" figure is the size of the whole circulating system (a common trap).
• From the centre outward: eye → eye wall → rain bands.
• Landfall means the centre of the cyclone crosses the coast. After landfall, the storm weakens because its moisture supply is cut and friction increases.
• Storm surge is caused by strong onshore winds plus low pressure. It is worst on shallow, funnel-shaped coasts such as the head of the Bay of Bengal.
• Cyclones crossing about 20° N generally recurve north-east and are more destructive.
• Tropical cyclones do not form within about 8° of the equator, because the Coriolis force is too weak there [2].
• Only tropical cyclones have an eye. Only extra-tropical cyclones have fronts.
• Energy source: latent heat for tropical cyclones; temperature contrast between air masses for extra-tropical cyclones.
• Movement: tropical cyclones move east to west; extra-tropical cyclones move west to east.

Mains Points

• Why the east coast suffers more (GS-I/III): Bay of Bengal storms are more frequent, recurve towards Odisha, West Bengal and Bangladesh, and meet a shallow, funnel-shaped head-bay, which magnifies storm surge (Bhola 1970; Amphan 2020 [3]).
• Landfall paradox: the storm starts weakening at landfall, yet the coastal strip takes the peak wind, rain and surge, of up to 10 m [2]. Disaster planning should therefore focus on a narrow coastal belt: early warning, evacuation to cyclone shelters, mangrove belts such as the Sundarbans, and coastal embankments.
• Structure and safety: the lull while the eye passes leads people to come out too early. Public awareness that winds return from the opposite direction saves lives.
• Comparison use: contrasting energy source, fronts, movement and the eye explains why tropical cyclones are smaller but far more destructive than extra-tropical cyclones such as the Western Disturbances.