Landfall

Topic: Air Masses, Fronts, Cyclones and Thunderstorms · NCERT: Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"

Meaning

Landfall is the place and time at which the centre (eye) of a cyclone crosses the coast and moves from sea onto land. It matters because two things happen at landfall. The storm starts to weaken. But the coastal belt suffers its worst winds, rain and storm surge at that moment.

Explanation

What exactly "crosses" the coast

• Landfall is counted when the centre, the eye, reaches the coast. The edge of the storm reaching the coast does not count.
• Eye: the calm centre of the storm. It has sinking air, often a clear sky and the lowest pressure. It is usually 20–50 km across.
• Eye wall: a ring of tall cumulonimbus clouds (dark thunderstorm clouds) around the eye. It has the strongest winds, up to about 250 km/h or more, and the heaviest rain.

• Rain bands: spiral cloud bands that reach outward from the eye wall. Their winds and rain are about one-half to two-thirds as strong as those in the eye wall [2].

• The order in which the storm reaches a coastal town:

• First the rain bands arrive. They bring bursts of rain and gusts well before the centre comes. This is an early warning sign.
• Next the eye wall hits with the strongest winds.
• Then the eye passes. The wind suddenly drops and the sky may clear.
• Finally the other side of the eye wall arrives. The winds come back at full force from the opposite direction.

Why the storm weakens after landfall

• The fuel is cut off.
• A tropical cyclone gets its energy from latent heat (heat stored in water vapour and released when it turns back into liquid water) [2].
• That energy depends on warm sea water of usually 26.5 °C or warmer [2].

• Over land there is no warm sea, so the moisture supply is cut and no more latent heat is released.

• Friction slows the winds.

• Land is rough because of hills, trees and buildings.

• This rough surface creates more friction, which slows the winds.

• Result: the storm loses energy and slowly dies away ("peters out").

Why the coast still suffers the most

• At landfall the eye wall comes ashore with its peak wind and rain.
• Storm surge (an abnormal rise in sea level along the coast during a cyclone) comes with it.
• Strong winds blowing onto the shore push sea water towards the land.
• The very low pressure at the centre lets the sea surface bulge upward.

• Together these can raise the sea up to 10 m, as a dome of water 80–160 km wide [2].

• Storm surge is worst on shallow, funnel-shaped coasts, where the water piles up higher.

What decides where landfall happens

• Tropical cyclones form between 8° and 20° latitude. At this distance from the equator the Coriolis force (the apparent turning of moving air caused by the Earth's rotation) is strong enough to make the winds spin [2].
• They move slowly, about 300–500 km a day. The trade winds (steady easterly winds of the tropics) carry them east to west.
• Recurvature: cyclones that cross about 20° N generally turn north-east when they reach the westerlies (winds blowing from west to east in the middle latitudes). These storms are more destructive.
• Because of recurvature, many Bay of Bengal storms make landfall on Odisha, West Bengal, Bangladesh and Myanmar.

Where it occurs

• Odisha coast (Bay of Bengal): Fani (2019) crossed the Odisha coast.
• West Bengal–Bangladesh coast, near the Sundarbans: Amphan (2020) 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, bringing storm surge and flooding [3].
• Kutch, Gujarat (Arabian Sea): Biparjoy (2023) struck here.
• Head of the Bay of Bengal: this coast is shallow and narrows like a funnel, so landfalls here bring very high storm surge.

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

• India in general: the east coast sees landfall more often than the west, because Bay of Bengal storms are more frequent and recurve towards it.

• Season in India: pre-monsoon (April–June) and post-monsoon (October–December).

Don't confuse with

• Storm surge: landfall is an event, the centre crossing the coast. Storm surge is a rise in sea level pushed onto the coast by onshore winds and low pressure. Landfall can bring a surge, but they are not the same thing.
• Passage of the eye (the lull): the calm while the eye passes over a place is not the end of the storm. The second half of the eye wall follows, with winds from the opposite direction.
• Recurvature: this is a change in the storm's track (turning north-east near about 20° N). It takes place over the sea. Landfall is the moment the storm reaches the coast.
• Extra-tropical cyclone: this kind of storm can form over land or sea and gets its energy from the temperature contrast between air masses. So moving over land does not cut off its fuel the way landfall does for a tropical cyclone.

Prelims Hooks

• Landfall is defined by the centre (eye) crossing the coast, not by the first rain bands reaching it.
• After landfall a tropical cyclone weakens for two reasons: its moisture/latent heat supply is cut and friction increases.
• At landfall, the strongest winds come from the eye wall, not the eye. The eye has the lowest pressure and sinking air.
• Match the cyclone to its landfall area: Fani (2019) – Odisha; Amphan (2020) – West Bengal/Bangladesh, near the Sundarbans [3]; Biparjoy (2023) – Kutch, Gujarat (Arabian Sea).
• Storm surge at landfall can reach up to 10 m [2]. It is worst on shallow, funnel-shaped coasts such as the head of the Bay of Bengal.
• Trap: "A tropical cyclone strengthens after landfall because of land heating." This is wrong. Tropical cyclones form only over warm seas and die over land.

Mains Points

• The landfall paradox (GS-III, disaster management):
• The storm starts to weaken at landfall.
• Yet the narrow coastal strip takes the peak wind, rain and storm surge of up to 10 m [2].

• So planning should focus on this belt: early warning, evacuation to cyclone shelters, mangrove belts such as the Sundarbans, and coastal embankments.

• Why the east coast suffers more (GS-I/III):

• Bay of Bengal storms are more frequent.
• They recurve towards Odisha, West Bengal and Bangladesh.
• They make landfall on a shallow, funnel-shaped head-bay that magnifies storm surge (Bhola 1970; Amphan 2020 [3]).

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

• Public awareness saves lives:

• When the eye passes during landfall, people think the storm is over and leave their shelters too early.
• Teaching that the winds come back from the opposite direction is a simple, low-cost safety step.

Related concepts

• Eye of the cyclone
• Eye wall
• Rain bands
• Recurvature of cyclones

Read more

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