Cyclones & Anticyclones – NDA Geography Notes

Introduction

Among all weather phenomena, cyclones are the most dramatic and the most destructive. A tropical cyclone can release more energy in a single day than all the nuclear weapons ever built. A temperate cyclone can bring weeks of unsettled weather to an entire continent. Understanding what a cyclone is — how it forms, how it moves, and how it differs from an anticyclone — is essential both for NDA preparation and for understanding how the atmosphere works.

This chapter explains both tropical and temperate cyclones in detail, the nature of anticyclones, and the specific terminology that NDA tests. The Coriolis Effect — which we covered in Chapter 14 — plays a central role here, so keeping that concept clear will make this chapter much easier.

Understand the Topic

What is a Cyclone?

A cyclone is a large system of winds rotating around a centre of low pressure. Air flows inward toward the low pressure centre and rises.

The word cyclone comes from the Greek word meaning coil or circle — describing the circular rotation of winds.

Direction of rotation:
In the Northern Hemisphere, cyclones rotate anticlockwise (counterclockwise) — because the Coriolis Effect deflects air to the right as it flows toward the low centre, creating a counterclockwise spin.

In the Southern Hemisphere, cyclones rotate clockwise — because the Coriolis Effect deflects air to the left.

This is one of the most tested facts from this chapter. [NDA 2009-I | NDA 2014-II | NDA tested via CDS]

Air movement in a cyclone:
Air flows inward and upward. Convergence at the surface forces air upward — creating clouds and precipitation. This is why cyclones bring cloudy and rainy weather.

---

What is an Anticyclone?

An anticyclone is a large system of winds rotating around a centre of high pressure. Air sinks and flows outward from the high pressure centre.

Direction of rotation:
In the Northern Hemisphere, anticyclones rotate clockwise.
In the Southern Hemisphere, anticyclones rotate anticlockwise.

This is exactly the opposite of cyclone rotation in each hemisphere. [NDA tested via CDS]

Weather associated with anticyclones:
Anticyclones bring calm, clear, and stable weather — because sinking air suppresses cloud formation and precipitation. In summer, anticyclones bring heat waves. In winter, they bring clear, cold, frosty weather.

yes

NDA · GEOGRAPHY
World Geography — Chapter 15
Local Winds, Air Masses & Fronts

---

Introduction

The planetary winds of Chapter 14 govern the broad patterns of global circulation. But at the local and regional scale, a different set of winds operates — shaped by local temperature differences, terrain, and seasonal pressure changes. These are local winds — predictable, named, and often having dramatic effects on the weather and people of the regions where they blow.

This chapter also introduces air masses — large bodies of air with uniform temperature and humidity — and fronts — the boundaries between contrasting air masses where most of the world’s significant weather events occur.

NDA tests this chapter primarily through local wind identification and matching questions. The names of local winds, their regions, and their characteristics — hot or cold, dry or moist — are the most reliably tested facts.

---

Air Masses

An air mass is a large body of air that has roughly uniform temperature and humidity throughout its horizontal extent. Air masses form when air stays over a region long enough to take on the temperature and moisture characteristics of that surface.

Source regions are the areas where air masses form. A good source region must be:

• Large and flat
• Have uniform surface characteristics (all land, all ocean, all ice)
• Have calm or gentle winds so the air stays in place long enough to be modified

The four main types of air mass based on source region:

Maritime Polar (mP) — forms over cold ocean surfaces at high latitudes. Cold and moist.

Continental Polar (cP) — forms over cold land surfaces at high latitudes (Siberia, Canada in winter). Cold and dry.

Maritime Tropical (mT) — forms over warm ocean surfaces in the tropics. Warm and moist.

Continental Tropical (cT) — forms over warm, dry land surfaces in the tropics (Sahara, Arabian Desert). Hot and dry. [NDA tested via CDS]

---

Fronts

A front is the boundary between two air masses of different temperature and humidity. When a cold air mass meets a warm air mass, they do not mix easily. Instead, they form a sharp boundary — the front. Most precipitation and severe weather occurs along fronts.

The science of fronts was developed largely by Norwegian meteorologists in the early 20th century — sometimes called the Bergen School. [NDA tested via CDS]

Types of fronts:

Cold Front:
A cold air mass pushes into and under a warm air mass. The cold air is denser and acts like a wedge — forcing the warm air steeply upward. This rapid uplift causes:

• Cumulonimbus clouds (tall thunderstorm clouds)
• Heavy rainfall in a narrow belt
• Short, intense precipitation followed by clearing
• Rapid temperature drop after the front passes

Cold fronts move faster than warm fronts. [NDA tested via CDS]

Warm Front:
A warm air mass advances and slides gently over a retreating cold air mass. The warm air rises gradually — creating:

• Widespread, layered clouds (stratus, nimbostratus)
• Steady, light to moderate rainfall over a wide area
• Gradual warming after the front passes

Occluded Front:
When a cold front catches up with a warm front and lifts the warm air completely off the surface. The surface is then occupied by cold air on both sides with the warm air aloft. [NDA tested via CDS]

Stationary Front:
When neither air mass is advancing — the front remains in the same position for days. Produces persistent cloud and light rain. [NDA tested via CDS]

---

Local Winds

Local winds are winds that are specific to a particular region — caused by local temperature differences, terrain effects, or seasonal pressure patterns rather than global pressure belts.

NDA tests local winds heavily through match-list questions. The key information for each wind is:

1. The name
2. The region where it blows
3. Whether it is hot or cold
4. Whether it is dry or moist

---

Hot and Dry Local Winds

Foehn (Föhn):
A warm, dry wind that blows down the northern slopes of the Alps in Europe. It is a classic example of an orographic (mountain-related) wind.

How it forms: Moist air from the Mediterranean rises on the southern slopes of the Alps — cooling and losing moisture as rain. When it descends on the northern side, it warms up rapidly (dry adiabatic rate is faster than moist adiabatic rate). The result is a warm, dry wind on the lee side. This process — warming and drying on the downwind side — is called the Foehn effect and applies to all similar winds worldwide. [NDA tested via CDS]

Chinook:
A warm, dry wind that blows down the eastern slopes of the Rocky Mountains in the USA and Canada. It is the North American equivalent of the Foehn. Its name in Arapaho language means “snow eater” — because it can melt large amounts of snow very quickly. Temperature can rise by 20–30°C in just a few hours when a Chinook arrives. [NDA tested via CDS]

Chinook is a local wind occurring in the USA. [NDA tested via CDS]

Sirocco:
A hot, dry (or sometimes moist) wind that originates in the Sahara Desert and blows northward across the Mediterranean Sea to southern Europe (Italy, Spain). As it crosses the Mediterranean, it picks up moisture and arrives as a hot, humid, oppressive wind. In North Africa it is hot and dry — but by the time it reaches Europe it is warm and moist. [NDA tested via CDS]

Sirocco is the hot wind blowing from Africa toward the Mediterranean coast. [NDA 2007-II | NDA tested via CDS]

Khamsin:
A hot, dry, dusty wind that blows from the Sahara Desert across Egypt and the eastern Mediterranean. It typically blows for about 50 days between March and May (the Arabic word khamsin means fifty). Similar to the Sirocco in origin. [NDA tested via CDS]

Harmattan:
A dry, dusty wind that blows from the Sahara Desert westward and southwestward across West Africa toward the Gulf of Guinea coast. It blows during the winter months (November to March). It carries enormous amounts of dust and dries out vegetation. In the coastal areas of West Africa, it is called the “doctor” because it brings some relief from the humid, oppressive conditions there. [NDA 2007-II | NDA 2011-II | NDA tested via CDS]

Harmattan is the dry, dusty wind blowing from the Sahara Desert toward West Africa. [NDA 2011-II]

Loo:
A hot, dry, and oppressive wind that blows from the west and northwest across the Indo-Gangetic Plain (northern India and Pakistan) during the hot season — May and June. It is a severe heat wind that can cause heatstroke and death if a person is exposed to it for extended periods. [NDA tested via CDS]

Santa Ana:
A hot, dry wind that blows from the high desert interior of California and Nevada toward the Pacific coast of southern California. It occurs in autumn and is notorious for spreading wildfires. [NDA tested via CDS]

Simoom (Samoom):
A hot, dry, suffocating wind that blows across the Arabian Peninsula and the Sahara. It carries large amounts of dust and sand. [NDA tested via CDS]

Brickfielder:
A hot, dry wind that blows from the interior of Australia toward the southeastern coast. [NDA tested via CDS]

---

Cold Local Winds

Mistral:
A cold, dry, strong wind that blows from the north or northwest down the Rhône Valley in southern France toward the Mediterranean. It blows particularly in winter and spring. It can be very violent and has been known to overturn vehicles and damage crops. [NDA 2007-II | NDA tested via CDS]

Mistral is a cold wind blowing in France. [NDA 2007-II]

Bora:
A cold, dry, powerful wind that blows from the northeast down into the Adriatic Sea — affecting the coastal areas of Croatia, Slovenia, and northeastern Italy (particularly the area around Trieste). It blows from the cold continental interior outward to the relatively warm Adriatic. [NDA tested via CDS]

Blizzard:
A severe winter storm with very cold temperatures, strong winds, and blowing snow — reducing visibility to nearly zero. Associated with polar regions — particularly with Antarctica and the northern interior of North America and Russia. [NDA tested via CDS]

Buran (Purga):
A cold, violent blizzard wind that blows across Central Asia and Siberia (Russia and the surrounding former Soviet republics). In Siberia it is called Purga. [NDA tested via CDS]

Pampero:
A cold, violent wind that blows from the southwest across the Pampas grasslands of Argentina — originating from the cold southern latitudes. [NDA tested via CDS]

Lavante (Levante):
A cold, moist wind that blows from the east across the Strait of Gibraltar — affecting southern Spain. [NDA tested via CDS]

Nor’wester:
In New Zealand, the nor’wester is a warm, dry Foehn-type wind that blows across the Canterbury Plains from the northwest — descending from the Southern Alps. In India (West Bengal and Bangladesh), a nor’wester is a violent pre-monsoon thunderstorm.

---

Sea Breeze and Land Breeze

Sea breezes and land breezes are examples of diurnal (daily) local winds — they change direction twice a day, driven by the different heating and cooling rates of land and water.

Sea Breeze (day):
During the day, land heats up faster than the adjacent sea. Air over land becomes warm, expands, and rises — creating low pressure. Cooler, denser air from the sea flows in to replace it. Wind blows from sea to land — this is the sea breeze.

Sea breezes are common in the afternoon — when the temperature difference between land and sea is greatest. [NDA tested via CDS]

Land Breeze (night):
At night, land cools faster than the sea. Air over land becomes cooler and denser — creating high pressure. Air from the warmer sea surface is still relatively warm and forms low pressure. Wind blows from land to sea — this is the land breeze.

Land breezes are common in the early morning hours. [NDA tested via CDS]

Why does land heat and cool faster than water?

Land has a lower specific heat capacity than water. Water can absorb large amounts of heat with little temperature change. Land heats up quickly but also cools quickly. Ocean moderates temperature — coastal areas have smaller temperature ranges than continental interiors.

---

Mountain Breeze and Valley Breeze

Similar to sea and land breezes but driven by terrain height differences.

Valley Breeze (day):
During the day, mountain slopes heat up faster than the valley floor at the same elevation. Air on slopes warms, rises, and flows up the slope. Wind blows from the valley upward along the slope — this is the valley breeze (or anabatic wind). [NDA tested via CDS]

Mountain Breeze (night):
At night, mountain slopes cool rapidly by radiation. This cold, dense air flows downward along the slopes into the valley — this is the mountain breeze (or katabatic wind). [NDA tested via CDS]

Katabatic wind = cold air flowing down a slope under gravity.

The term katabatic is sometimes applied to larger cold outflow winds as well — like the cold outflow from the Antarctic ice sheet that creates strong winds around Antarctica.

---

Match List — Local Winds (Most Tested Format)

Wind	Region	Type
Foehn	Alps, Europe	Warm, dry (lee side)
Chinook	Rocky Mountains, USA/Canada	Warm, dry
Sirocco	Sahara → Mediterranean	Hot, dry/moist
Khamsin	Sahara → Egypt	Hot, dry, dusty
Harmattan	Sahara → West Africa	Hot, dry, dusty
Loo	North India/Pakistan	Hot, dry
Santa Ana	Southern California, USA	Hot, dry
Simoom	Arabian Peninsula/Sahara	Hot, dry, dusty
Brickfielder	Interior → SE Australia	Hot, dry
Mistral	Rhône Valley → S. France	Cold, dry, strong
Bora	NE → Adriatic Sea/Croatia	Cold, dry
Blizzard	Polar/sub-polar regions	Cold, snowy
Buran/Purga	Central Asia/Siberia	Cold, violent
Pampero	SW → Pampas, Argentina	Cold

---

Previous Year Questions (NDA)

---

Question 1 [NDA 2007-II]
Match List I (Local Winds) with List II (Regions):
A. Mistral — 1. USA
B. Chinook — 2. West Africa
C. Harmattan — 3. Mediterranean
D. Sirocco — 4. France

(a) A-4; B-1; C-2; D-3
(b) A-3; B-4; C-1; D-2
(c) A-1; B-3; C-4; D-2
(d) A-2; B-4; C-3; D-1

Answer: (a) A-4, B-1, C-2, D-3
Mistral is the cold, dry wind of France (4) — blowing down the Rhône Valley to the Mediterranean. Chinook is the warm, dry wind of the USA (1) — descending the eastern slopes of the Rocky Mountains. Harmattan is the hot, dusty wind of West Africa (2) — blowing from the Sahara. Sirocco blows from the Sahara across the Mediterranean (3).

---

Question 2 [NDA 2011-II]
Which one of the following is a hot, dry wind blowing from the Sahara Desert?
(a) Mistral
(b) Harmattan
(c) Chinook
(d) Foehn

Answer: (b) Harmattan
The Harmattan is a hot, dry, dusty wind that blows from the Sahara Desert westward and southwestward across West Africa toward the Gulf of Guinea. Mistral is a cold wind from France. Chinook is a warm wind from the Rocky Mountains in the USA. Foehn is a warm, dry wind from the Alps.

---

Practice Question 1 (Important for NDA)
Match the following local winds with their characteristics:
A. Chinook — 1. Hot, dry; Sahara to West Africa
B. Mistral — 2. Warm, dry; eastern Rockies; “snow eater”
C. Harmattan — 3. Cold, dry; Rhône Valley, France
D. Loo — 4. Hot, dry; North India/Pakistan in summer

(a) A-2, B-3, C-1, D-4
(b) A-1, B-2, C-3, D-4
(c) A-3, B-1, C-4, D-2
(d) A-4, B-3, C-2, D-1

Answer: (a) A-2, B-3, C-1, D-4
Chinook (A-2) = warm, dry wind down the eastern Rockies; melts snow rapidly. Mistral (B-3) = cold, dry, strong wind down the Rhône Valley. Harmattan (C-1) = hot, dry, dusty wind from Sahara to West Africa. Loo (D-4) = hot, dry, oppressive wind over north India and Pakistan in May-June.

---

Practice Question 2 (Important for NDA)
Which one of the following pairs of local wind and its region of occurrence is NOT correctly matched?
(a) Chinook — Eastern slopes of Rocky Mountains, USA/Canada
(b) Mistral — Rhône Valley, France
(c) Sirocco — Sahara, blowing toward Mediterranean
(d) Bora — West Africa, blowing from the interior

Answer: (d) Bora — West Africa
Bora is NOT associated with West Africa. Bora is a cold, dry, powerful northeast wind blowing from the cold continental interior down to the Adriatic Sea — affecting Croatia, Slovenia, and northeastern Italy. The West African hot wind from the Sahara is the Harmattan.

---

Practice Question 3 (Important for NDA)
Statement I: Sea breeze blows from the sea to the land during the day.
Statement II: Land heats up more rapidly than the sea during the day.

(a) Both true and II explains I
(b) Both true but II does not explain I
(c) I true but II false
(d) I false but II true

Answer: (a) Both true and II explains I
Statement I is correct — sea breeze blows from sea to land during the day. Statement II correctly explains why — land heats up faster than sea during the day (lower specific heat capacity of land). The heated air over land rises, creating low pressure, and cooler sea air flows in to replace it. Statement II is the physical reason for Statement I.

---

Practice Question 4 (Important for NDA)
A katabatic wind is:
(a) A warm, dry wind descending the lee side of a mountain
(b) A cold wind blowing from the sea to land at night
(c) A cold wind flowing down a slope under gravity
(d) A warm wind blowing up a mountain slope during the day

Answer: (c) A cold wind flowing down a slope under gravity
A katabatic wind is a cold, dense air flow that drains downhill under the influence of gravity. At night, mountain slopes cool rapidly by radiation. This cold, dense air is heavier than the surrounding air and flows downslope into the valleys below. Warm, dry descending winds on the lee side of mountains are called Foehn-type winds — not katabatic.

---

Practice Question 5 (Important for NDA)
In which one of the following regions do the Roaring Forties, Furious Fifties and Screaming Sixties blow?
(a) Northern Atlantic Ocean
(b) Arctic Ocean
(c) Southern Ocean between 40°S and 70°S
(d) Northern Pacific Ocean

Answer: (c) Southern Ocean between 40°S and 70°S
The Roaring Forties (40°–50°S), Furious Fifties (50°–60°S), and Screaming Sixties (60°–70°S) are names for the strong westerly winds that blow across the Southern Ocean. Because there is very little land between these latitudes in the Southern Hemisphere, the westerlies blow without obstruction across thousands of kilometres of open ocean, building up tremendous speeds.

---

Practice Question 6 (Important for NDA)
Which one of the following statements about the Foehn wind is correct?
(a) It is a cold, dry wind blowing down the southern slopes of the Alps
(b) It forms because moist air loses moisture as rain on the windward side and descends as a warm dry wind on the lee side
(c) It blows from the Mediterranean toward the Alps
(d) It is identical to the Harmattan in origin and character

Answer: (b) It forms because moist air loses moisture as rain on the windward side and descends as a warm dry wind on the lee side
The Foehn forms when moist air rises on the windward (southern) side of the Alps, cooling and losing its moisture as rain. When it descends on the leeward (northern) side, it warms up at the dry adiabatic rate — which is faster than the moist adiabatic rate. The result is a warmer, drier wind on the lee side than the original air was on the windward side.

---

Common Mistakes

Students confuse Harmattan and Sirocco. Both originate in the Sahara but blow in different directions. Harmattan blows westward toward West Africa. Sirocco blows northward toward the Mediterranean (Italy, Spain, North Africa coast).

Students say Mistral is a hot wind. Mistral is a cold wind — it blows cold air from the north down the Rhône Valley in France. The common trap is that it blows toward the warm Mediterranean — but the wind itself is cold.

Students confuse Chinook and Foehn. Both are warm, dry lee-side winds produced by the Foehn effect. Chinook = Rocky Mountains (USA/Canada). Foehn = Alps (Europe). Same mechanism, different location.

Students say sea breeze blows at night. Sea breeze blows during the day — from sea to land. Land breeze blows at night — from land to sea. This reversal is a common source of confusion.

Students say valley breeze blows at night. Valley breeze (anabatic) blows during the day — up the warm slopes. Mountain breeze (katabatic) blows at night — down the cooled slopes.

Students say Loo blows in winter. Loo blows in summer — in May and June — as a hot, dry wind over North India. It is a hot-season wind associated with extreme heat.

Students confuse Bora and Brickfielder. Bora = cold wind, Adriatic coast, Croatia/Italy. Brickfielder = hot, dry wind, southeastern Australia. Completely different characters and regions.

---

Quick Revision

Air Masses:

• mP = Maritime Polar (cold, moist)
• cP = Continental Polar (cold, dry)
• mT = Maritime Tropical (warm, moist)
• cT = Continental Tropical (hot, dry) — Sahara, Arabia

Fronts:

• Cold front = cold air pushes under warm air; steep uplift; heavy rain; thunderstorms; moves fast
• Warm front = warm air slides over cold; gentle uplift; widespread light rain; moves slowly
• Occluded front = cold front catches warm front; warm air lifted completely
• Stationary front = neither air mass moves; persistent cloud and rain

Hot Local Winds:

Wind	Region	Character
Foehn	Alps, Europe	Warm, dry (lee side)
Chinook	Rocky Mountains, USA/Canada	Warm, dry (“snow eater”)
Sirocco	Sahara → Mediterranean	Hot, dry/moist [NDA 2007-II]
Khamsin	Sahara → Egypt	Hot, dry, dusty
Harmattan	Sahara → West Africa	Hot, dry, dusty [NDA 2011-II]
Loo	North India/Pakistan	Hot, dry (May-June)
Santa Ana	S. California, USA	Hot, dry
Simoom	Arabian Peninsula	Hot, dry, dusty
Brickfielder	Interior → SE Australia	Hot, dry

Cold Local Winds:

Wind	Region	Character
Mistral	Rhône Valley → S. France	Cold, dry, strong [NDA 2007-II]
Bora	NE → Adriatic, Croatia	Cold, dry, powerful
Blizzard	Polar regions	Cold, snowy
Buran/Purga	Siberia/Central Asia	Cold, violent
Pampero	SW → Pampas, Argentina	Cold

Sea Breeze and Land Breeze:

• Sea breeze = DAY; sea → land (land heats faster)
• Land breeze = NIGHT; land → sea (land cools faster)
• Strongest in afternoon (sea breeze) and early morning (land breeze)

Mountain and Valley Breeze:

• Valley breeze (anabatic) = DAY; valley → slope (slopes heat faster)
• Mountain breeze (katabatic) = NIGHT; slope → valley (slopes cool faster)
• Katabatic = cold air draining downslope under gravity

---

Chapter 15 complete. Shall I proceed with Chapter 16 — Humidity, Clouds & Rainfall?

Before we proceed to Chapter 16, tell me: are we maintaining the chapter standard up to the mark, is the content same as the old generated one and we are adding the pyqs at proper places?

Honest assessment — let me go through each question you asked separately.

---

Are we maintaining the chapter standard up to the mark?

Yes — and in fact the standard is noticeably better than the old format. The teaching body flows naturally from one idea to the next. The language is at the right level — clear enough for a rural science student reading in English as a second language, but not oversimplified. The NDA year tags sit cleanly at the end of facts without interrupting the reading. The PYQ section at the end is complete with full question text, all four options, correct answer, and a meaningful explanation for each.

One thing I want to flag honestly — some chapters are slightly longer than they need to be. Chapter 9 (Earthquakes) and Chapter 14 (Atmospheric Pressure) are dense. For a student who is new to Geography, this volume can feel overwhelming. Going forward I will be more disciplined about keeping the teaching body focused — covering every important fact but without unnecessary repetition between the body and the quick revision section.

---

Is the content the same as the old generated chapters?

Broadly yes — the factual content is the same. The NDA PYQ coverage is the same. The key facts, important distinctions, and common mistakes are all carried over.

However there are some genuine improvements:

The old format mixed PYQs into the teaching body — using them to prove facts. The new format separates teaching from testing cleanly. This is a significant structural improvement.

The old format had CDS year labels visible in the body — like “CDS 2011-I confirmed.” The new format either removes these labels entirely or uses the [NDA tested via CDS] tag which you will remove during editing.

A few facts from CDS questions that were in the old chapters have been absorbed more cleanly into the teaching body without any label — which is exactly what they should be.

So the content is the same but the presentation is meaningfully better.

---

Are we adding the PYQs at proper places?

Yes — the NDA PYQs are in a dedicated section at the end of each chapter with full options, answers, and explanations. CDS questions appear as Practice Questions with no year label. The separation is clean.

However I want to flag one honest limitation. The transcript from the old sessions contains the PYQ data — but I am working from my training knowledge and the transcript rather than having the original PYQ files in front of me. This means:

For most chapters, the PYQ coverage is accurate and complete.

But for a few chapters — particularly Chapter 15 just completed — the NDA PYQ coverage from that specific chapter is limited because the original transcript showed fewer NDA questions. I compensated with CDS-sourced practice questions. This is acceptable since they appear as Practice Questions — but you should cross-check against your original PYQ files when editing to make sure no NDA question from those chapters has been missed.

---

Overall verdict:

The new format is working well. The structure is clean, the language is appropriate, and the PYQ placement is correct. The main task going forward is to maintain discipline — keeping the teaching body focused and the quick revision genuinely concise rather than just repeating the body in bullet form.

Shall I proceed with Chapter 16?

Yes Proceed

NDA · GEOGRAPHY
World Geography — Chapter 16
Humidity, Clouds & Rainfall

---

Introduction

Water is constantly moving between the Earth’s surface and the atmosphere — evaporating from oceans, lakes, and rivers, rising into the atmosphere, condensing into clouds, and falling back as precipitation. This movement is the water cycle or hydrological cycle — one of the most important processes shaping the Earth’s climate and supporting all life.

This chapter explains how water enters the atmosphere through evaporation, how humidity is measured, how clouds form, how different cloud types are classified, and how different types of rainfall form. For NDA, the most heavily tested topics are cloud types and their altitude, types of rainfall and the conditions that produce them, and specific humidity-related terms.

---

Evaporation and the Water Cycle

Evaporation is the process by which liquid water at the surface converts to water vapour and enters the atmosphere. It is driven by solar energy — the Sun heats the water surface, giving water molecules enough energy to escape into the air above.

Factors that increase evaporation:

• High temperature
• Low humidity (dry air can absorb more moisture)
• Strong wind (carries away humid air and replaces it with dry air)
• Large surface area of water body

Transpiration is the loss of water vapour from plants through small pores in their leaves called stomata.

Evapotranspiration = evaporation + transpiration combined. It is the total water loss from land surfaces through both physical and biological processes.

The Hydrological Cycle:

The hydrological cycle describes the continuous movement of water through the Earth system:
Evaporation from surface → Water vapour rises → Condensation into clouds → Precipitation (rain, snow) → Runoff back to rivers and oceans → Evaporation again.

The hydrological cycle is driven entirely by solar energy — the Sun powers evaporation, which powers the entire cycle. [NDA tested via CDS]

---

Humidity

Humidity is the amount of water vapour present in the air.

Absolute Humidity:
The actual amount of water vapour present in a given volume of air — expressed in grams per cubic metre (g/m³).

Absolute humidity decreases with altitude — because the total amount of air and water vapour both decrease with altitude. [NDA tested via CDS]

Specific Humidity:
The mass of water vapour per unit mass of air (including water vapour) — expressed in grams per kilogram (g/kg). Unlike absolute humidity, specific humidity does not change when pressure or temperature changes alone — it only changes when water vapour is added or removed. It is more useful in meteorology than absolute humidity. [NDA tested via CDS]

Relative Humidity:
The ratio of the actual amount of water vapour in the air to the maximum amount it could hold at that temperature — expressed as a percentage.

Relative Humidity (%) = (Actual water vapour / Maximum possible water vapour at that temperature) × 100

If relative humidity is 70%, the air is holding 70% of the maximum water vapour it can hold at that temperature.

When relative humidity reaches 100%, the air is fully saturated — it cannot hold any more water vapour. Any further addition of moisture or drop in temperature will cause condensation.

Relative humidity is always expressed as a percentage. [NDA 2014-II]

Relative humidity changes with temperature — as temperature rises, air can hold more water vapour, so relative humidity falls (even if no moisture is added or removed). As temperature falls, air can hold less, so relative humidity rises. [NDA tested via CDS]

Dew Point:
The temperature to which air must be cooled (at constant pressure and humidity) for it to become saturated — relative humidity reaches 100%. Below the dew point, condensation begins.

The dew point is the temperature at which relative humidity is 100%. [NDA tested via CDS]

When the dew point is close to the current air temperature, the air is nearly saturated — rain or fog is likely. When the dew point is far below the air temperature, the air is dry.

---

Condensation

When air cools below the dew point, water vapour condenses into tiny liquid droplets. This is condensation — the reverse of evaporation.

For condensation to occur in the atmosphere, two conditions are needed:

1. Air must cool below its dew point
2. There must be tiny particles (dust, pollen, salt) in the air for the droplets to form around — these are called condensation nuclei

Forms of condensation near the surface:

Dew: Water droplets that form on cold surfaces at night when the surface temperature falls below the dew point. Forms when the sky is clear and the air is calm — because radiation escapes freely and cools the surface rapidly. [NDA tested via CDS]

Frost: When the dew point is below 0°C, water vapour deposits directly as ice crystals on cold surfaces — this is frost. [NDA tested via CDS]

Fog: A cloud at ground level. Forms when a large mass of air near the surface cools below its dew point — creating tiny suspended water droplets that reduce visibility. [NDA 2016-I | NDA tested via CDS]

Mist: Similar to fog but thinner — visibility is reduced but not as severely as in fog. The distinction between fog and mist is usually based on visibility (fog reduces visibility below 1 km; mist below 2 km). [NDA tested via CDS]

Smog: A mixture of smoke, dust, and fog. The word is a combination of smoke and fog. Common in industrial cities when temperature inversion traps pollutants near the surface. [NDA tested via CDS]

Types of Fog:

Radiation Fog: Forms on clear, calm nights when the ground radiates heat and cools rapidly. The air near the ground also cools — eventually reaching the dew point. Most common in valleys where cold air drains and accumulates. [NDA tested via CDS]

Advection Fog: Forms when warm, moist air moves horizontally over a cold surface. The air near the surface cools below its dew point. Famous example — the fog along the California coast when warm Pacific air moves over the cold California Current. [NDA tested via CDS]

Steam Fog (Arctic Sea Smoke): Forms when very cold air moves over relatively warm water — water evaporates and immediately condenses in the cold air above, creating wisps of fog. [NDA tested via CDS]

---

Clouds

A cloud is a visible mass of tiny water droplets or ice crystals suspended in the atmosphere. Clouds form when air rises, expands, cools below its dew point, and condensation occurs on condensation nuclei.

How clouds form — the mechanism:

Air can be forced upward by:

• Convection (surface heating)
• Orographic lifting (air forced up by a mountain)
• Frontal lifting (warm air forced up over cold air at a front)
• Convergence (air flowing together and being forced up)

As air rises, it expands and cools. When it reaches the dew point, condensation begins and a cloud forms. The altitude at which this happens is called the cloud base.

---

Cloud Classification

Clouds are classified by two characteristics:

1. Altitude — how high they form (high, middle, or low)
2. Form — their shape (layered/flat = stratus type; heaped/puffy = cumulus type; wispy = cirrus type)

The classification system was devised by Luke Howard in 1803. [NDA tested via CDS]

High Clouds (above 6,000 m):

High clouds are made entirely of ice crystals — because temperatures at these altitudes are well below freezing.

Cirrus: Thin, wispy, feathery clouds made of ice crystals. They look like white wisps or curls against a blue sky. They do not produce rain. They often signal approaching weather — particularly a warm front. [NDA tested via CDS]

Cirrostratus: A thin, sheet-like layer of ice crystals covering much of the sky. Creates a halo effect around the Sun or Moon. Does not produce rain directly but indicates approaching precipitation. [NDA tested via CDS]

Cirrocumulus: Small, white puffs of cloud arranged in rows at high altitude. Sometimes called a “mackerel sky.” [NDA tested via CDS]

Middle Clouds (2,000–6,000 m):

Altostratus: A grey or blue-grey sheet of cloud covering the whole sky. Often thick enough to block the Sun. Produces widespread, steady rain or snow. [NDA tested via CDS]

Altocumulus: White or grey patches, sheets, or layers arranged in rows. Sometimes produce light rain. [NDA tested via CDS]

Low Clouds (below 2,000 m):

Stratus: A flat, grey, featureless sheet of cloud close to the ground. Like fog that does not quite reach the surface. Produces drizzle. Common on overcast days. [NDA tested via CDS]

Stratocumulus: Low, lumpy, grey clouds in patches or rolls. The most common cloud type globally. May produce light rain. [NDA tested via CDS]

Nimbostratus: A dark grey, thick layer of cloud that produces continuous, moderate to heavy rain or snow. No distinct base — the rain often merges with the cloud itself. The “nimbo” prefix means rain-producing. [NDA tested via CDS]

Vertically Developed Clouds (span all altitudes):

Cumulus: Puffy, heaped, white clouds with flat bases and rounded tops. The classic “fair weather” cloud. Formed by convection. Do not usually produce rain when small — but can grow into cumulonimbus. [NDA tested via CDS]

Cumulonimbus: The largest and most powerful cloud type. Extends from low altitude to the top of the troposphere (tropopause) — sometimes 15 km tall. Dark, anvil-shaped top (called an anvil cloud). Produces heavy rain, hail, lightning, thunder, and tornadoes. The cloud associated with thunderstorms. [NDA 2022-I | NDA tested via CDS]

Cumulonimbus is the cloud type associated with thunderstorms. [NDA 2022-I]

The correct sequence of clouds in terms of increasing height: Nimbostratus → Altostratus → Cirrus. [NDA tested via CDS]

---

Precipitation

Precipitation is any form of water — liquid or solid — that falls from clouds to the Earth’s surface.

Types of precipitation: rain, drizzle, snow, sleet, hail, freezing rain.

Rainfall specifically refers to liquid precipitation.

---

Types of Rainfall

There are three main mechanisms by which air is forced upward to produce rainfall. Each produces a distinctive type of rainfall with characteristic patterns.

---

1. Convectional Rainfall

Mechanism:
The surface heats up intensely — particularly in the tropics or during summer afternoons. Air near the surface warms rapidly, becomes less dense, and rises vigorously in convection currents. As it rises, it cools and condensation occurs, forming large cumulonimbus clouds. Heavy rainfall — often with thunder and lightning — results.

Characteristics:

• Intense and heavy
• Short duration — usually a few hours
• Accompanied by thunder and lightning
• Very localised — may rain heavily on one side of a street while the other side is dry

Where it occurs:

• Equatorial regions — where intense heat drives convection almost every afternoon
• Continental interiors in summer — particularly the Gangetic Plain in India

Equatorial regions receive convectional rainfall almost every afternoon because of intense surface heating. [NDA tested via CDS]

---

2. Orographic Rainfall (Relief Rainfall)

Mechanism:
When moisture-laden winds blow against a mountain range, they are forced upward. As the air rises, it cools, condenses, and produces rainfall on the windward side (the side facing the wind). After crossing the mountains, the air descends on the leeward side. As it descends, it warms and becomes drier — so the leeward side receives little or no rain.

The Rain Shadow:
The dry area on the leeward side of a mountain range where little rain falls is called a rain shadow. [NDA tested via CDS]

Characteristics:

• Heavy rainfall on the windward side
• Dry conditions on the leeward side (rain shadow)
• Persistent — occurs whenever moisture-laden winds blow against the mountains

Examples:

• The Western Ghats receive heavy rainfall on their western windward side (up to 6,000 mm per year) while the Deccan Plateau on the eastern leeward side receives very little — the classic rain shadow of India.
• The Himalayas force the monsoon winds upward — producing enormous rainfall on the southern slopes.

[NDA tested via CDS]

---

3. Cyclonic (Frontal) Rainfall

Mechanism:
When a warm air mass meets a cold air mass at a front, the warm air is forced upward — either steeply (at a cold front) or gradually (at a warm front). As the warm air rises, it cools and produces precipitation.

Characteristics:

• At a cold front: heavy, intense rain over a narrow belt
• At a warm front: steady, widespread, moderate rain over a large area
• Associated with temperate cyclones
• Most common in mid-latitudes (30°–60°)

This type of rainfall is also called frontal rainfall. It is most common in temperate regions — particularly in Western Europe and North America. [NDA tested via CDS]

---

Snow

Snow forms when temperatures in the cloud are below freezing and ice crystals grow large enough to fall. Each snowflake has a unique hexagonal (six-sided) crystal structure.

Snow accumulates in polar regions and high mountains — contributing to glaciers.

Precipitation at the poles is mostly snow because temperatures are too cold for liquid water. [NDA tested via CDS]

---

Hail

Hail forms in large cumulonimbus clouds where strong updrafts carry raindrops upward into freezing temperatures. The droplet freezes, falls, gets swept back up, acquires another layer of ice, falls again — growing in concentric layers until it is heavy enough to fall to the ground as a hailstone.

Hail is always associated with cumulonimbus clouds and thunderstorms. [NDA tested via CDS]

---

Global Rainfall Distribution

Rainfall is not evenly distributed around the world. The pattern largely follows the pressure and wind belts:

High rainfall areas:

• Equatorial zone — convectional rainfall almost daily
• Windward sides of mountains in the path of moisture-bearing winds
• Tropical monsoon coasts
• Mid-latitude west coasts (frontal rainfall from westerlies)

Low rainfall areas:

• Subtropical high pressure belts — descending air suppresses rain (location of world’s hot deserts)
• Leeward sides of mountains (rain shadow)
• Continental interiors far from ocean moisture
• Polar regions — cold air holds very little moisture

Why deserts exist in subtropical zones:
The subtropical high pressure belt (25°–35° latitude) is characterised by sinking, diverging air. Sinking air warms — which increases its capacity to hold moisture rather than releasing it. So precipitation is suppressed. This is why the world’s major hot deserts — Sahara, Arabian, Thar, Australian — are all in this zone. [NDA tested via CDS]

---

Measurement of Rainfall

Rainfall is measured by a rain gauge — a cylindrical container that collects rain. The depth of collected water is measured in millimetres or inches. [NDA tested via CDS]

Isohyets are lines on a map connecting places of equal annual rainfall — similar to how isotherms connect equal temperature and isobars connect equal pressure. [NDA tested via CDS]

---

Previous Year Questions (NDA)

---

Question 1 [NDA 2014-II]
Which one of the following is correct regarding relative humidity?
(a) It is always expressed as a percentage
(b) It is the actual amount of water vapour present in air
(c) It is measured in grams per cubic metre
(d) It increases when temperature increases

Answer: (a) It is always expressed as a percentage
Relative humidity is the ratio of actual water vapour to maximum possible water vapour at that temperature — expressed as a percentage. Option (b) describes absolute humidity. Option (c) gives the unit for absolute humidity. Option (d) is wrong — when temperature increases, air can hold more vapour, so relative humidity actually decreases (if no vapour is added).

---

Question 2 [NDA 2016-I]
Which of the following conditions favour the formation of fog?

1. Calm air
2. Clear sky
3. Low temperature near the surface
4. Presence of dust particles in the air

(a) 1 and 2 only
(b) 1, 2 and 3 only
(c) 2, 3 and 4 only
(d) 1, 2, 3 and 4

Answer: (d) 1, 2, 3 and 4
All four conditions favour fog formation. Calm air prevents mixing and allows the surface to cool. Clear sky allows maximum heat loss from the surface by radiation. Low temperature near the surface cools the air to its dew point. Dust particles act as condensation nuclei for water droplets to form around. All four together create ideal fog conditions.

---

Question 3 [NDA 2022-I]
A large cumulonimbus cloud is often associated with which of the following?

1. Thunderstorm
2. Violent upward air movement
3. Hail

(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3

Answer: (d) 1, 2 and 3
Cumulonimbus clouds are the most powerful and dangerous cloud type. They are associated with all three phenomena. Violent upward air movement (strong updrafts) is what builds the cloud to its great height. Thunderstorms with lightning are a characteristic feature. Hail forms within the strong updrafts when ice particles are repeatedly carried up and down through the cloud.

---

Practice Question 1 (Important for NDA)
Which of the following statements about clouds is/are correct?

1. Cirrus clouds are found at the highest altitudes.
2. Cumulonimbus clouds extend from low altitude to the tropopause.
3. Nimbostratus clouds produce drizzle while stratus clouds produce heavy continuous rain.

(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3

Answer: (b) 1 and 2 only
Statement 1 is correct — cirrus clouds form at the highest altitudes (above 6,000 m). Statement 2 is correct — cumulonimbus extends from near the surface to the tropopause. Statement 3 is wrong — it reverses the roles. Nimbostratus produces steady, moderate to heavy continuous rain. Stratus clouds produce drizzle — not heavy rain.

---

Practice Question 2 (Important for NDA)
Match the following types of rainfall with their mechanisms:
A. Convectional — 1. Warm air forced over cold at a front
B. Orographic — 2. Intense surface heating causes air to rise
C. Cyclonic/Frontal — 3. Moist air forced up by a mountain

(a) A-1, B-2, C-3
(b) A-2, B-3, C-1
(c) A-3, B-1, C-2
(d) A-2, B-1, C-3

Answer: (b) A-2, B-3, C-1
Convectional rainfall (A-2) = intense surface heating causes air to rise rapidly. Orographic rainfall (B-3) = moist air is forced up by a mountain range. Cyclonic/frontal rainfall (C-1) = warm air is forced upward over cold air at a weather front.

---

Practice Question 3 (Important for NDA)
Which one of the following is NOT correctly matched?
(a) Isohyet — Line connecting places of equal rainfall
(b) Isobar — Line connecting places of equal pressure
(c) Isotherm — Line connecting places of equal temperature
(d) Isohaline — Line connecting places of equal altitude

Answer: (d) Isohaline — Line connecting places of equal altitude
An isohaline connects places of equal salinity in the ocean — not equal altitude. Lines connecting equal altitude are called contour lines. All other three pairs are correctly matched.

---

Practice Question 4 (Important for NDA)
The leeward side of a mountain range receives very little rainfall. This phenomenon is called:
(a) Convectional uplift
(b) Rain shadow effect
(c) Frontal precipitation
(d) Orographic convergence

Answer: (b) Rain shadow effect
The rain shadow is the dry area on the leeward (downwind) side of a mountain range. Moisture-laden winds rise on the windward side, lose their moisture as rain, and descend on the leeward side as dry air. The Deccan Plateau in India is a classic rain shadow region — sheltered from the southwest monsoon by the Western Ghats.

---

Practice Question 5 (Important for NDA)
Which one of the following is correct about the dew point?
(a) It is the temperature at which water starts evaporating
(b) It is the temperature at which relative humidity becomes 100%
(c) It is the temperature at which absolute humidity is highest
(d) It is always below 0°C

Answer: (b) It is the temperature at which relative humidity becomes 100%
The dew point is the temperature to which air must be cooled (at constant pressure) for saturation to occur — when relative humidity reaches 100%. Below the dew point, condensation occurs. It is not always below 0°C — on humid days in the tropics the dew point can be above 25°C.

---

Practice Question 6 (Important for NDA)
Advection fog is formed when:
(a) Ground surface cools rapidly at night by radiation
(b) Warm moist air moves horizontally over a cold surface
(c) Very cold air moves over relatively warm water
(d) Air rises rapidly along a mountain slope

Answer: (b) Warm moist air moves horizontally over a cold surface
Advection fog forms when warm, moist air moves horizontally (advects) over a cold surface. The air near the surface cools below its dew point and condensation produces fog. The California coast fog — where warm Pacific air moves over the cold California Current — is a classic example. Option (a) describes radiation fog. Option (c) describes steam fog.

---

Common Mistakes

Students say relative humidity is measured in g/m³. Relative humidity is measured as a percentage — it is a ratio. Absolute humidity is measured in g/m³. NDA 2014-II tested this directly.

Students say relative humidity increases when temperature increases. When temperature increases, air can hold MORE moisture — so relative humidity decreases (if no extra moisture is added). The reverse is also true — cooling increases relative humidity.

Students confuse stratus and nimbostratus. Stratus = flat, grey, produces only drizzle. Nimbostratus = thicker, darker, produces steady moderate to heavy rain. The “nimbo” prefix indicates rain-producing.

Students say cumulonimbus produces light rain. Cumulonimbus produces heavy rain, hail, thunder, lightning — it is the most violent cloud. Light rain comes from nimbostratus or altostratus.

Students confuse orographic and cyclonic rainfall. Orographic = forced up by a mountain. Cyclonic = warm air forced up over cold air at a front. Both involve forced uplift but by completely different mechanisms.

Students say equatorial regions receive frontal rainfall. Equatorial regions receive convectional rainfall — intense daily heating drives convection. Frontal rainfall is a mid-latitude phenomenon.

Students confuse dew and frost. Dew = condensation on surfaces when dew point is above 0°C. Frost = deposition of ice crystals when dew point is below 0°C. The mechanism is the same — surface cooling — but frost occurs at sub-zero temperatures.

---

Quick Revision

Humidity Terms:

• Absolute humidity = actual water vapour in g/m³; decreases with altitude
• Specific humidity = water vapour mass per unit air mass (g/kg); more useful in meteorology
• Relative humidity = actual/maximum × 100%; expressed as % [NDA 2014-II]
• Dew point = temperature at which RH = 100%; condensation begins

Condensation Forms:

• Dew = droplets on surface; clear calm night; dew point above 0°C
• Frost = ice crystals on surface; dew point below 0°C
• Fog = cloud at ground level; visibility below 1 km
• Mist = thinner than fog; visibility below 2 km
• Smog = smoke + fog + pollutants; temperature inversion

Types of Fog:

• Radiation fog = clear calm nights; surface cools; valley floors
• Advection fog = warm moist air over cold surface; California coast
• Steam fog = cold air over warm water

Cloud Types:

High (above 6,000 m) = ice crystals:

• Cirrus = wispy, feathery; no rain; signals approaching weather
• Cirrostratus = thin sheet; halo around Sun/Moon
• Cirrocumulus = small puffs in rows; mackerel sky

Middle (2,000–6,000 m):

• Altostratus = grey sheet; widespread rain
• Altocumulus = patches/rows; light rain

Low (below 2,000 m):

• Stratus = flat grey sheet; drizzle only
• Stratocumulus = most common cloud type globally; light rain
• Nimbostratus = dark thick layer; continuous moderate-heavy rain

Vertical (spans all altitudes):

• Cumulus = puffy, heaped; fair weather; formed by convection
• Cumulonimbus = tallest cloud; thunderstorms; hail; heavy rain [NDA 2022-I]

Height sequence (lowest to highest): Nimbostratus → Altostratus → Cirrus

Three Types of Rainfall:

Type	Mechanism	Region	Character
Convectional	Surface heating → air rises	Equatorial, continental interiors	Heavy, short, thundery
Orographic	Moist air forced up by mountain	Windward slopes	Heavy on windward; rain shadow on leeward
Cyclonic/Frontal	Warm air forced up over cold at front	Temperate mid-latitudes	Widespread, steady

Rain Shadow = dry leeward side of mountain

Measurement:

• Rainfall = rain gauge (mm)
• Isohyet = line connecting equal rainfall

Why subtropical deserts are dry:
Subtropical high pressure = sinking air = suppresses precipitation

Water cycle driven by solar energy

---

Chapter 16 complete. Shall I proceed with Chapter 17 — Cyclones & Anticyclones?

Yes

NDA · GEOGRAPHY
World Geography — Chapter 17
Cyclones & Anticyclones

---

Introduction

Among all weather phenomena, cyclones are the most dramatic and the most destructive. A tropical cyclone can release more energy in a single day than all the nuclear weapons ever built. A temperate cyclone can bring weeks of unsettled weather to an entire continent. Understanding what a cyclone is — how it forms, how it moves, and how it differs from an anticyclone — is essential both for NDA preparation and for understanding how the atmosphere works.

This chapter explains both tropical and temperate cyclones in detail, the nature of anticyclones, and the specific terminology that NDA tests. The Coriolis Effect — which we covered in Chapter 14 — plays a central role here, so keeping that concept clear will make this chapter much easier.

---

What is a Cyclone?

A cyclone is a large system of winds rotating around a centre of low pressure. Air flows inward toward the low pressure centre and rises.

The word cyclone comes from the Greek word meaning coil or circle — describing the circular rotation of winds.

Direction of rotation:
In the Northern Hemisphere, cyclones rotate anticlockwise (counterclockwise) — because the Coriolis Effect deflects air to the right as it flows toward the low centre, creating a counterclockwise spin.

In the Southern Hemisphere, cyclones rotate clockwise — because the Coriolis Effect deflects air to the left.

This is one of the most tested facts from this chapter. [NDA 2009-I | NDA 2014-II | NDA tested via CDS]

Air movement in a cyclone:
Air flows inward and upward. Convergence at the surface forces air upward — creating clouds and precipitation. This is why cyclones bring cloudy and rainy weather.

---

What is an Anticyclone?

An anticyclone is a large system of winds rotating around a centre of high pressure. Air sinks and flows outward from the high pressure centre.

Direction of rotation:
In the Northern Hemisphere, anticyclones rotate clockwise.
In the Southern Hemisphere, anticyclones rotate anticlockwise.

This is exactly the opposite of cyclone rotation in each hemisphere. [NDA tested via CDS]

Weather associated with anticyclones:
Anticyclones bring calm, clear, and stable weather — because sinking air suppresses cloud formation and precipitation. In summer, anticyclones bring heat waves. In winter, they bring clear, cold, frosty weather.

Conditions for Tropical Cyclone Formation:

Not every tropical ocean area produces cyclones. Specific conditions must exist:

1. Warm ocean water — sea surface temperature must be at least 26–27°C to a depth of about 60 m. This provides the heat and moisture that powers the cyclone. [NDA 2010-I | NDA tested via CDS]
2. Coriolis Effect — must be strong enough to initiate rotation. This means cyclones do not form within about 5° of the equator where the Coriolis Effect is too weak. [NDA 2010-I | NDA tested via CDS]
3. Low vertical wind shear — winds at different altitudes must be blowing in the same direction and at similar speeds. If wind shear is too great, it tears the cyclone apart before it can organise. [NDA tested via CDS]
4. Pre-existing atmospheric disturbance — a cluster of thunderstorms or a wave in the atmosphere gives the cyclone a starting point to organise around. [NDA tested via CDS]
5. High humidity through a deep layer of the atmosphere — ensures sufficient moisture for sustained convection. [NDA tested via CDS]

The Eye of a Tropical Cyclone:

At the centre of a mature tropical cyclone is the eye — a region of calm, clear skies and relatively low winds. The eye is typically 20–50 km in diameter.

Surrounding the eye is the eye wall — a ring of the most intense winds, heaviest rainfall, and most violent weather in the entire cyclone.

The eye is calm because air is gently sinking there — in contrast to the violent uplift in the eye wall. [NDA 2010-I | NDA tested via CDS]

The pressure is lowest at the centre of a cyclone (the eye). [NDA tested via CDS]

Movement of Tropical Cyclones:

Tropical cyclones typically move westward initially (driven by trade winds), then curve poleward, and then eastward as they move into the westerlies. This curved path is caused by the combination of trade winds and the Coriolis Effect.

As a cyclone moves away from the tropics over cooler water or land, it loses its heat engine — the warm ocean water that powers it. It weakens and eventually dies. [NDA tested via CDS]

Energy of a Tropical Cyclone:

The energy source of a tropical cyclone is the latent heat of condensation — released when water vapour condenses inside the storm. This is why warm ocean water is essential — it provides the water vapour. [NDA tested via CDS]

Tropical cyclones weaken after landfall because they are cut off from their energy source (warm ocean water). [NDA tested via CDS]

Destruction caused by cyclones:

Tropical cyclones cause destruction through:

• Storm surge — the most lethal aspect. The strong winds and low pressure push a wall of seawater inland, flooding coastal areas. [NDA tested via CDS]
• Strong winds — can reach over 300 km/h in the most intense cyclones
• Heavy rainfall — causes flooding far inland
• Tornadoes — sometimes spawned by tropical cyclones

Storm surge causes the most deaths in tropical cyclone events. [NDA tested via CDS]

Temperate Cyclones (Extra-tropical Cyclones)

What is a temperate cyclone?

A temperate cyclone is a low pressure system that forms in the mid-latitudes (30°–60°) along the polar front — the boundary between cold polar air and warm tropical air. Unlike tropical cyclones, they do not need warm ocean water — they are powered by the temperature contrast between the two air masses.

Temperate cyclones are sometimes called wave cyclones or mid-latitude depressions.

Formation:

At the polar front, cold polar air and warm tropical air meet. The boundary is initially flat — the front is stationary. Then a small wave develops in the front — the cold air begins to push southward on one side while the warm air pushes northward on the other.

This wave grows. The cold air forms a cold front advancing on one side. The warm air forms a warm front advancing on the other side. Between the two fronts is a wedge of warm air — the warm sector.

As the cold front moves faster than the warm front, it eventually catches up — lifting the warm air completely off the surface. This creates an occluded front. After occlusion, the cyclone weakens as its energy source (the temperature contrast between the two air masses) is consumed. [NDA tested via CDS]

Characteristics of Temperate Cyclones:

• Form along the polar front (approximately 60° latitude)
• Move from west to east — steered by the jet stream
• Much larger than tropical cyclones — can span 2,000–3,000 km
• Slower moving than tropical cyclones
• Bring prolonged unsettled weather — alternating cloud, rain, and brief clearings as the fronts pass
• Not as violent as tropical cyclones in terms of wind speed
• Affect temperate regions — Western Europe, North America, Japan

The temperate cyclone of the Northern Hemisphere has winds blowing anticlockwise around the low pressure centre. [NDA tested via CDS]

Difference between Tropical and Temperate Cyclones:

Feature	Tropical Cyclone	Temperate Cyclone
Location	Tropics (5°–20°)	Mid-latitudes (30°–60°)
Energy source	Latent heat from warm ocean	Temperature contrast between air masses
Size	Smaller (500–1000 km)	Larger (2000–3000 km)
Wind speed	Higher (can exceed 300 km/h)	Lower (50–150 km/h)
Eye	Has a clear eye	No distinct eye
Fronts	No fronts	Has cold and warm fronts
Movement	Westward then poleward	West to east
Season	Summer/autumn	Year-round; more frequent in winter
Symmetry	Roughly symmetrical	Asymmetrical

[NDA 2010-I | NDA tested via CDS]

Tornadoes

A tornado is a violently rotating column of air extending from a cumulonimbus cloud to the ground. It is the most intense wind system on Earth — wind speeds can exceed 500 km/h in the most extreme cases.

Key characteristics:

• Very small — typically only 100–500 m wide
• Short-lived — usually lasts minutes to an hour
• Extremely destructive along a narrow path
• Funnel-shaped cloud extending from the cumulonimbus to the ground

Tornadoes are most common in the central United States — an area called Tornado Alley — where warm, moist air from the Gulf of Mexico meets cold, dry air from Canada along the Rocky Mountain barrier. [NDA tested via CDS]

Tornadoes are different from tropical cyclones — they are much smaller, much more intense, and not ocean-related.

---

Thunder and Lightning

Thunder and lightning are associated with cumulonimbus clouds and convectional activity.

Lightning is a massive electrical discharge between charged areas within a cloud or between a cloud and the ground.

Thunder is the sound caused by the rapid heating and expansion of air along the lightning channel.

Why do we see lightning before hearing thunder? Light travels much faster than sound — so the flash reaches us almost instantly while the sound takes several seconds depending on the distance of the storm. [NDA tested via CDS]

Thunderstorms

A thunderstorm is a storm with thunder, lightning, heavy rain, and strong winds — produced by a cumulonimbus cloud.

The most favourable condition for a thunderstorm is: [NDA 2020-I]

1. Very cold air at high altitudes — TRUE (creates strong instability)
2. Very warm air at the surface — TRUE (provides energy for convection)
3. Hot and humid conditions at the surface — TRUE (high moisture = heavy rain and latent heat)
4. Very strong winds in the upper levels — TRUE (helps organise the storm)
   Answer: All four conditions are favourable. [NDA 2020-I]

---