UPSC NCERT Notes Geography - Air Masses, Fronts and Cyclone

Air masses, fronts, and cyclones are the dynamic trio of weather systems that orchestrate the ever-changing show in our skies. Imagine Earth’s atmosphere as a vast ocean, not of water, but of air. Within this ocean swim giant currents of air called air masses, each with its own distinct temperature, humidity, and pressure. These air masses, like ships on the high seas, can collide and interact, forming boundaries called fronts. Fronts are like weather battle lines, where warm and cold air masses clash, bringing about dramatic changes in temperature, precipitation, and wind.

Table of Contents

Air Masses

An air mass is a sizable body of air characterized by relatively uniform temperature and humidity properties. It attains these characteristics by remaining stationary over a specific area for an extended period, allowing it to reach equilibrium with the underlying surface. A front marks the boundary where two distinct air masses meet on Earth’s surface, each possessing unique temperature and humidity features.
On the other hand, a cyclone is a weather phenomenon or system of winds that revolves around a center of low atmospheric pressure, moving at a speed of 30-50 km/h.
Air masses play a role in modifying the temperature and moisture content along their path, undergoing modifications themselves. An air mass forms when it lingers over a homogenous area for a significant duration, adopting the characteristics of that region.

An air mass exhibits two fundamental traits:

Vertical temperature distribution, known as lapse rate, a measure indicating its warmth or coldness, influencing its stability.
Homogeneous moisture content, serves as an indicator of latent heat.

Classification of Air Masses

Air masses can be categorized as follows:

Arctic and Antarctica Air Masses

These air masses originate over expansive snow and ice-covered regions, typically near the poles in both hemispheres.
Forming predominantly during winter (December to March in the Northern hemisphere and June to September in the Southern hemisphere), when the poles experience minimal insolation and remain tranquil without significant heating.

Polar Air Masses

Source regions for polar air masses are generally located between 55° and 65° latitudes in both hemispheres. Various regions contribute to the formation of polar air masses in the Northern Hemisphere, including Siberia in winter, the Gulf of Alaska, Northern Canada, the North Pacific to the South of the Atlantic, and West of the British Isles.

Tropical Air Masses

There is a network of tropical air mass source regions encircling the Northern hemisphere and another in the Southern hemisphere near the Tropics of Cancer and Capricorn.
These sources are associated with the subtropical high-pressure belt and large-scale subsidence, leading to adiabatic warming. These high-pressure belts are responsible for the world’s deserts, and tropical air masses form over regions such as the Sahara, Azores, Southwest USA (summer), and Northern Mexico (summer).

Equatorial Air Masses

The convergence of trade winds at the Equator forms the ‘Inter-Tropical Convergence Zone (ITCZ),’ a trough of low pressure. Stagnant air regions in some areas serve as sources for equatorial air mass formation, occurring only over water in these latitudes.

Fronts

The sloping boundary that separates two opposing air masses with differing characteristics in terms of temperature, density, pressure, humidity, and wind direction is known as a front.
Fronts are most pronounced in middle latitudes where polar cold air masses and tropical warm air masses converge, characterized by steep temperature and pressure gradients.
Frontogenesis refers to the process of front formation or intensification, while frontolysis is the weakening or destruction of fronts, leading to the birth of cyclones and anticyclones.

Classification of Fronts

Fronts can be categorized as follows:

Stationary Front

When a front remains stationary, it is called a stationary front. This type of front exhibits no noticeable weather disturbances, as two different air masses stand face to face without significant movement. Later, it may transform into another type of front.

Warm Front

A well-defined boundary between a warm and cold air mass, where the warm air advances and overrides the cold air, marking the leading edge of the warm sector of a depression.
The gradient of the warm front is gentler than that of the cold front, resulting in the slow ascent of warm moist air, cooling, condensation, and the development of nimbus clouds.

Cold Front

A clearly defined boundary between a warm and cold air mass, where the cold air advances and undercuts the warm air (rear of the warm sector of a frontal depression).
This results in a significant temperature drop, extensive cumulonimbus cloud development, heavy rainfall (sometimes with thunder), and the wind blowing from a Northern to Northwestern direction in the Northern hemisphere.
The gradually rising warm air along the gently sloping warm front is cooled adiabatically, leading to saturation, condensation, and precipitation over a relatively large area for several hours in the form of moderate to gentle precipitation.

Occluded Front

A front that develops during the later stages of the warm sector is no longer at the ground surface. Here, warm air mass is uplifted completely by the cold air mass in the rear of the depression.
As the cold front normally travels more quickly than the warm front, it slowly reduces the area of the warm sector, until it merges with the preceding front to complete the occlusion process. The occlusion is therefore, a compound zone with warm and cold front characteristics.
The associated weather has characteristics of both warm front and cold front weather

Frontal Zones

There are three types of frontal zones which are as follows.

Arctic Frontal Zone

Arctic marine and continental winds collide to form Arctic Frontal Zones. Due to the nearly same temperature of the convergent winds, they are not very active.
The extension of the Arctic Frontal Zone is found in Eurasia and North America.

Polar Frontal Zone

When the cold polar air mass and the warm tropical air mass collide, a polar front is created.
Fronts in these regions are more active in winter than they are in summer.
The North Atlantic and North Pacific oceans include a large amount of the polar frontal zone.

Inter-Tropical Frontal Zone

This front extends over the Mediterranean low-pressure belt. The Inter-Tropical Frontal Zone is created by the convergence of the North and South-Eastern Trade Winds.
Trade winds converge, causing air to climb upward and provide heavy rains.
This frontal zone moves South during the winter and North during the summer

**Diagrammatic Representation of Frontal Zones**

Cyclone

A cyclone is a low-pressure area surrounded by high pressure areas from all sides, having an elliptical or circular shape.
They assume anti-clockwise direction in the .Northern hemisphere and the Southern hemisphere due to clockwise direction Coriolis effect.
The cyclones move in different directions and affect the weather conditions of the areas which fall in their path of movements.
Cyclones are also termed as atmospheric disturbances when the velocity of wind increases to such an extent, that they attain great force, the atmospheric disturbance or cyclone is called cyclonic storm
Cyclones range in shape from circular, elliptical to V-shaped.
Cyclones have immense influence on the climate and weather of the region. Cyclones are divided into two types i.e., temperate and tropical cyclone.

Temperate Cyclone

Temperate cyclones are also termed as extra tropical cyclones or wave cyclones or simply depressions or low or troughs.
They are formed in the regions extending between 35°- 65° latitudes in both the hemisp- heres, due to convergence of two contrasting air masses e.g. warm, moist and light tropical air masses with cold and dense polar air masses

Following their formation, temperate cyclones travel eastward, guided by the influence of westerly winds, shaping weather conditions in the middle latitudes. The vertical span of an average cyclone is approximately 10-12 km, boasting an average velocity of 32-48 km/h and a diameter of about 1000 km.
More active in winter in the Northern hemisphere, temperate cyclones maintain year-round activity in the Southern hemisphere due to the prevalence of water. Extra-tropical cyclones have a broader impact, affecting a much larger area compared to tropical cyclones.

Life Cycle of Temperate Cyclone

The life cycle of a temperate cyclone encompasses six stages:

Stages of Temperate Cyclone

First Stage Involving the convergence of two contrasting air masses, initially moving parallel to each other, leading to the formation of a stationary front—referred to as the initial stage.
The second Stage Also known as the incipient stage, where warm and cold air masses penetrate each other’s territories, creating a wave-like front.
Third Stage The mature stage sees the cyclone fully developed, with isobars adopting an almost circular configuration.
Fourth Stage The warm sector contracts due to the advancing cold front approaching the warm front.
Fifth Stage Marked by the occlusion of the cyclone, as the advancing cold front overtakes the warm front, forming an occluded front.
Sixth Stage The warm sector vanishes, the occluded front dissipates, and the cyclone ultimately dies out—a process known as frontolysis.

Size and Extent of Temperate Cyclone

Assuming various shapes such as circular, semi-circular, elongated, or V-shaped during its growth, most temperate cyclones tend to be elliptical when fully matured.
Cyclones exhibit a diameter range spanning from 150 km to a whopping 3000 km, with certain temperate cyclones covering expansive areas exceeding one million sq km.
The height of an average temperate cyclone measures between 10 to 12 km, while the average winter wind velocity ranges from 40-60 km/h, decreasing to 15-20 km/h during summer.

Areas Visited by Temperate Cyclones

North Atlantic Ocean: Originating from the convergence of cold air masses from Greenland and Iceland with warm air masses from lower latitudes, these temperate cyclones impact large regions including Britain, Sweden, Norway, and other parts of Europe.
Mediterranean Sea: Formed by the convergence of cold and dry air masses from Central Europe with the Westerlies during the Western season, these cyclones originate in the Mediterranean Sea and, after crossing the Middle East countries, reach Pakistan and Northwest India.
North Pacific Ocean: Originating near the Aleutian Islands, temperate cyclones in this region cause widespread rainfall in the USA and Canada after crossing the Rockies.
China Sea: Temperate cyclones in this area originate near the Japan Sea and bring rainfall to North China after crossing the China Sea.

Tropical Cyclones

Tropical cyclones, powerful storms characterized by violent winds, heavy rainfall, and storm surges, originate over tropical oceans and move towards coastal areas, resulting in extensive destruction.
These cyclones originate and intensify over warm tropical oceans, gaining strength from a continuous supply of moisture.
Upon reaching the land, the moisture source is severed, leading to the dissipation of the storm.
The point where a tropical cyclone crosses the coast is referred to as the landfall of the cyclone.

**Diagrammatic Representation of Tropical Cyclone**

In this region, wind attains its maximum velocity, reaching up to 250 km/h.
Tropical cyclones typically form during the summer season near the Inter-Tropical Convergence Zone (ITCZ) over warm ocean surfaces. Subsequently, they shift to latitudes between 5° and 30° in the Northern hemisphere.
These cyclones emerge over ocean basins in lower latitudes across all oceans, excluding the South Atlantic and Southeast Pacific.
They also develop over the warm waters of the Bay of Bengal and the Arabian Sea, known as Cyclones in the Indian Ocean, Hurricanes in the Atlantic Ocean, Typhoons in the Western Pacific Ocean and the South China Sea, Willy-willies in Western Australia, Baguio in the Philippines, Taifu in Japan, and more.
Tornadoes, formed over land, are more destructive than cyclones, with wind speeds often exceeding 320 km/hr. Despite their intensity, tornadoes are relatively small, measuring only a few hundred meters across. When a tornado passes over a sea or lake, it can suck up water towards its center, forming a water spout. Tornadoes are frequently observed in the USA.
Cyclones that sustain a wind speed of 39 mph are given names. If the storm’s speed reaches or surpasses 74 mph, it is categorized as a hurricane, cyclone, or typhoon.

Structure of Tropical Cyclone

The innermost part of a tropical cyclone is known as the ‘eye,’ with a diameter ranging from 20-50 km. This region is characterized by calm conditions, subsiding air, the lowest pressure, and the highest temperature.
Eye Wall: The area surrounding the eye is referred to as the ‘eye wall.’ It is characterized by a robust, spiraling ascent of air that reaches higher altitudes, often extending to the tropopause. Winds in this region reach their maximum velocity, reaching speeds as high as 250 km/h, accompanied by torrential rainfall.
Rain Bands: Adjacent to the eye wall, the region known as Rain Bands experiences relatively low levels of rainfall.
Annular Area: This outer boundary of the cyclone is marked by the descent of air, creating unfavorable conditions for rain.
External Vascular Belt: The outer region adjacent to the annular area experiences connective rainfall due to the rising of air in the form of a convective wave, induced by field effects.

Regions of Tropical Cyclone

Hurricanes are the term for tropical cyclones in the Caribbean Sea region.
In the Philippines, the coasts of China, and Japan, tropical cyclones are known as Typhoons.
The Indian Ocean region refers to such cyclones as ‘cyclones.’
The North-East and North-West coasts of Australia are familiar with them as willy-willies.

Comparison Between Temperate Cyclone and Tropical Cyclone

Aspect	Tropical Cyclone	Temperate Cyclone
Area and Movement	Affects a relatively smaller area and moves East to West.	Affects a much larger area and moves West to East.
Isobars and Pressure	Isobars usually form complete circles, pressure gradient is zero.	Isobars are usually ‘V-shaped, pressure gradient is low.
Wind Velocity and Rainfall	Higher wind velocity, heavy rainfall.	Lower wind velocity, slow and continuous rainfall.
Formation and Dissipation	Forms on seas with temperature > 26°-27°C, dissipates on reaching land.	Can form on both land and sea, lasting for many days.
Duration	Typically lasts for no more than 7 days.	Can last for 15 to 20 days.

Naming of Cyclone

Cyclones are named to make identification easier, as using numbers and technical terms would be challenging. It becomes crucial to assign a name to a cyclone when its speed exceeds 34 nautical miles. A storm earns the designation of a hurricane, cyclone, or typhoon if its speed surpasses 74 mph.
Previously, cyclones were categorized based on latitude and longitude. Presently, factors such as a cyclone’s origin, features, and extent are taken into consideration during the naming process.
The practice of naming cyclones was introduced in 1953 and has been followed since then. Until 1979, cyclones were exclusively named after women, but male names were introduced that year. The World Meteorological Organization (WMO) has compiled six lists of names, which can be reused after a 6-year cycle.
Cyclone Tauktae, for example, was named by Myanmar after a reptile, signifying “Highly Local Lizard.”

Anti-Cyclones

An anti-cyclone is a high-pressure area surrounded by low-pressure zones on all sides, with winds flowing outward from the center.
Due to the Earth’s rotation, winds move clockwise in the Northern hemisphere and counterclockwise in the Southern hemisphere.
Isobars in an anti-cyclone are generally circular, but they can occasionally take a V-shaped form.
The pressure difference between the center and periphery ranges from 10 to 20 mb, sometimes reaching 35 mb. Anti-cyclones lack fronts, and descending winds from above bring clear and rainless weather, promoting atmospheric stability.
The temperature in anti-cyclones depends on the nature of the air mass and humidity in the air. Wind velocity is comparatively lower, and rainfall is slow and continuous over an extended period.
Seasonal Phenomena: During the season of warm air masses, anti-cyclones develop, characterized by an average velocity of 30 to 50 km/hr.

Thunderstorms:

Intense convection on hot and moist days leads to thunderstorms, featuring well-grown cumulonimbus clouds that produce thunder and lightning. When these clouds reach heights with sub-zero temperatures, they generate hail, resulting in hailstorms. Insufficient moisture can cause a thunderstorm to transform into a dust storm.

Tornadoes:

Typically occurring in middle latitudes, tornadoes over the sea are referred to as waterspouts. Measured by the Enhanced Fujita Scale ranging from 0 to 5, tornadoes are prevalent in the USA, Australia, France, the UK, Germany, Argentina, South Africa, and India.

Waterspouts:

An intense columnar vortex, appearing as a funnel-shaped cloud, occurs over bodies of water. Waterspouts are a natural phenomenon common in tropical and subtropical regions.

Types of Anti-Cyclones:

Cold Anti-Cyclones: Also known as thermal anti-cyclones, these form above polar regions due to sinking air and the post-subsidence outflows in an Easterly and South-Easterly direction.
Warm Anti-Cyclones: Referred to as dynamic anti-cyclones, they form above warm subtropical regions with air sinking from the upper troposphere to the lower troposphere, resulting in air divergence.
Blocking Anti-Cyclones: Developing due to obstruction in the upper troposphere’s air circulation, these mid-latitude blocking anti-cyclones hinder the flow of temperate cyclones.

Color Codes for Cyclone Stages:

Cyclone Alert: Yellow
Cyclone Warning: Orange
Post-Landfall Outlook: Red

Wind Speed and Destruction:

119-153 kph: Mild impact
154-177 kph: Extensive impact
178-208 kph: Heavy loss, building collapse, electricity supply disruption
209-251 kph: Greater loss, flood situations
252 kph or more: Maximum destruction, infrastructural damage

Cyclone Forecasting System:

Cyclone forecasting in India is conducted by the Regional Meteorological Centre under the Ministry of Earth Sciences. The process involves four steps:

Advance Cyclone Surveillance: A 72-hour advance notice is given about the cyclone in the Northern Indian Ocean.
Cyclone Danger Warning: A 48-hour advance warning is issued regarding changes in weather conditions.
Cyclone Warning: An exact 24-hour advance warning is issued, including the prediction of the landfall point.
After Landfall: A warning is issued 12 hours before the onset of the cyclone.

Prelims facts

The tropical cyclone of Phillippines is termed as – Bagyo [APSC (Pre) 2016)
Which region of the USA is known as Tornado Alley’? – Mississippi Plains/UPPSC (Pre) 2005)
Tornadoes are very strong tropical cyclones originating in – Caribbean sea [UPPSC (Pre) 1996)
In the South Atlantic and South-Eastern Pacific regions in tropical latitudes, cyclones does not originate because – Sea surface temperatures are low (IAS (Pre) 2015]
The diameter of temperate cyclones is bigger than – Tropical cyclone MPSC (Pre) 2024)
The term ‘Baguios’ stands for tropical cyclone is which – Philippines MPSC (Pe) 2020
Dorian, Hagibis, Lekima, and Mitag are the names of Willy-Willy is a tropical cyclone or – the Nots-West Australi 114S PrOl 1905, LIKPSC (Mains)

UPSC NCERT Practice Questions

1. Cyclones of the Bay of Bengal region mainly occur during

(a) the monsoon period

(b) the pre-monsoon period

(d) the pre and post-monsoon period

2. Consider the following statements. GAS (Pre) 2020

1. Jet Streams occur in the Northern hemisphere only.

2. Only some cyclones develop an eye.

3. The temperature inside the eye of a cyclone is nearly 10°C lesser than that of the surroundings.

Which of the statements) given above is/are correct?

(a) Only 1

(b) 2 and 3

(d) 1 and 3

3. The cyclonic storm of China sea is popularly known as

(a) Cyclone

(b) Tornado

(d) Hurricane

4. In the South Atlantic and South-Eastern Pacific regions in tropical latitudes, cyclone does not originate. What is the reason? IAS (Pre) 2015

(a) Sea surface temperatures are low.

(b) Inter-tropical convergence zone seldom occurs.

(d) Absence of land in those regions.

5. The diameters of temperate cyclones are MPSC (Pre) 2014

(a) equal to those of tropical cyclones.

(b) sometimes bigger and sometimes smaller than those of tropical cyclones.

(d) smaller than those of tropical cyclones.

6. The ‘eye’ of the cyclone has

(a) abnormally high temperature and lowest pressure.

(b) abnormally low temperature and pressure.

(d) dense cloud cover and low pressure.

7. Match List I with List Il and select the correct answer by using the codes given below the lists. UPPSC (Pre) 2014, 2020

List II	List I
A. Willy-Willy	2. Australia
B. Hurricanes	1. USA
C. Typhoons	3. Philippines
D. Baguio	China

Codes

(a) 1 2 3 4

(b) 21:43

(d) 2 1 3 4

8. Nisarga, Gati, Nivar, Tauktae and Yaas are names of

(a) new fighter aircrafts

(b) tourist places

(d) cyclones

9. To measure the intensity of tornadoes, we use

(a) Mercalli scale

(b) Fujita scale

(d) Richter scale

Know Right Answer

1 (d)

2 (c)

3 (c)

4 (a)

5 (c)

6 (a)

7 (b)

8 (d)

9 (b)

Frequently Asked Questions (FAQs)

1. What are air masses and how do they affect weather?

Air masses are large bodies of air with relatively uniform temperature and humidity. They form over large areas with similar surface properties, like oceans or deserts. When air masses move, they carry their weather characteristics to new regions. For example, a warm, moist air mass from the tropics can bring heavy rain to a colder area. Different types of air masses (arctic, tropical, polar, equatorial) influence weather patterns depending on their temperature and moisture content.

2. What are fronts and how do they cause stormy weather?

Fronts are the boundaries between two different air masses. When air masses with contrasting temperatures and humidities meet, they don’t readily mix. The denser, colder air wedges under the warmer air, forcing it to rise. As the warm air rises, it cools and condenses, forming clouds and precipitation. The type of front (cold, warm, occluded) determines the intensity and speed of the weather changes. Cold fronts typically bring sudden, heavy rain or snow, while warm fronts usually lead to gradual, lighter precipitation.

3. What are cyclones and how do they cause wind and rain?

Cyclones are areas of low pressure in the atmosphere, often associated with stormy weather. Winds blow counterclockwise (clockwise in the Southern Hemisphere) around the center of a cyclone, converging towards the low pressure area. This convergence also forces air upwards, leading to cloud formation and precipitation. Depending on their strength and location, cyclones can bring everything from light drizzle to strong winds, hurricanes, and typhoons.

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UPSC NCERT Notes – Geography – Air Masses, Fronts and Cyclone