Wednesday, October 28, 2009

Indian Ocean

Indian Ocean The current circulation in the Indian Ocean is different from those of the Atlantic and the Pacific Oceans. Important, Indian Ocean currents are:
South-West Morisoon Drift In summer, when the domi­nant wind is the south-west monsoon, the currents are blown from a south-westerly direction as the South-West Monsoon Drift.
North-East Monsoon Drift In winter, the dominant north­east monsoon blows the currents from the north-east as the North-East Monsoon Drift.
West Wind Drift A cold current, the West Wind Drift moves across the ocean in the higher latitudes from west to east. One of the branches of this cold current turns northwards along the west coast of Australia. This current, known as West Australian Current, flows northward to feed the South Equatorial Current.
Regional Currents in the Indian Ocean include the Agulhas current and Mozambique current.

Atlantic Ocean

Atlantic Ocean In the Atlantic Ocean, the steady trade winds in north and south of the equator drive two streams of surface water westward. They are known as North and South Equatorial Currents. They are warm cur­rents. Other important currents of the Atlantic Ocean are as follows:
Guinea Current A current called the Equatorial Counter Current flows from west to east between the two main equatorial currents. It is known as the Guinea current off the West African coast.

Brazil Current The South Equatorial Current is split into two branches near Cape de Sao Roque (Brazil). One branch turns south as the warm Brazilian current.
Gulf Stream It is a warm current of sea water. It originates in the eastern Gulf of Mexico before flowing past Florida and the eastern seaboard of the USA. The warm Gulf stream combines with the prevailing south-westerly winds to produce the temperate climate of north-west Europe and keeps the Norwegian coast ice-free during the winter months.

Canary Current It is a cold current which flows south­wards past Madeira and the Canaries off the Atlantic coast of North Africa. It is caused by the upwelling of colder water from greater depths owing to the southward deflec­tion of the west wind drift in the North Atlantic and the transference of surface water back across the Atlantic by the North Equatorial Current.

Labrador Current The Labrador current (warm) flows southwards along part of the east coast of Canada and meets the warm Gulf Stream. The confluence of these two currents, one hot and the other cold, produces the famous fogs around Newfoundland.
Benguela Current It is a cold ocean current flowing northwards off the coast of South West Africa. It is characterised by the upwelling of relatively cold water and has an associated effect on the coastal climate of the region.

Florida Current It is one of the important ocean currents of the North Atlantic. It commences as a branch of the North Equatorial Current, which enters the Caribbean and then returns to the Atlantic Ocean through the Florida Straits as the Florida current.

Pacific Ocean

Pacific Ocean North Equatorial Current A warm current, it flows westwiirds in the north of the equator. This current is produced by the north-east trades.
South Equatorial Current A warm. current, it flows westwards in the south of the equator. This current is produced by the south-east trades.

Kuro Siwo Current The north-east trade winds blow the north equatorial current off the coasts of the Philippine~ and Formosa into the East China Sea as the Kuro Siwo 01 Kuroshio or Japan current. It is a warm current.

North Pacific Current From the south-east coast oj Japan, the current flows right across the ocean from wes1 to east as the North Pacific current. North Pacific curren1 is a warm current.

Alaska Current A cold current, Alaska current flows anti-clockwise along the coast of British Columbia and Alaska. This current is a branch of the North Pacific current.
Californian Current A cold current, the Californian current flows southwards along the Pacific coastline of the USA. This current is caused by the upwelling of colder water from greater depths due to the southward deflection of the North Pacific Current. The Californian current eventually joins the North Equatorial Current, thus com­pleting the cycle.

Peru or Humboldt Current Reaching the south western coast of South America, the South Pacific Current turns northward as Peru current. It is a cold current.
Counter Equatorial Current Between the North and the South Equatorial Currents, a current flows from west to east which is known as the Counter Equatorial Current, a warm current.

Oya Siwo or Oyashio Current The Cold Alaskan current or Bering current creeps southwards from the narrow Bering strait and is joined by the Okhotsk current (a cold current) to meet the warm Japan current as the Oya Siwo or Oyashio (a cold current).

Movements in the Oceanic Waters

Movements in the Oceanic Waters The movement of ocean waters takes place in three different ways: waves, currents and tides.

Ocean Waves Waves are oscillatory movements in water, manifested by an alternate rise and fall of the sea surface. Waves are mainly produced by winds. When blowing winds impart their energy to the water in the form of friction Md pressure on the surface of the sea, waves are produced. There are three types of wind-generated waves:
(i) Sea Several trains of differing wave lengths and directional movements of sea waves occur simultaneously in the oceans. It results into an overall irregular and chaotic wave pattern called the sea.
(H) Swell Swells are the regular undulating movement of the surface of the open sea. They result from waves that have moved out of the area in which they were generated and have ceased increasing in size.
(iii) Surf The breaking waves in coastal regions are called surf.

Ocean Currents
The ocean currents are the most important of the movements in the oceanic water. The ocean current is the general movement of a mass of water in a fairly defined direction over great distances. Ocean currents can broadly be divided into two types: (i) warm current, and (H) cool or cold currents. Those that flow from equatorial regions polewards have a higher surface tem­perature and are warm currents. Those that flow from polar regions equatorwards have a lower surface temperature and are cold currents.
The origin and nature of the movement of the currents are related to four sets of factors. (i) factors related to earth's rotation (gravitational force and force of deflection), (ii) factors originating outside the sea (atmospheric pres­sure, winds, etc.), (Hi) factors originating within the sea (pressure, salinity, etc.), and (iv) factors modifying the ocean currents (direction and shape of the coast, seasonal variations, etc.).

In the Northern Hemisphere, the currents move to' their right and in the Southern Hemisphere to their left. However, as an exception, in the northern part of the Indian Ocean, the direction of currents changes in response to the reversal of the monsoon winds. Warm currents move towards the cold; seas and cold currents towards the warm oceans. The direc\ion of currents in the Northern and Southern Hemispheres is determined by Coriolis force or Ferrel's law.

OCEAN WATERS AND THEIR CIRCULATION

OCEAN WATERS AND THEIR CIRCULATION
The temperature and salinity are two important properties of the ocean water, which determine the movements of large masses of water, their characteristics and the types, and' also the types of marine flora and fauna.

Temperature decreases according to the increasing depth of the ocean. In general, temperature in ocean waters varies from below -5°C to over 33 "c. In the oceans, temperature of the surface water varies from one part to another. Wa!er is hottest near the equator and temperature gradually decreases polewards. The annual range of tem­perature is greater in the Atlantic than in the Pacific Ocean due to the difference in their size. The highest temperatures are recorded in the enclosed tropical areas. For example, the average summer temperature of the surface water of the Red Sea is about 30 "c.

The amount of salinity also varies from one part of 1200 the ocean to another" The areas of the highest salinity are found near the tropics. In the Atlantic Ocean, the salinity near the tropics is about 37 per cent. From the tropical areas of the oceans, the salinity decreases both towards the equator and towards the poles. In the polar seas, there is less salinity - between 20 per cent and 32 per cent - due to very low amount of evaporation (in the tropics, there is active evaporation owing to clear skies).

Ocean and Oceanography

RELIEF FEATURES OF OCEANS
The floors of the ocean are rugged and complex with long mountain ranges, deep trenches and large plains. The development of the sonic depth recorder has made it possible to measure depth for mapping the ocean floor indirectly with the help of sound waves. The ocean floor can be divided into four major divisions.
(i) Continental Shelf The continental shelf is the gently sloping part of a continent that lies submerged below the sea. The shelf is generally formed due to a relative rise in sea level or marine deposition beneath the water. The average width of continental shelf is about 70 km, while the average depth is about 200 m. In all, about 7.5 per cent of the total area of the oceans is covered by the continental shelves. The North Sea and the Baltic Sea are examples of seas that lie on the continental shelf. Such seas are known as epicontinental or shelf seas.

About 20 per cent of the world production of petro­leum and gas comes from shelves.
(ii) Continental Slope The continental slope is the comparatively steep slope that descends from the edge of the continental shelf to the deep-sea platform. There are five type of slopes: (i) fairly steep with the surface dissected by canyons, (ii) gentle slope with elongated hills and basins, (iii) faulted slopes, (iv) slopes with terraces, and (v) slopes with sea-mounts.
(iji) Continental Rise The continental rise or apron is the gently inclined slope which leads up from the deep­sea floor (abyssal zone) to the foot of the continental slope. In other words, where the continental slope ends, the continental rise begins. The continental rise has an average slope of between 0.5° to 1° and its general relief is low.
(iv) Abyssal Plain The abyssal plains or abyssal floors are the areas of deep-ocean floor found at a depth of 3,000 to 6,000 metres.

They occupy about 40 per cent of the ocean floor and are present in all major oceans and several seas of the world. Besides, these four broad features, there are many associated features. Submarine ridges are the mountain ranges on the sea floor that rise above the general level of the abyssal plain. These ridges are either broad like a plateau, or gently sloping or steep-sided narrow mountains. The mid-ocean ridges are the largest. Areas of upland smaller than a mountain are found on the deep sea floor.

These upland areas are called abyssal hills. Seamount is an isolated mountain rising abruptly some 1,000 m from the ocean floor but. without extending above sea level. Seamounts are of volcanic origin, and are com­mon in the Pacific Ocean. Flat-topped sea-mounts are known as guyots. The summits of guyots are almost entirely at depths of more than 1,000 m but rise upto 3 km from the ocean floor. The conical shape of the guyot suggests a volcanic origin. Like seamounts, guyots are found in abundance in the Pacific Ocean. A long, narrow and steep-sided depression on the bottom of the ocean is called a trench or a deep. Trenches or deeps are the deepest parts of the oceans with their bottoms far below the average level of the ocean floors.

They are most common in the Pacific Ocean. Submarine canyons are deep gorges on the ocean floor. They occur around all the coasts of the world and are mainly restricted to the continental shelf, slope and rise. Hudson Canyon is the best known canyon in the world. The largest canyons in the world occur in the Bering Sea off Alaska. They are the Bering, Pribilof and Zhemchung canyons.

A bank is a more or less flat-topped elevation located in the continental margins. The depth of water over a bank is relatively small. The Dogger Bank in the North Sea and the Grand Bank in the Atlantic off Newfoundland are famous examples. A shoal is a detached elevation with shallow depths. The shoal is not composed of a rock or coral.

A reef is predominantly organic deposit made by living or dead organisms that forms a mound or a rocky elevation like a ridge. It may be permanently submerged, submerged at high tide, or normally just above the water. The word reef is most commonly used for coral reefs built up in tropical seas by the hard calcareous material secreted by minute sea animals. The largest reef in the world is found off the Queensland coast of Australia.

Facts About Position of States

Facts About Position of States
. Jammu and Kashmir, Himachal Pradesh, Sikkim and Arunachal Pradesh are the states of India lying cor,npletely in the Himalayas, while the state of Uttaranchal lies partly in the Himalayas and partly in the northern plains.

. Madhya Pradesh, Chhattisgarh, Jharkhand, Maharashtra, Orissa, Andhra Pradesh, Karnataka, Kerala and Tamil Nadu together make the great peninsular plateau.

. Arunachal Pradesh, Nagaland, Manipur, Mizoram, Tripura and Meghalaya are the l:1dian provinces made up of hills in the north-east India.

. Gujarat, Maharashtra, Goa, Karnataka, Kerala, Tamil Nadu, Andhra Pradesh, Orissa and West Bengal are states on the coast of India. Among our Union Territories, Daman and Pondicherry have sea boundaries.

. The Indian states on international boundaries (other country/ countries within brackets) are: Gujarat (Pakistan), Rajasthan (Pakistan), Punjab (Pakistan), Jammu and Kashmir (China and Pakistan), Himachal Pradesh (China), Bihar (Nepal), Uttaranchal (China and Nepal), Uttar Pradesh (Nepal), West Bengal (Bhutan and Bangladesh), Sikkim (China, Bhutan and Nepal), Arunachal Pradesh (Bhutan, China and Myanmar), Nagaland (Myanmar), Manipur (Myanmar); Mizoram (Bangladesh and Myanmar), Meghalaya (Bangladesh), Tripura (Bangladesh), and Assam (Bhutan, Bangladesh and Myanmar).

. The states of Haryana, Madhya Pradesh, Chhattisgarh and Jharkhand are the only land-locked states. They are not on an international border either.

INDIA: LOCATION AND SIZE

Situated in south-central Asia, the Indian subcon­tine':!t extends southward into the Indian Ocean, with the Arabian Sea to its west and the Bay of Bengal to its east. It is located north of the equator, hence in the northern hemisphere. Lying between 80 4' Nand 370 18' N latitude and 680 7' E and 970 25' E longitude, the country is divided into almost equal parts by the Tropic of Cancer (230 30' N).

The southern half which coincides with peninsular India lies in the tropical zone, while the northern half belongs to the subtropical zone and is somewhat continental in nature. India also belongs to the eastern hemisphere as it lies to the east of the Prime Meridian.

The 820 30' E longitude is taken as the Standard Time Meridian of India, as it passes through the middle of India (near Allahabad). It means that when it is noon in Allahabad, the time for the whole of India is taken as noon. This is so despite the fact that at places lying east of 820 30' E, I.e., the Standard Meridian, it is already past noon, and at places lying west of Allahabad noon is yet to come. (India, for such a large country, is not divided into different time zones.)

In terms of area, India is the seventh largest country in the world occupying 2.2 per cent of the world area. It is 12 times as big as the United Kingdom and nearly one-third of Canada. Population-wise, it is second in the world, after China; 16 per cent of world population is said to inhabit India.

The country covers an area of 32,87,263 sq km. It measures about 3,214 km from north to south and about 2,933 km. from east to west.
. Mainland India has a coastline of 6,100 km. Including the Lakshadweep and Andaman and Nicobar, the coastline measures about 7,516.6 km. To the north-west, north and north-east, India has land fron­tiers measuring about 15,200 km in length.
The southernmost point of the Indian territory is
the Indira Point in Great Nicobar, while Kanyakumari, also known as Cape of Comorin, is the southernmost point of the Indian mainland. T.he Bay of Bengal, the Indian Ocean and the Arabian Sea meet at Kanyakumari.

India's international boundaries are mostly natural and historically determined. Our country is bound in three directions by the Arabian Sea, the Indian Ocean and the Bay of Bengal. In the north and east, the mighty Himalayas and its offshoots form the international boundary of the country. However, India shares its frontiers with many countries such as Pakistan, China, Nepal, Bhutan, Myanmar and Bangladesh. If we take into consideration Pakistan-occupied Kashmir, Afghanistan is also one of India's neighbours.

. The nearest neighbour of India across the sea is Sri Lanka which is separated by the narrow Palk Strait from the mainland of India. Maldive island is another neighbour situated to the south of Lakshadweep islands. Myanmar is our immediate neighbour across the Purvanchal Hills and the Bay of Bengal. Other neighbours on these frontiers include Malaysia, Indonesia, Thailand, Laos and Vietnam. To the west are Pakistan, Afghani­stan, Iran, Iraq and the Arabian countries. Egypt, Sudan, Ethiopia, Somalia, Kenya and Tanzania are our neighbours across the ocean. Five countries of Asia-Ghina, Uzbekistan, Afghanistan, Pakistan and India-meet at the apex of the North Indian Triangle. At the easternmost point of the Indian border is the north-eastern tri-junction, where India, China and Myanmar meet.

THE COASTAL PLAINS

THE COASTAL PLAINS The 4,500-km long coastline of India has the Arabian Sea on its west, the Bay of Bengal on its east and the Indian Ocean on its south. It runs from the Rann of Kutch in the west to the delta of the Ganga - Brahmaputra in the east. The coastal plain in India has been divided into the following two types.

The East Coastal Plains These extend between the source of the Subarnarekha and Kanyakumari and contain deltas of almost all rivers in the region except Tapti and Narmada as these flow towards the Bay of Bengal. The Eastern Coastal Plains include Tamil Nadu Plain, Andhra
Plain, and Utkal Plain.

The West Coastal Plains
About 10 to 25 km wide, they run from the Rann of Kutch to Kanyakumari, They comprise the sandy plains of the Kutch peninsula and the Kathiawar peninsula in the north.

ISLANDS India has a number of islands (247) both in the Bay of Bengal, (204 islands) and the Arabian Sea (43 islands). The Andaman and Nicobar Islands in the Bay of Bengal represent the elevated portions of submarine mountains. Mount Harriet (460 m) in the South Andaman
Island is the highest point. The Andamans have a physical. base derived from sandstones, limestones and clays. ~he Nicobar group contains 19 islands. The Great Nicobar is the largest island in the group.

THE GREAT PLAINS

THE GREAT PLAINS Between the Himalayas and thE Peninsular Plateau are found the Great Northern Pljiins tha stretch in an east-west direction for about 2,400 km. The) cover more than seven lakh sq km and have a widtt ranging from less than 200 km (as in Bihar) to 500 km (af in Punjab and Rajasthan). The depth is about 1,300-1,400n on an average. Alluvial in nature, these plains are composec of older alluvium (bhangar) or the newer alluvium (khadai or bet). The rivers flowing from the Himalaya deposi pebbles and coarse sand in forelands ca~led bhabar or ghar Small rivers that disappear below the ground in thE bhabars reappear in the plains, creating large swamps callec terais.

The Great Plains have a number of sub-p~ains, such as
The Punjab-Haryana Plains With a distance of 640 km from north-east to south-west and 300 km from west t< east, these flat plains occupy 1.75 lakh sq km and have c height ranging from 200 m to 300m. They comprise tht Bist Doab (between Sutlej and Beas rivers) the Upper Bar Doab (between Beas and Ravi rivers), and the south-eas Malwa Plain.

The Rajasthan Plains These comprise the marusthal of Thar and the nearly Bagar areas and occupy an area 0 1.75 lakh sq km. Sand dunes are longitudinal (in the wes and south) and transverse (in the east). The Luni rive flowing towards the south-west is the only river in thl region.' The Rajasthan desert is sloping towards two direc tions: (a) westwards to the Indus Valley in Pakistan, an( (b) southwards to the Rann of Kutch.

The Ganga Plains Spreading across the states of Uttar Pradesh, West Bengal and Bihar for 3.57 lakh sq km, the plains comprise the raised bhangar areas and khadar areas. The Ganga Plain is divided into the (a) Ganga-Yamuna Doab, (b) Rohilkhand Plain, (c) Avadh Plain (covering the eastern half of Uttar Pradesh), (d) Bihar Plain, and (e) Bengal Plain. The Ganga delta, which constitutes the Bengal basin, has part of it stretching along the sea and covered with tidal forests (the Sunderbans).

The Brahmaputra Plains The low-level plains formed by deposits carried by mainly the Brahmaputra river but also the Diband, the Sesiri and the Luhit are bordered by high mountains. Moist soil conditions and thick forests form the northern extreme.

THE PENINSULAR PLATEAU

THE PENINSULAR PLATEAU The peninsular plateau is the largest and the oldest of all the physiographic divisions. Its north-west limit is marked by the Aravalli range and its northern extreme has the raised Bundelkhand. At its western and eastern ends are the Western Ghats and the Eastern Ghats respectively. It has the shape of an inverted triangle.

The plateau can be divided as follows.
The Central Plateaus The Central Plateaus are the
upland of Central India with the River Ganga to its north, Vindhya and Satpura ranges to its south, the Kaimur­Maikal range to its east, and the Aravallis and Kutch to its west. The Central Plateaus comprise Mewar, Malwa and Vindhya plateaus. Malwa Plateau in Madhya Pradesh formed of lava flows lies to the north of the Vindhya range.

The Eastern Plateaus These lie to the north-east of the Malwa plateau. The topography of the Eastern Plateaus is diversified, ranging from the low-lying Mahanadi basin to undulating plateaus of the Baghelkhand, Chhota Nagpur and Dandakaranya. Beyond the lava-ridden Rajmahal Hills that form the eastern end of Chota Nagpur Plateau is the Meghalaya Plateau.

The Kathiawar and Kutch Plateaus
These plateaus are joined to the peninsular plateaus by Gujarat's plains. Deccan lavas make up the Kathiawar region, while the Kutch ha~ an abundance of tertiary rocks.

The Deccan Plateaus A triangular plateau, they OCCUp) land between the Western and 'Eastern Ghats and south 0 the Mahadeo, Maikal and Satpura ranges. The Deccar Plateaus comprise the Maharashtra Plateau (formed entirel) of basalt) and Karnataka and Telangana (Andhra) Plateaw (both composed of Archaean gneisses. Deccan Plateaus an India's largest plateaus.

The Northern Mountains

The Northern Mountains These are the Himalayas that stretch across northern India from Jammu and Kashmir to Arunachal Pradesh for about 2,400 km. The Himalayan range has the shape of a crescent. The Himalayas, the worlds's youngest fold mountains, achieve their greatest heights in eastern Nepal.

Breadthwise, the Himalayas may be classified into the following four zones.
(i) The Outer Himalayas or the' Siwaliks With a height ranging from 600 to 1,500m and a width of 15 to 50km, the S'iwaliks are composed of tertiary sediments mainly containing boulder and clay. They emerged in the most recent phase in the Himalayan orogeny, i.e., during the Mi­ocene-Pleistocene epoch.

The Outer Himalayas are separated from the Lesser Himalayas by duns - valleys resulting from draining away of the temporary lakes formed by ridges when these obstructed the course of rivers.

The Lesser or Middle Himalayas (The Himachal) The intricate and rugged hills with deeply-cut rivers are made of rocks of the Algonkian-Eocene times. The Lesser or Middle Himalayan hills have a hogback appearance. The ranges include the 400-km Pir Panjal range, the longest range of the Himalayas.

The Greater Himalayas or the Himadri
The innermost Himalayan range is the world's highest, with an average height of 6,000 m. It is the most continuous of the Himalayan ranges and forms the Himalayas' northernmost part. The Himadri's highest peaks include the world's highest peak, Mt Everest.

The Trans-Himalayan Zone With a width of 40 km in its eastern and western ends and a width of 222 km in its central part, it has important ranges such as the Zaskar Range and the Great Karakoram range. The Karakoram extends towards the south-east to form the Kailash Range (Tibet). The highest peak in the Karakoram range is ~ (8,611 m). The longest glacier is Siachen in the Nubra Valley which is more than 72km long.

PHYSIOGRAPHIC DIVISION OF INDIA

PHYSIOGRAPHIC DIVISION OF INDIA
India has a variety of physical features ranging from small islands to one of the highest peaks of the world. Physiographically, India can be divided into three major physiographic units-the mountain, the peninsular pla­teaus, and the plains. To these may be added the fourth, namely, the coasts and islands.
MOUNTAINS The Peninsular Mountains The moun­tains of peninsular India can be classified as follows:

The Western Ghats (The Sahyadri) About 1,600 km long and with an average elevation of 1,000-1,300 m, they run along the west coast from the south of the Tapti river valley to Kanyakumari. The Deccan Trap constitutes the northern portion of the Ghats. The southern part of the Ghats is composed of Archaean gneisses, schists and charnockites.

The Eastern Ghats These irregular hill ranges stretch from northern Orissa to the Nilgiris in Kerala and Tamil Nadu across coastal Andhra. They are mainly ranges of garnetiferous sillimanite gneisses or khondalite and charnockites. The highest peak is at 1,680 m in Vishakhapatnam district of Andhra Pradesh.
The meeting point of the Western and Eastern Ghats in the Nilgiri Hills has the highest point at Doda Betta peak.

The Vindhya Range Running from Sasaram (Bihar) in the east to Jobat (Gujarat) in the west, the Vindhya range separates northern India from the southern mainland. Mostly composed of sandstones and quartzites, it is lava­ridden in the west.

The Satpura Ra,zge Covered with lava, the range ex­tends from the Narmada valley in the north to the Tapti valley in the south. Its northern and southern borders are composed of sandstone, while the eastern section has Gondwana and Archaen gneisses. The average elevation is 1,030 m above sea level, with the highest point at Dhupgarh (1,350 m) near Pachmarhi (Madhya Pradesh).

The Aravallis M~de of the rocks of quartzites, gneisses and schists belonging to the Pre-Cambrian age, the Aravalli ranges constitute one of the most ancient fold mountain ranges of the world. Stretch­ing 800 km from the north-east to south-west of India, they separate the semi-desert and fertile regions of Rajasthan. The Gurusikhar peak (1,722 m) of the Abu Hills is the highest point of the range.

Australia

Australia is the smallest continent. It is also the only country that is also a continent. It covers 7,682,300 sq km, or about 5 per cent of the earth's land area. Mainland Australia extends from 12°S to 38°S latitudes (12°S to 44"S if we include Tasmania) and 114° E to 154° E longitudes. Australia lies entirely in the Southern Hemisphere.

Australia can be divided into three major physical divisions-from west to east, the Western Plateau, the Central Lowlands, and the Eastern Highlands. The Western Plateau covers the western two-thirds of Australia. Most of the land is flat and covered with small shrubs, though at places isolated mountain ranges rise above the general level. A vast, dry, treeless plateau called the Nullarbor Plain extends along the southern edge of the Western Plateau.

The Central lowlands have the lowest elevations in Aus­tralia. Lake Eyre, the lowest point in Australia (16 metres ,below sea level), lies in the Central Lowlands. The Lowlands extend from the Gulf of Carpentaria in the north, across the continent to the southern shores. The region receives scanty rains, except in areas along the north and south coasts imd near the Eastern Highlands. The Eastern High­lands run almost parallel to the east coast of Australia, extending from Cape York Peninsula in the north to Tasmania in the south.

The region includes the highest elevations in Australia. Along the Pacific Coast lies a low plain bordered by sandy beaches and rocky cliffs. The Eastern Highlands are known as the Great Dividing Range because their slopes divide the flow of the rivers in the region. Those flowing down the eastern slopes drain into the ocean. Those rurming down the western slopes flow to the Central Lowlands. The Eastern Highlands are broad and low in the north and narrow and high in the south.

Australia's highest mountains rise in the Australian Alps in the extreme southern part of Eastern Highlands. Mount Kosciusky (2,228 m), in the Snowy Range of. the Alps, is the highest peak in Australia. The highlands drop steeply in the east towards the Pacific Ocean and more gently in the west towards the Central Lowlands. The Highlands are not a single range; they consist mainly of high plateaus that are broken in many places by gorges, hills, and low mountain ranges.

Most of the rivers of Australia go dry in some parts of the year and fill with water only during the rainy season. River Murray, originating from the Snowy Mountains in the Alps and flowing westward, is the continent's longest permanently flowing river. During the dry season in southern Australia, the Murray is fed by River Darling, Austalia's longest river. River Darling begins in the central part of the Eastern Highlands and flows southwestwards.
Deserts cover about a third of Australia. The four major deserts are the Simpson (western edge of the Central Lowlands), the Gibson, the Great Sandy and Great Victoria (central part of the Western Plateau) Off the north-east coast of Australia exists the Great Barrier Reef-the world's largest coral reef, extending more than 1900 km in length.

Australia lies south of the equator and so its seasons are opposite those in the Northern Hemisphere. Northern Australia lies in the tropics and so experiences warm season throughout the year. The wet season lasts from November through April and the dry season lasts from May through October. Southern Australia has four seasons-winter (June through August) is the coolest and wettest season and summer (December through February) is the hottest and driest season.

Europe

Europe ranks sixth in terms of area of continents. It covers 10,521,100 sq km or about one-fifteenth of the world's land area. It extends between 25° Wand 65° E longitudes and 35° Nand 73° N latitudes-from the Arctic Ocean in the north to the Mediterranean Sea, the Black Sea and the Caucasus Mountains in the south, and from the Atlantic Ocean in the west to the western part of Russia in the east. Europe is a westward extension from north­western Asia. Asia and Europe are not separated com­pletely, and the two regions are sometimes regarded as one continent-Eurasia. However, because of its certain distinc­tive characteristics, Europe is considered to be a continent by itself.

Europe is divided into four major physical divisions­the Northwest Mountains, the North European Plains, the Central Uplands and the Alpine System. The Northwest Mountains extend from Finland through Sweden, Norway and the British Isles to Iceland, and include some of the oldest rock formations on earth. The mountains have worn down over the years and the average elevation is not very high. The area is rich in metallic minerals such as iron and copper.

The North European Plains extend from the Urals in the east to the Atlantic coast in the west. The region also includes part of south-eastern England. The plains are broadest in the eastern part and narrow down towards the west. To the north of the North European Plains lie the Northwest Mountains, the White Sea and the Baltic Sea, and to the south lie the Central Uplands. The plains comprise flat and rolling land and some hills. They constitute some of the world's most fertile farmlands. In some places beneath the plains lie deposits of high-grade fossil fuels such as coal, oil and natural gas.

The Central Uplands region is constituted of a variety of low mountains, hills and plateaus that run through the central part of Europe. The uplands include the Meseta in Spain and Portugal, the Massif Central and Juna mountains in France, the Black Forest in Germany and .several low ranges in the Czech and Slovak republics. Most of the land is rocky and has poor soil for farming, but some river valleys in the region are suitable to agriculture. Parts of the region are rich in mineral deposits.
The Alpine Mountain System comprises chains of high mountains stretching from the Atlantic Ocean in the west to the Caspian Sea in the east. Important mountain ranges include the Alps (covering part of south-eastern France and northern Italy, most of Switzerland, and parts of southern Germany, Austria and northern Yugoslavia), the Apennines (covering much of Italy), the Alpines (including the Dinaric Alps of Yugoslavia and Greece; the Balkans of Bulgaria; and the Carpathians; and the Caucasus).

The highest mountain peak of Europe is Mount Elbrus (5,633 m) in the Caucasus.
Europe has an irregular and a long coastline of about 60,957 km. Large portions of sea have penetrated deep into the land, with the result that most of Europe (except the heart of Russia) are within about 480 km of a seacoast. The land curves in and out in a series of large and small peninsulas-the Scandinavian peninsula (Norway and Sweden), Jutland (Denmark), the Iberian peninsula (Portu­gal and Spain), the Apennine peninsula (Italy) and the Balkan peninsula (Albania, Bulgaria, Greece, and parts of Turkey and Yugoslavia).
Thousands of islands lie off the coast of Europe. The largest and most important of these is Great Britain, one of the British Isles.
The Volga is Europe's longest river, flowing 3,531 km through Russia to the Caspian Sea. River Rhine flows through the Alps through western Germany and the Netherlands to the North Sea. The Danube, Europe's second longest river, flows from southern Germany through Austria, Czechoslavakia, Hungary, Yugoslavia, Bulgaria and Romania to the Black Sea.
The salt water Caspian Sea, which lies partly in Europe and partly in Asia, is also the world's largest lake. Its northern shore, lying 28 metres below sea level, is Europe's lowest point. Europe's largest freshwater lake is the Ladoga in Russia. Finland has about 60,000 lakes-the country is known as the 'land of thousand lakes'.
Most of Europe experiences mild weather. Northern Europe has longer, colder winters and shorter cooler summers than southern Europe. Eastern Europe experi­ences longer and colder winters and shorter and hotter summers than western Europe.

Antarctica

Antarctica, with an area of 14,000,000 sq km, surrounds and covers the South Pole. The continent is the coldest and iciest region on earth. The Antarctic icecap, which has been formed from layers of snow pressed together over millions of years, buries 98 per cent of the continent and forms the largest body of fresh water or ice in the world. The average thickness of ice on this continent is approximately two to five kilometres. Underneath the ice, Antarctica has moun­tains, lowlands and valleys.

The Transantarctic Mountains, made up of several ranges, cross the entire continent and divide it into two natural land regions-East Antarctica and West Antarctica. East Antarctica faces the Atlantic and Indian Oceans and covers more than half the continent. The region consists of rocks that are more than 570 million years old. The central part of East Antarctica is a plateau about 3000 metres above the sea level. The South Pole, the southern-most point on the earth, lies on this plateau. The south magnetic pole, the southern part indicated by compass needles, is also located in East Antarctica.

West Antarctica faces the Pacific Ocean. Much of the region lies below sea level and does not contain the old rock of East Antarctica. West Antarctica developed later as part of the Ring of Fire, a string of volcanoes encircling the Pacific Ocean. West Antarctica includes several moun­tain ranges and volcanoes. The Antarctic Peninsula is a mountainous strip of land pointing towards South America. The peninsula is said to be a continuation of the Andes mountain chain of South America. Near the peninsula are several islands; the South Shetland Islands include the Deception Island, an active volcano. Vinson Massif, the highest point in Antarctic (5,140 m), is part of the Ellsworth Mountains near the peninsula. Mount Erebus, Antarctica's most active volcano, lies on the Ross Island.

The Ross Sea and the Weddell Sea are gulfs that cut into Antarctica from opposite ends of the Transantarctic Mountains. There are smaller bays indenting the coastline.
Temperatures in Antarctica rarely reach above O°c. At the South Pole, temperatures as low as -95°C have been recorded during winters. The Antarctic winter lasts from May until August. Summer lasts from December until February. Extremely cold dry conditions exist on the inland plateau. The plateau has one of the driest climates on earth; it receives no rain and little new snow each year. Relatively mild and moist conditions mark the coastal regions.
­

South America

South America is the fourth largest continent in terms of area. It covers 17,806,000 sq km or about 12 per cent of the world's land area. It extends from 12° N to 55° S latitudes and 34° W to 81° W longitudes. It is almost completely surrounded by water-the Caribbean Sea on the north, the Atlantic Ocean on the northeast and east, the Pacific Ocean on the west and the Drake Passage on the south. The continent is connected to Central America by a narrow strip of land-the Isthmus of Panama.

The major physical divisions of South America are 'the Western Coastal Strip, the Western Mountains, the Central Plains and the Eastern Highlands. The Western Costal Strip is a narrow. strip of lowland along the Pacific Coast in the western part of South America. The Western Mountains, called the Andes Mountains, stretch for about 7200 kilometres from Venezuela in the north to Tierra del Fuego in the south. The Andes, which are young fold mountains, are the second highest mountain systems in the world, next only to the Himalayas. Mt Aconcagua (6,959 m) in Argentina is the tallest mountain in the entire western hemisphere. The Andes are also the world's longest mountain ranges above sea level. (The longest range is the Mid-Atlantic Ridge which rises from the bottom of the Atlantic Ocean.) Between the ranges of the Andes are some high plateaus. Lake Titicac~, one of the largest lakes of South America, is located in the Bolivian Plateau. The Andes also have several volcanic peaks, some active, others dormant or extinct. Cotopaxi in Ecuador is the highest active volcano in the world.

The Central Plains, covering about three-fifths of South America, lie between the Andes and the Eastern highlands. They are made up of the Llamos, rolling grasslands in the Orinico river basin of Colombia and Venezuela; the Selva, tropical rain forest of the Amazon river basin in Bolivia, Brazil and Peru; the Gran Chaco, hardwood serial forest in north-central Argentina, western Paraguay and southern Bolivia; the Pampas, vast grasslands in Argentina.

The Eastern Highlands consist of the Guyana High­lands and the Brazilian Highlands separated by the Amazon basin. Mountains of the Eastern Highlands are lower and older than the Andes.

The Amazon, the Rio de Ie Plata, the Magdalena-Cauca the Orinoco and the Sao Francisco are the major rive systems of South America. The Amazon is the world' second longest river, but has the largest drainage basin Lake Maracaibo in Venezuela is the continent's largest lake The Angel Falls in South-eastern Venezuela is the highes water fall in the world.

There is a broad zone of desert running southward: from coastal Eucador along the coasts of Peru and Chill and through southern Argentina.

South America has a long coastline-32,000 km. Then are several major island groups- Tierra del Fuego grouF (the largest); the Juan Fernandez Islands; the Falklanc Islands; and the Galapagos Islands.

The continent experiences a variety of climates-from dry desert conditions of northern Chile to the heavy raim along the south-western coast.

North America

North America is the third largest continent, extending from the Arctic Ocean in the north to South America in the south. It covers about 24,249,000 sq km or about a'sixth 6f the world's land area, and extends between (approxi­mately) 84° N to 7° N latitudes and 180° W to 20° W longi­tudes.
The continent has four major physical divisions-the Canadian Shield, the Appalachian Mountains or Eastern Highlands, the Central Plains, and the Western Cordilleras. The Canadian Shield, a huge area of ancient rocks, covers most of Canada.

It is a reservoir of valuable mineral resources like gold, silver, nickel, iron, copper, platinum, radium, cobalt and uranium. A large part of the shield is made up of swamps and a number of lakes such as the Great Bear, Winnipeg and the Great Lakes (Lakes Superior, Michigan, Huron, Erie and Ontario). The famous Niagara Falls is located between Lake Erie and Lake Ontario.

The Appalachian Mountains, located in the north-east, are constituted of low rounded mountains, plateaus and valleys. A number of minerals are found here.
The Central Plains cover central Canada, mid-western United States and the coastal plains in Texas. The mid and southern parts are a vast low and flat river basin of the Missouri-Mississippi.

The Western Cordilleras are a mountainous region along the north to south of the western part of the continent. There are several parallel ranges. The Rocky mountains are North America's largest mountain range. The Alaska Range, the Coastal Range, the Sierra Nevada, the Cascade Range, etc., are the other ranges. Mt McKinley (6,194 m) in the Alaska Range is North America's highest mountain. Fertile valleys between the mountain ranges are major agricultural centres. The ranges also enclose inter­montane plateaus-the Great Basin is the largest in termon­tane plateau ot the continent. Most of the rivers of North America rise in the Western Cordilleras. There are many acttve volcanoes in the Western Cordilleras, especially in Alaska and Mexico.

The Mississippi-Missouri-Ohio is the continent's longest river system. The Grand Canyon of River Colorado is the largest of its kind in the world. Lake Superior is the largest freshwater lake in the world. Great Salt Lake in Utah is even saltier than the ocean. Most North American deserts lie in the southwestern United States and northern Mexico.

North America's coastline-about 300,000 km-is longer than that of any other continent. A number of countries lie off the coasts of the continent-Greenland (part of the North American continent, but governed as a province of Denmark), Newfoundland, Vancouver, the Aleutian Is­lands, and the West Indies.
North America experiences the most varied range of climate-from extremely dry cold of the Arctic type to the hot tropic.

African Continent

Africa is the second largest continent in terms of size, covering about one-fifth of the land area of the earth. Spread over 30,330,000 sq km, Africa is located between 35° 5 and 37° N latitudes and 50° E and 17° W longitudes. The African continent is separated from Europe by the Mediterranean Sea and from Asia by the Red Sea. The Strait of Gibralter in the north-west, the Suez Canal in the north­east and the Strait of Bab-el Mandeb connect Africa to Eurasia.

Africa is an enormous plateau,. most of which is covered by deserts, forests and grasslands. The continent can be divided into two major land regions-Low Africa and High Africa. Low Africa, consisting of northern, west­ern and central Africa, is further subdivided into the Coastal Lowlands, the Northern Highlands, the Saharan Plateau, the Western Plateau, the Nile Basin and the Congo (Zaire) Basin. High Africa covers eastern and southern Africa and may be further subdivided into the Rift System, the Eastern Highlands, the Southern Plateau, the Coastal Lowlands and Madagascar. The Rift System, a spetial physical feature of Africa, consists of the Great Rift Valley, which is a series of parallel cracks in the earth that form deep, steep-sided valleys. Many of these valleys house lakes. There are several large lakes in High Africa. Lake Victoria is the largest lake in Africa and is the source of River Nile, the longest river in the world. Other large rift lakes include the Tanganyika, Nyasa, Albert and Turkana. The Rift System extends from Ethiopia in the east to Mozambique in the south-east. It is rich in volcanic soil and has some of Africa's best farmlands.

Deserts cover about two-fifths of Africa. The Sahara, the world's largest desert, stretches across northern Africa from the Atlantic Sea to the Red Sea. Other deserts include the Namib desert and the Kalahari desert in southern Africa. Grasslands cover more than two-fifths of Africa. Forests cover less than a fifth of the continent.
Other major African rivers include the Congo (Zaire)­which carries the greatest volume of water among all the rivers of Africa-and the Niger, which discharge into the Atlantic, and the Limpopo and the Zambezi which drain into the Indian Ocean. The Nile flows northward from east­central Africa to the Mediterranean Sea.
Most of Africa's highest mountains have been created by volcanic activity. Mt Kilimanjaro (5895 m) in Tanzania and Mt Kenye (5199) in Kenya are extinct volcanoes. The Atlas mountain range, extending from Morocco to Tunisia, forms Africa's longest mountain chain and is of non­volcanic nature.
About 90 per cent of the African continent lies within the tropics-Africa has the largest tropical area in the world.

Asian Continent

Asia is the largest contine] covering almost a third of the world's land area. Asia, wi an area of 43,947,000 square kilometres, extends betwel 80° Nand 10° S latitudes, and 25°E and 170° W longitudE The entire Asian mainland lies north of the equator, b the islands associated with Asia extend into the southel
hemisphere. Most of Asia lies in the eastern hemispher and only a part of it extends to the western hemispher Asia extends from the Arabian peninsula, Turkey and Ur mountains eastward to the Pacific Ocean. From the Aret Ocean, it reaches south to the Indian Ocean.
Asia is home to a variety of natural features, rangin from the hot deserts of the south-west, the cold deser1 in the interior and the frozen north; and from barre un9ultivable regions to the fertile river valleys, coast" pl,ins and delta regions. The highest and lowest places OJ thr earth are in Asia. Mount Everest, the highest, rises 8,84: metres above sea level along the Nepal-Tibet border. Th,
Dead Sea shore, the world's lowest land, lies about 39' metres below sea level on the border between Jordan and Israel.

The five major physical divisions of Asia are the Northern Lowlands, the Central Mountains, the Southerr Plateaus, the Great River Valleys and the Island Groups The Northern Lowlands comprise the Siberian plain whid extends between the Ural mountains in the west and River Lena in the east.

Lake Baikal in Siberia is the deepest lakE in the world. The interior heartland of Asia is surrounded by mountains and deserts. The Central Mountains, com­prising fold mountains and plateaus, lie south of the Northern Lowlands. Asia has more mountains than any other continent. The major Asian mountain systems meet to form a large group of rugged peaks and deep valleys, called the Pamir Knot. Sometimes referred to as the 'roof of the world', the Pamir Knot lies where Afghanistan, China, Pakistan and the Central Asian republics meet. From the Pamir Knot, the Hindukush range extends to the west, the Tian Shan to the north-east, the Kunlun to the east and the Karakoram and the Himalayas to the south-east. The Karakoram and the Himalayan ranges contain some of the highest peaks of the world. Some of the largest glaciers of the world are also found in the Karakoram range. Between the Karakoram and the Himalayas in the south and the Kunlun in the north lies the Tibetan plateau. Between the Kunlun and the Tien Shan in the further north lies the Tarim basin. In between the Tien Shan in the west and the mountain chains of north-east Siberia lie the ancient fold mountain ranges-the Altai, the Yablonoi and the Stanovoi-which house the Tarim basin and the vast cold desert of Gobi.

The Southern Plateaus lie to the south of the Central Mountains. Formed of very old rocks, the Southern Pla­teaus (comprising the plateau of Arabia, the Deccan plateau and the plateau of Yunnan) constitute the major part of the peninsulas projecting southward from mainland Asia. The Great River Valleys lie in-between the mountains and plateaus. Some of these river systems-the Tigris-Euphrates, the Indus, the Ganga-Brahmaputra, the Ayeyarwaddy, the Mekong, the Sikiang, the Chang Jiang (Yangtze-Kiang) and the Huang He-are very old. The Yangtze is Asia's longest river.
Three major island groups are located to the south­east and east of Asia-Indonesia, the Philippines and Japan. Most of these islands have a mountainous core surrounded by narrow coastal plains. Some islands still have active volcanoes.
Asia's coastline measures about 129,077 km.

Because of its tremendous size and varied physical
features, Asia experiences a variety of climates-the bitter cold of the polar north; the hot, dry desert environment of the centre and the southwest; and the hot, humid conditions of the tropical south.

Distribution of Continents and Oceans

About 71 per cent of the globe is water and about 29 per cent is land surface. The continents into which the land surface is broadly divided are: Asia, Africa, North America, South America, Europe, Australia, and Antarctica. If the seas were to drain away, it appears that broad areas lying close to continental shores are actually covered by shallow water less than 180 m deep. Disregarding the earth's curvature, the continents can be visualised as platform-like masses; the oceans as broad, flat-floored basins. Most of the land surface of the continents is less than 1 km above sea-level, while a predominant part of the ocean floor lies between 3 and 6 km below sea level.
Relief features of continents are being discussed here, while those of oceans will be dealt with in a separate chapter Ocean and Oceanography.
RELIEF FEATURES OF CONTINENTS
The continents consist of two fundamental kinds of geological units: (i) the shields, and (ii) the mountain belts or orogenic belts.
The shields are extensive, rigid and permanent part of the earth's crust, composed of Precambrian rocks (more than 590 million years old). These are the oldest rocks on the earth's surface. The mountains of earlier periods were eroded to their roots. The shields today are, therefore, largely plains and low plateaus and are highly stable parts of the earth's continental crust. Crustal movement of
1. Laurentian Shield 2. Brazilian Shield 3. Baltic Shield
4. African Shield
5. Arabian Shield
6. Siberian Shield 7. Deccan Shield
8. China Shield
9. Austra.lian Shield

shields in the later geologic time have been epeirogenic movements, i.e., rise and fall of the crust over broad areas without breaking or bending the rocks. Negative epeirogenic movement resulted in submerging of shields, shallow seas and continental shelves. Later, positive epeirogenic move­ment brought the sedimentary cover that had gathered over time above sea level where it has been carved up by streams into hills and plateaus. The Canadian Shield in North America and the Russian-Baltic Shield of Europe are well-known shields of the northern hemisphere. Similar shield areas occupy parts of Australia, Africa, South America, India and Antarctica. Rocks in these areas date back to the oldest geological time from one to three billion years ago.

Mountain belts of continents are along narrow a genic zones in which crust has been compressed and fon to buckle into tight folds and at the same time to
strongly elevated. The older, inactive mountain ranges he usually been reduced to hill belts. In some places, they i still moderately elevated possessing a rugged relief. 1 young ranges are still active zones of crustal deformati and volcanic activity in some areas. They rise as speetacu alpine mountains.

Distribution of Continents and Oceans

About 71 per cent of the globe is water and about 29 per cent is land surface. The continents into which the land surface is broadly divided are: Asia, Africa, North America, South America, Europe, Australia, and Antarctica. If the seas were to drain away, it appears that broad areas lying close to continental shores are actually covered by shallow water less than 180 m deep. Disregarding the earth's curvature, the continents can be visualised as platform-like masses; the oceans as broad, flat-floored basins. Most of the land surface of the continents is less than 1 km above sea-level, while a predominant part of the ocean floor lies between 3 and 6 km below sea level.
Relief features of continents are being discussed here, while those of oceans will be dealt with in a separate chapter Ocean and Oceanography.
RELIEF FEATURES OF CONTINENTS
The continents consist of two fundamental kinds of geological units: (i) the shields, and (ii) the mountain belts or orogenic belts.
The shields are extensive, rigid and permanent part of the earth's crust, composed of Precambrian rocks (more than 590 million years old). These are the oldest rocks on the earth's surface. The mountains of earlier periods were eroded to their roots. The shields today are, therefore, largely plains and low plateaus and are highly stable parts of the earth's continental crust. Crustal movement of
1. Laurentian Shield 2. Brazilian Shield 3. Baltic Shield
4. African Shield
5. Arabian Shield
6. Siberian Shield 7. Deccan Shield
8. China Shield
9. Austra.lian Shield

shields in the later geologic time have been epeirogenic movements, i.e., rise and fall of the crust over broad areas without breaking or bending the rocks. Negative epeirogenic movement resulted in submerging of shields, shallow seas and continental shelves. Later, positive epeirogenic move­ment brought the sedimentary cover that had gathered over time above sea level where it has been carved up by streams into hills and plateaus. The Canadian Shield in North America and the Russian-Baltic Shield of Europe are well-known shields of the northern hemisphere. Similar shield areas occupy parts of Australia, Africa, South America, India and Antarctica. Rocks in these areas date back to the oldest geological time from one to three billion years ago.

Mountain belts of continents are along narrow a genic zones in which crust has been compressed and fon to buckle into tight folds and at the same time to
strongly elevated. The older, inactive mountain ranges he usually been reduced to hill belts. In some places, they i still moderately elevated possessing a rugged relief. 1 young ranges are still active zones of crustal deformati and volcanic activity in some areas. They rise as speetacu alpine mountains.

Air Masses and Fronts

AIR MASSES AND FRONTS
AIR MASSES An air mass is a mass of air with similar properties of temperature and moisture covering a large area of the earth's surface and bounded by fronts. Regions where homogenous air masses tend to be created are known as source regions. Some of the well known source regions are sub-tropical and tropical oceans, low-latitude deserts, and the continental interiors. An air mass is said to be cold when it is colder than the surface over which it rests or is moving. An air mass is said to be warm when it is warmer than the surface over which it rests or is moving.

On the basis of source region, air masses are of three types: tropical air mass, polar air mass, and Arctic or Antarctic air mass. Tropical air mass is an air mass that has its source region within the sub-tropical high pressure areas between 20° and 40° north and south of the equator. Polar air mass is an air mass having its source region between latitudes 40° and 60° north and south of the equator. Arctic or Antarctic air mass is an air mass with its source area between latitude 60° and the poles. On the basis of humidity, tropical and polar air masses are further divided into tropical maritime, tropical continental, polar maritime and polar continental. Maritime air masses originate over an ocean, while continental air masses originate over a con­tinent or landmass.

FRONTS The boundary zones between air masses that have originated in different source areas have differing temperature and humidity characteristics. Where the bound­ary zone intersects the earth's surface, it forms a line of separation, called a front. It is represented by a line in weather charts. A front is usually associated with a trough of low pressure and is very important as a considerable amount of weather is generated along it. Fronts may be nearly vertical as in the case of air masses having little motion relative to one another. They may -be almost horizontal as in cases where one air mass slides over
,another.

Cold Front is a front separating a retreating warm air mass and an advancing cold air mass; the cold air, being heavier, remains near the surface, forcing the warm air to rise over it. The cold front is steeper than the warm front. Another is associated with strong atmospheric disturbances. At a cold front, pressure rises, temperature falls, the wind, veers and there are heavy showers often accompanied by thunder.
Wann Front is a front separating a retreating cold air mass and an advancing warm air mass. Warm air is forced to rise here also. Warm fronts have lower slopes and are commonly attended by stable atmospheric conditions, lack­ing the turbulent air motion of the cold front. If the wal1I\ air is unstable, it results in heavy showers and thunder­storm. The weather associated with warm fronts is seen long before the front reaches the observer. The first indication of its approach is high cirrus cloud, which gradually lowers and thickens into low cirrostratus, fol­lowed by altostratus and then rain-producing nimbostratus at the front itself, frequently a distance of 1000 km over
World Geography 25
all. As the front passes, temperature and humidity rise, pressure stops falling and winds veer.
Occluded Front forms when, during the later stages of the evolution of a cyclonic storm, the more rapidly moving cold front catches up with the warm front and lifts the warm sector above the surface. This is fairly common over Western Europe. The weather associated with an occlusion is similar to that occurring at the original fronts, but does not last as long and eventually dies out as the air in the warm sector riSes and cools.
Stationary Front is formed when neither the warm nor the cold front are able to replace each other. At the stationary fronts rain occurs, sometimes lasting for several days.