www.iqraias.com

General Studies

Geography Notes Part -1

By – Rohit Wazir

www.iqraias.com

Atmosphere

The atmosphere is a thin envelop of gases surrounding the Earth that forms a protective boundary between the outer space and the biosphere generally considered to be below480 km (300 mi). Earth's atmosphere is formed by gases from the crust and interior and the exhalations of all life over the time. Atmosphere is a mixture of gases that is odourless, colourless, tasteless, and formless, blended so thoroughly that it behaves like a single gas. Earth's atmosphere is unique because, it supports life. The stabilisation of atmosphere in its present form took place in the Cambrian Period (about 600 million years ago). The gases of the present atmosphere are not direct residue of the earliest form of the planet rather they are evolutionary products of the volcanic eruptions, hot springs, chemical breakdown of solid matter and distribution from the biosphere including photosynthesis and human activity. Composition of Atmosphere A unit mass of dry air is made up of 78.084 percent nitrogen (N,), 20.946 percent oxygen (O2), 0.934 percent argon (A), 0.036 percent carbon dioxide (CO,), and smaller proportions of rare gases such as neon, helium, methane and hydrogen. On the basis of composition of gases, thermosphere may be divided into (i) Heterosphere and (ii) Homosphere Homosphere It extends from the Earth's surface up to an altitude of 80 km (50 miles). Even though the atmosphere rapidly decrease in density with increasing altitude; the blend of gases is nearly uniform throughout the homosphere. The only exceptions are the concentration of ozone (O3) in the stratosphere (ozone layer) from 19 to 50 km, and the variation in water vapour and pollutants in the lowest portion of the atmosphere near the Earth's surface. The stable mixture of gases throughout the homosphere has evolved slowly. The present proportion, which includes oxygen was attained approximately 600 million years ago (Canmbrian Period). The homosphere may be divided into: (1) Troposphere, (2) Stratosphere and (3) Mesosphere Troposphere Troposphere extends up to the tropopaue,defined by a temperature of -57°C, occurring at an altitude of 19 km at the equator, 13 km in the middle latitudes, and about 8 km near the poles. It contains approximately 90 per cent of the total mass of the atmosphere and the bulk of all water vapour, clouds, veather, and air pollution. The height of the tropopause with season, latitude, surface temperatures and pressures. The temperature in the troposphere decreases at the rate of 6.4°C per 1000 meters. This decrease in temperature is known as the normal lapse rate. Stratosphere Above the troposphere lies the stratosphere.The troposphere, ranges from about 19 to 50km above the Earth's surface on equator. The temperature in the troposphere varies from-57 C at the tropopause to 0°Cat the stratopause(50 km). This sphere is characterised by the presence of ozonosphere, or ozone layer. Ozone is highly reactive. oxygen molecule made up of three oxygen atoms (O). That make up most of the oxygen gas. Ozone absorbs wavelengths of ultraviolet light: Through this process, the most harmful ultraviolet radiation is effectively 'filtered' from the incoming solar radiation safeguarding the Earth's surface. Mesosphere

www.iqraias.com

The mesosphere is the area from 50 to 80 km. It’s upper boundary, the mesopause, is the coldest portion of the atmosphere, averaging -90C although temp may vary. Heterosphere The gases in this part are not evenly mixed. The heterosphere begins at around 80 km (50mi) altitude and extend outward some 10,000 km. The solar constant is, however, measured at the altitude of 480 km. Above that point, the atmosphere is rarified (nearly a vacuum) and is called exosphere, which means 'outer space'.It contains individual atoms of the light gases hydrogen And helium, weakly bound by gravity. The heterosphere has thermal region (called the thermosphere) and within it functional layers (called the ionosphere). Thermosphere (the Heatosphere') This is a region extending from 80 km to 480 km in altitude. It contains the functional ionosphere layer. High temperatures are generated into the thermosphere because the gas molecules in this sphere absorbs shortwave solar radiation. The temperatures rise sharply in the thermosphere, up to 1200°C (2200°F) and higher. Despite such high temperatures, the thermosphere is not 'hot' in the way you might expect. Temperature and heat are two different things. The intense solar radiation in this portion of the atmosphere excites individual molecules and atoms (principally nitrogen and oxygen) to high levels of vibration. The density of molecules is so low that little actual heat is produced (heat is the quantity of thermal energy). Heating in the lower atmosphere near the Earth's surface is different because the greater number of molecules in the denser atmosphere transmit kinetic energy as sensible heat, meaning that we can sense it . lonosphere This is a zone of the upper atmosphere (Heterosphere) characterised by gases that have been ionized by solar radiation. The ionosphere is composed of atoms that acquired electrical charges when they absorbed cosmic.rays, gammarays, x-rays, and shorter wavelengths of ultraviolet radiations. These charged atoms are called ions, giving the ionosphere constantly producing a constant flux (flow) of electrons and charged atoms.The incoming solar radiation begins to interact with the atmosphere at altitudes beyond 480 km (300 miles). This outer region is also where gaseous particles escape the Earth’s gravity. Particles are far apart that some or them have a high enough velocity in a direction away from the Earth to escape to space. The atmosphere is very thin at this altitude, but it is here that incoming space vehicles and meteorites begin to heat due to friction. Above this, to perhaps 1125 kilometres (675miles) atomic oxygen is prevalent. Beyond this layer of atomic oxygen, helium is most common out to 3540 km. Still farther out, hydrogen atoms predominate. The boundaries among the ozone is not clearly defined.

www.iqraias.com

ATMOSPHERIC PRESSURE

The mass of atmosphere exerts or creates pressure on surface of earth by the effect of gravity called as atmospheric pressure. It is also defined as the force exerted in all directions as the consequence of weight of all air above it. It is a mixture of several gases , to understand the concept of air pressure the behaviour of all gases and laws governing it must be understood. Atmosphere exerts 1034 grams/sq cm at sea level. It is also expressed as 1013.25 mb. It is calculated as the 760 mms compression of mercury column used in barometer. It is at a temperature of 15 degree Celsius at a latitude of 45ᵒ. Atmospheric pressure is equally exerted on living and non living objects. Human body does not feel the atmospheric pressure because of the inner air pressure of the body. It ranges between 982mb -1033mb . The highest sea level pressure ever recorded was at Irkutsk in Siberia. It was 1075.2 mb . All weather changes are associated with the fluctuations in atmospheric pressure. It emerges that there are in all seven alternating low and high pressure belts on the earth’s surface. These pressure belts are as follows:- 1. Equatorial trough of low pressure

2. Subtropical high pressure belt (northern hemisphere)

3. Subtropical high pressure belt (southern hemisphere)

4. Subpolar low pressure belt (northern hemisphere)

5. Subpolar low pressure belt (southern hemisphere)

6. Polar high (northern hemisphere)

7. Polar high (southern hemisphere)

Equatorial trough of low pressure:

The equatorial trough of low pressure is located in the vicinity of the geographical equator between latitudes 5ᵒN and

5ᵒS. This is the mean position of this pressure belt. In fact, like the belt of maximum temperature this trough of low

pressure extends more to the north than to the south of the equator. Width of the equatorial trough is not uniform

in different parts of the equatorial region.

Since the maximum insolation is available in the equatorial region, the earth’s surface is intensely heated during the

day so that the lowermost layers of air get warmed. The heated air expands, becomes lighter , and rises upward.

Thus, conventional currents are set up in the atmosphere throughout the year. Because of the warm and moist air

moving upward, it has great potential for tremendous amount of energy in the form of latent heat of condensation.

In the equatorial low pressure belt the air is warm and moist.

www.iqraias.com

Equatorial trough of low pressure is the zone of convergence of trade winds blowing equator-ward from the sub-

tropical belts of high pressure in the northern and southern hemispheres. Within this the winds are light and variable

with frequent calms. That is why this belt is called the

doldrums. This belt is quite distinct on maps showing

the distribution of mean annual pressure for the

world and average distribution of pressure for

January. But on the pressure distribution map for July

the equatorial trough of low pressure appears to be

highly irregular and indistinct.

The equatorial trough of low pressure is, as it were,

tied with the sun. Therefore it shifts towards the

north and south of equator with the apparent

movement of the sun. During the month of July this

low pressure belt extends upto latitude 20 ᵒN in

North Africa and to the north of the Tropic of Cancer

in the vast continent of Asia. In January it migrates to the south of equator. In the summer of southern hemisphere

it extends to latitudes 10ᵒ and 20ᵒS.

Sub-tropical high pressure belt: The areas of sub- tropical high pressure are located between latitudes 25ᵒ and 35ᵒN

and S. The most important feature of this pressure belt is that it is broken into a number of high pressure centres or

cells. These high pressure cells or centers of action’ are the key points in the distribution pattern of air pressure over

the globe. But the exact causes of their origin are not yet fully known.

These high pressure areas have no prevailing winds. The winds are light and variable. There are occasional calms.

However, sometimes these belts are invaded by extra-tropical or tropical disturbances attended by stormy winds.

These high pressure winds are called the horse latitudes.

The cells of high pressure persist throughout the year over the large ocean basin where there is a slight seasonal

change in their position. However , there are marked variations in their intensity and size.

• In the southern hemisphere higher pressures are observed during the winter seasons. In this hemisphere there is

an almost continuous belt of high pressure, because the high pressure cells spread to adjacent continental areas

during the winter season.

www.iqraias.com

• In the northern hemisphere, during the warmer part of the year, the subtropical high pressure areas record higher

pressures and become larger in area.

As regards the origin of subtropical high pressure cells on the poleward margin of the tropical regions, there is a

lot of controversy.

classical theory

The classical theories ascribe their origin to the piling up of poleward moving air in the so called anti- trades which,

when at about latitudes 20 degrees, are deflected into the westerlies. Convergence at higher levels results in

downward movement of air and high pressure near the earth’s surface. However, this dynamic explanation of the

origin of sub-tropical high pressure cells has one weak point

against it, and that is the frequent absence of anti- trades.

Hence this factor alone cannot account for the origin of the

semi permanent high pressure cells in subtropical regions.

dynamic theory

There is another dynamic theory according to which polar

masses are the main cause of subtropical highs. Anticyclones

near the Polar Front have a tendency to move equator-wards,

while cyclones usually move polewards. These moving cold

anticyclones are said to regenerate the subtropical highs

occasionally. This fact is corroborated by the pulses in the

intensity of the subtropical highs that are observed on a series of daily weather maps. Further, the cold polar air

masses have a strong preference for eastern parts of the oceans where there are cold ocean currents. This explains

as to why at low levels , over the eastern oceans , high pressures are observed.

Both the dynamic and the thermal factors are responsible for the origin of subtropical highs, each reinforcing the

other. That is why there are seasonal variations in the intensity and location of subtropical high pressure cells.

Subpolar low pressure belts: In the southern hemisphere there is an uninterrupted belt of low pressure between

latitudes 60ᵒ and 70ᵒ, where there is a vast expanse of the oceans. But in the northern hemisphere there are large

land masses between these latitudes which are very cold. Therefore the pressures over these land masses are

increased. Thus, the continuity of subpolar low pressure belt in the northern hemisphere is broken. However, there

are well defined low pressure cells over the northern oceans. The centres of these low pressure systems lie in the

vicinity of Aleutian Islands in the Pacific Ocean and between Greenland and Iceland in the Atlantic.

www.iqraias.com

During the winter season, there is a great contrast between the temperatures of the continents and adjacent oceans.

This helps in reinforcing the Aleutian Low and the Icelandic Low. In summer the temperature contrast between the

continents and oceans is much reduced, so that the belt of sub-polar low is more developed and becomes more

regular.

Polar highs: Pressures at the poles are consistently high throughout the year. In the northern hemisphere the high

pressure area is not centered at the pole, but it is believed to extend from northern Greenland westward across the

islands situated in northern part of Canada. From the reports of explorers of Antarctica the existence of relatively

higher pressure near the South Pole is confirmed.

www.iqraias.com

WINDS

On the earth’s surface, certain winds blow constantly in a particular direction throughout the year. These are known

as the ‘Prevailing Winds’. They are also called the Permanent or the Planetary Winds. Certain winds blow in one

direction in one season and in the opposite direction in another. They are known as Periodic Winds. Then, there are

Local Winds in different parts of the world.

Planetary or Permanent Winds

The planetary wind system of the world accompanies the presence of the High and Low-Pressure Belts. We know

that winds tend to blow from the high-pressure centres to the low-pressure centres. The effect of the earth’s rotation

(Coriolis Force) tends to deflect the direction of these winds. The deflection in the direction of these winds take place

according to Ferrel’s Law. Two sets of surface winds, the Trades and the Westerlies are the main planetary winds of

the world.

Trade Winds

North and South of the Equatorial Belt of Calms, are the Trade Winds covering roughly the zone lying between 5° and

30° North and South ie. they cover almost the entire area between 30°N and 30°S latitudes on both sides of the

equator. The Trade Winds are a result of a pressure gradient from the Sub-Tropical Belt of High Pressure to the

Equatorial Belt of Low Pressure.

In the Northern Hemisphere, the wind moving equator-ward, is deflected by the earth’s rotation to flow south-

westward. Thus, the prevailing wind there is from the North-East, and it has been named as the ‘North-East Trades’.

In the Southern Hemisphere, deflection of the wind is towards the left, this causes the ‘South-East Trades’. Trade

Winds are noted for their steadiness and persistent direction. But the system of Doldrums and trades shifts seasonally

north and south, through several degrees of latitudes, as do the pressure belts that causes them.

The trades are best developed over the Pacific and the Atlantic Oceans, but are upset in the Indian Ocean because

of nearness of the great Asian landmass.

They are named after the Latin word ‘trado’ which means blowing steadily in a constant direction; hence, the name

Trade Winds. As these Trade Winds blow from the warmer, sub-tropical latitudes to the hot tropics, they have a great

capacity for holding water-vapor or moisture. When they cross the open oceans, they pick up a lot of moisture. They

bring heavy rainfall to the eastern coasts of continents lying within the tropics because they blow on-shore. On the

www.iqraias.com

western coasts of continents, these Trade Winds do not bring any rainfall. It is because here there are ‘offshore’ winds

or winds blowing just parallel to the shores, as they blow off-shore. As such, the western areas within the tropics

suffer from aridity. The great deserts of the Sahara, Kalahari, Atacama and the Great Australian Deserts all lie on the

western margins of the continents, lying within the tropical latitudes.

Westerlies

The Westerlies or the Prevailing Westerly Winds blow between 35° and 60° North and South latitudes from the Sub-

Tropical High-Pressure Belts towards the Sub-Polar Low-Pressure Belts. We know that the high-pressure belt is a zone

of divergence for these outgoing winds. In the Northern Hemisphere, the Westerlies generally blow from the south-

west to the north-east, and in the Southern Hemisphere from the north-west to the south-east. These are on-shore

winds on the west coasts and off-shore winds on their east coasts. The on-shore winds bring rainfall while the off-

shore winds are lacking in it. These winds are not as constant in

strength and direction as the Trade Winds.

They are rather stormy and variable though the main direction

remains from west to east. But as their general direction is from

the west, they are called the “Westerlies”. They are also known

as “Anti-Trade Winds”, because their movement is in the

opposite direction from that of the Trade Winds. In the Northern

Hemisphere, land-masses cause considerable disruption in the

westerly winds. But in the Southern Hemisphere, between 40°S

and 60°S, the westerlies gain great strength and persistence

because of the vast expanse of oceans in their belt.

This made the mariners of old call them the “Roaring Forties”, the “Furious Fifties” and the “Screaming Sixties”.

In olden days, sailing vessels had to face great danger while sailing in the opposite direction in the face of the

prevailing westerly winds. It is to be rioted that the westerlies bring warmth and rainfall throughout the year to all

the western coasts of the Temperate Zone. But the areas, which lies in the Mediterranean type of region, get rainfall

only in winter. At that time, in December, the Mediterranean parts of Europe and California (U.S.A.) come under the

influence of the westerlies and receive rainfall. In the Southern Hemisphere, in this month, the Mediterranean regions

(Central Chile, Southern Africa, S.W. Australian coast) do not receive any rainfall, as they shift away from the influence

of the westerlies. In June, the Mediterranean parts of the southern continents come under the influence of the

www.iqraias.com

westerlies and receive rainfall. At that time, the Mediterranean areas of the Northern Hemisphere do not receive any

rainfall from the westerlies, because they shift away from their influence.

Polar Winds

The winds blowing in the Arctic and the Antarctic latitudes are known as the Polar Winds. They have been termed

the ‘Polar Easterlies’, as they blow from the Polar High Pressure Centres towards the Sub-Polar Low-pressure Belts.

In the Northern Hemisphere, they blow in general from the north-east, and are called the Northeast Polar Winds; and

in the Southern Hemisphere, they blow from the south-east and are called the South-East Polar Winds. As these winds

blow from the ice-capped landmass, they are extremely cold. They are more regular in the Southern Hemisphere than

in the Northern Hemisphere.

Periodic Winds/Local winds-

Land and sea breezes, Mountain and valley breezes and monsoon winds are winds of a periodic type.

See Breeze

During the day, the greater heating of the land causes the air to ascend, causing a low pressure over land and the

cool heavy air from the sea moves in to take its place. The strength of the sea breeze depends on the topography of

the coast and the regions.

Land Breeze

During the night the land cools quickly so that it is colder than the sea. A low pressure area is caused over the sea and

the cooler heavier air from the land begins to flow towards the sea. The general effect of the contrast in heating of

land and sea is to produce cooler winters and warmer summers in the centres of continents than along coasts.

Mountain and Valley Breezes

Mountain and valley breezes are common in regions with great topographic relief. A valley breeze develops during

the day as the Sun heats the land surface and air at the valley bottom and sides.

As the air heats it becomes less dense and buoyant and begins to flow gently up the valley sides. Vertical ascent of

the air rising along the sides of the mountain is usually limited by the presence of a temperature inversion layer.

When the ascending air currents encounter the inversion, they are forced to move horizontally and then back down

to the valley floor. This creates a self-contained circulation system. If conditions are right, the rising air can condense

and form into cumuliform clouds.

www.iqraias.com

www.iqraias.com

TEMPERATURE

The earth’s atmosphere receives very little amount of heat by direct solar radiations. Most of its temperature is because of the absorption of outgoing terrestrial radiations. The constant interaction of atmospheric gases such as greenhouse gases and reflected infra-red rays heat the lower atmosphere. The transfer of heat is an ongoing process caused by phenomenon like conduction, convection, radiation and absorption.

Heat Transfer: The solar energy received gets converted into heat energy and is temporarily stored which is then radiated from the ground and the water surface in the form of long terrestrial radiation called as ground or surface radiation. Water vapor’s, clouds cause re- radiation towards the earth’s surface.

Process of heating as under :

1. Direct insolation: The atmosphere absorbs around 14% of total incoming solar radiation. Around 50% of this energy is spread upto the height of 2 kms where most of the water vapors are found.

2. Conduction: This process involves transfer of energy when two bodies are in contact to each other. It can occur between the same bodies or touching other surfaces. It is a very slow process as air and rocks are poor conductors of heat. On the whole it has less importance with regard to heat transfer.

3. Terrestrial Radiation: It is the most significant contributor to heat of atmosphere. Earth absorbs very little amount of direct solar radiation or short wave radiation only about 19% of short wave radiation is absorbed. Around 85% is absorbed through long wave terrestrial radiation. Earth acts just like a black body. It absorbs short waves and radiates heat back to atmosphere. During the day time the net loss is less than incoming radiation. During night time loss exceeds. According to Kirchoff’s Law earth radiates heat in the form of long

waves. It follows laws of radiation such as:

1. Wein’s displacement law: It states that the wavelength of radiation is inversely proportional to temperature of the body ie. Higher the temperature lower is the wavelength. This law helps to determine the temperature and color of the Stars.

www.iqraias.com

2. Steffen-Boltzman law: It states that flux is directly proportional to fourth power of absolute temperature of the radiating body. This leads to the most important phenomenon known as Green House Effect.

4. Convection: The transfer of heat caused when mass of body moves. The vertical transport is called as convection whereas horizontal transport is called as Advection. Convection is responsible for redistribution of heat from equatorial region to polar region. Warm packet of air expands becomes less dense than surrounding air therefore rises. Advection is more seen in mid latitude regions with high diurnal variations. The warm wind loo is outcome of advectional expansion.

5. Latent heat of Condensation: The latent heat of Condensation plays an important role in heating of atmosphere. During the process of evaporation the heat is absorbed referred as latent heat of vapourisation. As condensation involves a reverse phenomenon, heat is released called as latent heat of condensation. It heats the surrounding air.

6. Compression of Air: A descending air mass is compressed by the virtue of higher atmospheric pressure in the lower region. Air molecules are set in motion producing friction and thereby the heat.

1. Horizontal distribution: Temperature in higher latitude is

far less than temperature in lower latitudes because of the

variation in the solar insolation which is higher towards

equatorial areas.

8. Vertical distribution: Vertically temperature shows a variation called as Vertical Temperature Gradient. This is

controlled by various factors like convection and transfer of heat through conduction and intricate processes

such as release of latent heat of condensation, radiation and re-radiation.

www.iqraias.com

Atmosphere generally follows lapse rate of 6.5 degree Celsius/km also referred as Environmental lapse rate.

It remains confined to troposphere. Do note that heating of the lower layer of air is not

caused because of nearness to earth surface but other factors such as denser atmosphere

in higher concentration of green house gases. Upper air acts transparent to incoming short

waves. Lower atmosphere acts opaque to outgoing/ long wave terrestrial radiations heating

therefore begins from lower atmosphere. Upper air is rarefied , dry therefore absorption is

negligible. Even on mountain surfaces the temperature of ground and that of free air. In the

same way there is large difference in temperature taken in shade and the sun. The condition

of atmosphere varies with respect to moisture and concentration of gases . Thus

environmental lapse rate is always different than normal lapse rate. In specific conditions

temperature change varies it reverses, the phenomenon is called as temperature inversion.

Effects of Temperature Inversion

• Inversions play an important role in determining cloud forms, precipitation, and visibility.

• An inversion acts as a cap on the upward movement of air from the layers below. As a

result, convection produced by the heating of air from below is limited to levels below the

inversion. Diffusion of dust, smoke, and other air pollutants is likewise limited.

• In regions where a pronounced low-level inversion is present, convective clouds cannot

grow high enough to produce showers.

• Visibility may be greatly reduced below the inversion due to the accumulation of dust and

smoke particles. Because air near the base of an inversion tends to be cool, fog is

frequently present there.

• Inversions also affect diurnal variations in temperature. Diurnal variations tend to be very

small.

Ideal Conditions for Temperature Inversion

1. Long nights, so that the outgoing radiation is greater than the incoming radiation.

2. Clear skies, which allow unobstructed escape of radiation.

3. Calm and stable air, so that there is no vertical mixing at lower levels.

Types of Temperature Inversion

Temperature Inversion in Intermontane Valley (Air Drainage Type of Inversion)

• Sometimes, the temperature in the lower layers of air increases instead of decreasing with elevation. This

happens commonly along a sloping surface.

www.iqraias.com

• Here, the surface radiates heat back to space rapidly and cools down at a faster rate than the upper layers.

As a result the lower cold layers get condensed and become heavy.

• The sloping surface underneath makes them move towards the bottom where the cold layer settles down

as a zone of low temperature while the upper layers are relatively warmer.

• This condition, opposite to normal vertical distribution of temperature, is known as Temperature Inversion.

• In other words, the vertical temperature gets inverted during temperature inversion.

• This kind of temperature inversion is very strong in the middle and higher latitudes. It can be strong in

regions with high mountains or deep valleys also.

Ground Inversion (Surface Temperature Inversion)

• A ground inversion develops when air is cooled by contact with a colder surface until it becomes cooler than

the overlying atmosphere; this occurs most often on clear nights, when the ground cools off rapidly by

radiation. If the temperature of surface air drops below its dew point, fog may result.

• This kind of temperature inversion is very common in the higher latitudes.

• Surface temperature inversion in lower and middle latitudes occurs during cold nights and gets destroyed

during daytime.

Subsidence Inversion (Upper Surface Temperature Inversion)

• A subsidence inversion develops when a widespread layer of air descends.

• The layer is compressed and heated by the resulting increase in atmospheric pressure, and as a result the

lapse rate of temperature is reduced.

• If the air mass sinks low enough, the air at higher altitudes becomes warmer than at lower altitudes,

producing a temperature inversion.

• Subsidence inversions are common over the northern continents in winter (dry atmosphere) and over the

subtropical oceans; these regions generally have subsiding air because they are located under large high-

pressure centers.

www.iqraias.com

• This temperature inversion is called upper surface temperature inversion because it takes place in the upper

parts of the atmosphere.

Frontal Inversion (Advectional type of Temperature Inversion )

• A frontal inversion occurs when a cold air mass undercuts a warm air mass (Cold and Warm Fronts: we will

study in detail later) and lifts it aloft; the front between the two air masses then has warm air above and cold

air below.

• This kind of inversion has considerable slope, whereas other inversions are nearly horizontal. In addition,

humidity may be high, and clouds may be present immediately above it.

• This types of inversion is unstable and is destroyed as the weather changes.

Economic Implications of Temperature Inversion

• Sometimes, the temperature of the air at the valley bottom reaches below freezing point, whereas the air at

higher altitude remains comparatively warm. As a result, the trees along the lower slopes are bitten by frost,

whereas those at higher levels are free from it.

• Due to inversion of temperature, air pollutants such as dust particles and smoke do not disperse in the valley

bottoms. Because of these factors, houses and farms in intermontane valleys are usually situated along the

upper slopes, avoiding the cold and foggy valley bottoms. For instance, coffee growers of Brazil and apple

growers and hoteliers of mountain states of Himalayas in India avoid lower slopes.

www.iqraias.com

• Fog lowers visibility affecting vegetation and human settlements.

• Less rainfall due to stable conditions.

www.iqraias.com

Humidity

• Water vapour present in the air is known as humidity.

Absolute Humidity

• The actual amount of the water vapour present in the atmosphere is known as the absolute humidity.

• It is the weight of water vapour per unit volume of air and is expressed in terms of grams per cubic metre.

• The absolute humidity differs from place to place on the surface of the earth.

• The ability of the air to hold water vapour depends entirely on its temperature. Warm air can hold more

moisture than cold air.

Relative Humidity

• The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature

is known as the relative humidity.

• With the change of air temperature, the capacity to retain moisture increases or decreases and the relative

humidity is also affected.

• It is greater over the oceans and least over the continents.

• Relative humidity can be changed in either of the two ways—

1. By adding moisture through evaporation: if moisture is added by evaporation, the relative humidity will

increase and vice versa.

2. By changing temperature of air: a decrease in temperature (hence, decrease in moisture-holding capacity)

will cause adecrease in relative humidity and vice versa.

• The relative humidity determines the amount and rate of evaporation and hence it is an important climatic

factor.

• Air containing moisture to its full capacity at a given temperature is said to be ‘saturated’. At this temperature,

the air cannot hold any additional amount of moisture. Thus, relative humidity of the saturated air is 100%.

• If the air has half the amount of moisture that it can carry, then it is unsaturated and its relative humidity is

only 50%.

Dew point

• The air containing moisture to its full capacity at a given temperature is said to be saturated.

• It means that the air at the given temperature is incapable of holding any additional amount of moisture at

that stage.

• The temperature at which saturation occurs in a given sample of air is known as dew point.

• Dew point occurs when Relative Humidity = 100%.

www.iqraias.com

Specific Humidity

• It is expressed as the weight of water vapour per unit weight of air.

• Since it is measured in units of weight (usually grams per kilogram), the specific humidity is not affected by

changes in pressure or temperature.

Absolute Humidity and Relative Humidity are Variable whereas Specific Humidity is a constant.

Evaporation

• Evaporation is a process by which water is transformed from liquid to gaseous state. Heat is the main cause for

evaporation.

• Movement of air replaces the saturated layer with the unsaturated layer. Hence, the greater the movement of

air, the greater is the evaporation.

Factors Affecting Rate of Evaporation

• Amount of water available.

• Temperature.

• Relative humidity. [explained in previous post]

• Area of evaporating surface.

• Wind speed: A high wind speed removes the saturated air from the evaporating surface and replaces it with

dry air which favors more evaporation.

• Whenever there is a combination of high temperature, very low relative humidity and strong winds, the rate

of evaporation is exceptionally high. This leads to dehydration of soil to a depth of several inches.

• Air Pressure: Evaporation is also affected by the atmospheric pressure exerted on the evaporating surface.

Lower pressure over open surface of the liquid results in a higher rate of evaporation.

• Composition of water: Evaporation is inversely proportional to salinity of water.

• Rate of evaporation is always greater over fresh water than over salt water.[Because of the reduction in the

water vapor pressure at the water surface due to salinity.]

• Under similar conditions, ocean water evaporates about 5% more slowly than fresh water.

• More evaporation by plants: Water from plants generally evaporates at a faster rate than from land.

Condensation

• The transformation of water vapour into water is called condensation.

•

www.iqraias.com

• Condensation is caused by the loss of heat (latent heat of condensation, opposite of latent heat of

vaporization).

• When moist air is cooled, it may reach a level when its capacity to hold water vapour ceases (Saturation Point

= 100% Relative Humidity = Dew Point reached). Then, the excess water vapour condenses into liquid form.

If it directly condenses into solid form, it is known as sublimation.

• In free air, condensation results from cooling around very small particles termed as hygroscopic

condensation nuclei. Particles of dust, smoke, pollen and salt from the ocean are particularly good nuclei

because they absorb water.

• Condensation also takes place when the moist air comes in contact with some colder object and it may also

take place when the temperature is close to the dew point.

• Condensation, therefore, depends upon the amount of cooling and the relative humidity of the air.

• Condensation takes place:

1. when the temperature of the air is reduced to dew point with its volume remaining constant

(adiabatically),

2. when both the volume and the temperature are reduced,

3. when moisture is added to the air through evaporation,

• After condensation the water vapour or the moisture in the atmosphere takes one of the following forms —

dew, frost, fog and clouds.

• Condensation takes place when the dew point is lower than the freezing point as well as higher than the

freezing point.

www.iqraias.com

Air Masses

The term air mass ,as used by meteorologists denotes a large volume of air which is mostly homogeneous in temperature and humidity and extends to a large area often covering over thousands of kms and reaching upto the top of the troposphere. An air mass may consist of several layers lying horizontally one above the other with each layer having uniformity of temperature and humidity. Thus the uniformity of temperature and humidity is not confined merely to the air in contact with earth’s surface but exists even in upper air up to a height of several kms. An air mass can be defined as:

“A body of air in which the upward gradients of temperature and moisture are fairly uniform over a large area is known as an air mass.”

Origin of Air Mass: An air mass originates over a period of time when a large volume of relatively stationary air remains in contact with a vast land or ocean surface which has nearly uniform characteristics of temperature and moisture. By doing so over a sufficiently long period of time ( several days or even weeks) , the air aquires the characteristics of the concerned surface.

For example: the vast ice cover in the polar regions will impart icy coldness to the air mass lying over it. Similarly, if the air mass is in contact with warm oceans within the tropics, it will be hot and humid.

Following two conditions are essential for the development of an air mass.

a. An extensive land or water surface with homogenous conditions of temperature and humidity.

b. A large, relatively stationery, volume of air over the surface with no divergent winds. Light winds ensure that the air stays over the source region for a sufficiently long time so that it acquires the temperature and humidity of the surface over which it stays.

Main Source Regions:

1. Arctic and Antarctic Source Regions: These source regions surround north pole and south pole respectively and represent extremely cold conditions. Obviously, very cold air masses originate in these source regions.

2. Continental Polar Source Region (cP): The northern parts of continents in the northern hemisphere are very close to Arctic source and are almost permanently covered by snow and ice. These are very cold areas where cyclonic conditions prevail. As such this source region offers good opportunities for air mass formation.

3. Maritime Polar Source Region (mP): The maritime polar sources are found over the oceans at about 60ᵒN and S. The northern part of the Atlantic Ocean and the eastern part of the Pacific Ocean provide suitable conditions for air mass formation.

4. Continental Tropical Source Region(cT): Source regions on the continent in the tropical zone are known as continental Tropical Source Region. These source regions specially develop in Asia and North Africa. They are confined to North Africa only in winter but spread over vast areas in Africa, Asia and southern Europe ion summer.

5. Maritime Tropical Source Region(mT): These source regions are found over oceans in the tropical zone . The anticyclonic conditions , both in summer and winter provide suitable opportunities for development of air mass source regions.

www.iqraias.com

6. Maritime Equatorial Source Region(mE): Trade winds from the north and south converge over the oceans at the equator and give rise to maritime equatorial sources region. Here, air masses keep on developing throughout the year.

7. Monsoon Source Region: In addition to the above mentioned source regions, there is monsoon source region which is centred over the Indian Ocean. It strengthens the summer monsoon and helps in bringing summer rainfall. This region is the source of warm, moist air masses in summer and cold, dry air masses in winter.

FRONTOGENESIS:

The process of front formation is known as frontogenesis.

It depends upon the following two factors:

1. Geographical Factor: Geographically a front is formed within two air masses of different temperature and humidity approach each other. In North America , for example, cold and dry air mass from the north meets warm and moist air mass from the south to form a front.

2. Dynamic Factor: Movement in different types of air masses is a necessary condition for the formation of a front. T.Bergeron and Petterson proved that movement of air masses enhances the formation of fronts. It is true for only converging air masses. Diverging air masses do not form fronts.

Areas of Frontogenesis and Frontolysis: There are certain areas in the world where air masses having different temperature and humidity converge and form fronts. These areas are known as ‘areas of frontogenesis’. Eastern part of Asia, East Coast of North America and Greenland etc. are good examples of areas of frontogenesis particularly in winter season. In contrast to this, there are areas where air masses move away from each other and donot provide suitable frontogenesis. Rather,the fronts suffer decay and such areas are called ‘areas of frontolysis’. Siberia, North Canada, South Africa etc. provide good examples of areas of frontolysis.

Conditions for Frontogenesis: The areal distribution of fronts clearly indicates that fronts require specific conditions for their formation. Following two conditions are essential for the formation of fronts:

1. Contrasting characteristics of air masses: Two air masses can form a front only when they have contrasting characteristics of temperature and humidity. If one of the air masses is cold and dry, the other mass must be warm and humid. When these two air masses of contrasting characteristics come close to each other, the warm and humid air, being light, rises over the heavy cold and dry air and a front is formed.

2. Convergence of air masses: A front can be formed only when two air masses of contrasting characteristics move towards each other and there is convergence between the two. Due to convergence, two air masses meet each other, the hot and humid air mass rises over the cold and dry air mass and a front is formed. As against this, there is no possibility of a front being formed if the air masses diverge away from each other.

Characteristics of Fronts:

1. Depth: Air masses forming fronts are generally found in the lower strata of the atmosphere. Very few fronts are formed above 3000m and practically no fronts are formed above 5000m.

2. Width: There is no definite limit to the width of fronts. Normally fronts are 5 to 80 km wide.

3. Horizontal Velocity: Air fronts normally move at the speed of 50 to 80 km per hour horizontally.

www.iqraias.com

4. Vertically velocity: There is generally ascending air at the front. Since warm and humid air is light, it ascends over cold and dry dense air which causes vertical movement in the air. When warm and humid air ascends, there is condensation and cloud formation as a result of which precipitation takes place.

5. Discontinuity: The characteristics of air masses on two sides of a front are different and they do not merge in each other so easily. As a result, there is a discontinuity of the characteristics such as temperature, humidity, pressure gradient, wind direction etc.

For further explanation on types of winds refer to lecture videos

Tropical Cyclone Temperate Cyclone Origin Thermal Origin Dynamic Origin – Coriolis Force, Movement of air

masses. Latitude Confined to 100 - 300N and S of equator. Confined to 350 - 650 N and S of equator. More

pronounced in Northern hemisphere due to greater temperature contrast.

Frontal system Absent The very cyclone formation is due to frontogenesis.[Occluded Front]

Formation They form only on seas with temperature more than 26-270 C. They dissipate on reaching the land.

Can form both on land as well as seas

Season Seasonal: Late summers (Aug - Oct) Irregular. But few in summers and more in winters.

Size Limited to small area. Typical size: 100 – 500 kms in diameter. Varies with the strength of the cyclone.

They cover a larger area. Typical size: 300 – 2000 kms in diameter. Varies from region to region.

Shape Elliptical Inverted ‘V’ Rainfall Heavy but does not last beyond a few hours. If the

cyclone stays at a place, the rainfall may continue for many days.

In a temperate cyclone, rainfall is slow and continues for many days, sometimes even weeks.

Wind Velocity and destruction

Much greater (100 – 250 kmph) (200 – 1200 kmph in upper troposphere) Greater destruction due to winds, storm surges and torrential rains.

Comparatively low. Typical range: 30 – 150 kmph. Less destruction due to winds but more destruction due to flooding.

Isobars Complete circles and the pressure gradient is steep Isobars are usually ‘V’ shaped and the pressure gradient is low.

Life time Doesn’t last for more than a week Last for 2-3 weeks.

www.iqraias.com

Path East – West. Turn North at 200 latitude and west at 300 latitude. Move away from equator. The movement of Cyclones in Arabian Sea and Bay of Bengal is a little different. Here, these storms are superimposed upon the monsoon circulation of the summer months, and they move in northerly direction along with the monsoon currents.

West – East (Westerlies – Jet Streams). Move away from equator.

Temperature distribution

The temperature at the center is almost equally distributed.

All the sectors of the cyclone have different temperatures

Calm region The center of a tropical cyclone is known as the eye. The wind is calm at the center with no rainfall.

In a temperate cyclone, there is not a single place where winds and rains are inactive.

Driving force The tropical cyclone derives its energy from the latent heat of condensation, and the difference in densities of the air masses does not contribute to the energy of the cyclone.

The energy of a temperate cyclone depends on the densities of air masses.

Influence of Jet streams

The relationship between tropical cyclones and the upper level air-flow is not very clear.

The temperate cyclones, in contrast, have a distinct relationship with upper level air flow (jet streams, Rossby waves etc.)

Clouds The tropical cyclones exhibit fewer varieties of clouds – cumulonimbus, nimbostratus, etc..

The temperate cyclones show a variety of cloud development at various elevations.

Surface anti-cyclones The tropical cyclones are not associated with surface anticyclones and they have a greater destructive capacity.

The temperate cyclones are associated with anticyclones which precede and succeed a cyclone. These cyclones are not very destructive.

Influence on India Both coasts effected. But east coast is the hot spot. Bring rains to North – West India. The associated instability is called ‘Western Disturbances’.

Tropical Cyclones and Temperate Cyclones Comparison

www.iqraias.com

Ocean Relief

• Ocean relief is largely due to tectonic, volcanic, erosional and depositional processes and their interactions. • Ocean relief features are divided into major and minor relief features.

Major Ocean Relief Features Four major divisions in the ocean relief are:

1. continental shelf, 2. continental slope, 3. Continental rise, 4. Deep Sea Plain or the abyssal plain.

Continental Shelf

• Continental Shelf is the gently sloping seaward extension of continental plate. • These extended margins of each continent are occupied by relatively shallow seas and gulfs. • Continental Shelf of all oceans together cover 7.5% of the total area of the oceans. • Gradient of continental is of 1° or even less. • The shelf typically ends at a very steep slope, called the shelf break. • The continental shelves are covered with variable thicknesses of sediments brought down by rivers, glaciers

etc.. • Massive sedimentary deposits received over a long time by the continental shelves, become the source of

fossil fuels [Petroleum]. • Examples: Continental Shelf of South-East Asia, Great Banks around Newfoundland, Submerged region

between Australia and New Guinea. Width

• The average width of continental shelves is between 70 – 80 km. • The shelves are almost absent or very narrow along some of the margins like the coasts of Chile, the west

coast of Sumatra, etc. [Ocean – Continent Convergence and Ocean – Ocean Convergence]. • It is up to 120 km wide along the eastern coast of USA. On the contrary, the Siberian shelf in the Arctic

Ocean, the largest in the world, stretches to 1,500 km in width. Depth

• The depth of the shelves also varies. It may be as shallow as 30 m in some areas while in some areas it is as deep as 600 m.

Importance of continent shelves

1. Marine food comes almost entirely from continental shelves; 2. They provide the richest fishing grounds; 3. They are potential sites for economic minerals [20% of the world production of petroleum and gas comes

from shelves. Polymetallic nodules (manganese nodules; concentric layers of iron and manganese hydroxides) etc. are good sources of various mineral ores like manganese, iron copper, gold etc.]

www.iqraias.com

www.iqraias.com

Continental Slope

• The continental slope connects the continental shelf and the ocean basins. • It begins where the bottom of the continental shelf sharply drops off into a steep slope. • The gradient of the slope region varies between 2-5°. • The depth of the slope region varies between 200 and 3,000 m. • The seaward edge of the continental slope loses gradient at this depth and gives rise to continental rise. • The continental slope boundary indicates the end of the continents. • Canyons and trenches are observed in this region.

Continental Rise

• The continental slope gradually loses its steepness with depth. • When the slope reaches a level of between 0.5° and 1°, it is referred to as the continental rise. • With increasing depth the rise becomes virtually flat and merges with the abyssal plain.

Deep Sea Plain or Abyssal Plain

• Deep sea planes are gently sloping areas of the ocean basins. • These are the flattest and smoothest regions of the world because of terrigenous [denoting marine sediment

eroded from the land] and shallow water sediments that buries the irregular topography. • It covers nearly 40% of the ocean floor. • The depths vary between 3,000 and 6,000m. • These plains are covered with fine-grained sediments like clay and silt.

Oceanic Deeps or Trenches

• The trenches are relatively steep sided, narrow basins (Depressions). These areas are the deepest parts of the oceans.

• They are of tectonic origin and are formed during ocean – ocean convergence and ocean continent convergence.

• They are some 3-5 km deeper than the surrounding ocean floor. • The trenches lie along the fringes of the deep-sea plain at the bases of continental slopes and along island

arcs. • The trenches run parallel to the bordering fold mountains or the island chains. • The trenches are very common in the Pacific Ocean and form an almost continuous ring along the western

and eastern margins of the Pacific. • The Mariana Trench off the Guam Islands in the Pacific Ocean is the deepest trench with, a depth of more

than 11 kilometres. • They are associated with active volcanoes and strong earthquakes (Deep Focus Earthquakes like in Japan).

This makes them very significant in the study of plate movements. • As many as 57 deeps have been explored so far; of which 32 are in the Pacific Ocean; 19 in the Atlantic Ocean

and 6 in the Indian Ocean. Abyssal Hills

• Seamount: It is a mountain with pointed summits, rising from the seafloor that does not reach the surface of the ocean. Seamounts are volcanic in origin. These can be 3,000-4,500 m tall.

www.iqraias.com

• The Emperor seamount, an extension of the Hawaiian Islands in the Pacific Ocean, is a good example. • Guyots: The flat-topped mountains (seamounts) are known as guyots. • Seamounts and guyots are very common in the Pacific Ocean where they are estimated to number around

10,000. Submarine Canyons

• CANYON: a deep gorge, especially one with a river flowing through it

• GORGE: a steep, narrow valley or ravine

• VALLEY: a low area between hills or mountains or a depression, typically with a river or stream flowing through it. - These are deep valleys, some comparable to the Grand Canyon of the Colorado river. - They are sometimes found cutting across the continental shelves and slopes, often extending from the

mouths of large rivers. - The Hudson Canyon is the best known canyon in the world.

Atoll

• These are low islands found in the tropical oceans consisting of coral reefs surrounding a central depression. • It may be a part of the sea (lagoon), or sometimes form enclosing a body of fresh, brackish, or highly saline

water. Bank, Shoal and Reef

• These marine features are formed as a result of erosional, depositional and biological activity. • These are produced upon features of diastrophic [Earth Movements] origin. Therefore, they are located on

upper parts of elevations. Bank

• These marine features are formed as' a result of erosional and depositional activity. • A bank is a flat topped elevation located in the continental margins. • The depth of water here is shallow but enough for navigational purposes. • The Dogger Bank in the North Sea and Grand Bank in the north-western Atlantic, Newfoundland are famous

examples. • The banks are sites of some of the most productive fisheries of the world.

Shoal

• A shoal is a detached elevation with shallow depths. Since they project out of water with moderate heights, they are dangerous for navigation.

Reef

• A reef is a predominantly organic deposit made by living or dead organisms that forms a mound or rocky elevation like a ridge.

• Coral reefs are a characteristic feature of the Pacific Ocean where they are associated with seamounts and guyots.

www.iqraias.com

• The largest reef in the world is found off the Queensland coast of Australia. Since the reefs may extend above the surface, they are generally dangerous for navigation.

www.iqraias.com

Ocean Currents

An ocean current is a continuous movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences

The warm currents move towards the cold seas and cool currents towards the warm seas. Effects of Ocean Currents Ocean currents have a number of direct and indirect influences on human activities.

For further explanation on ocean currents refer to class notes. Pacific Ocean Currents Previous post: Ocean Currents – Factors Responsible for the Formation of Ocean Currents – Effects of Ocean Currents on climate, fishing, navigation, tropical cyclones. Equatorial Pacific Ocean Currents

• Under the influence of prevailing trade winds [tropical easterlies], the north equatorial current and the south equatorial current start from the eastern pacific (west coast of Central America) and traverses a distance of 14,500 km moving from east to west.

Counter equatorial current

• This raises the level of western pacific (near Indonesia and Australia) ocean by few centimeters. And this creates a counter-equatorial current which flows between the north equatorial current and the south equatorial current in west-east direction.

Three factors aid the formation of Counter-Equatorial current

1. Piling up of water in the western pacific due to trade winds. 2. The presence of doldrums (equatorial low-pressure belt) in between the north equatorial current and the

south equatorial current. Doldrums are narrow regions with calm (lower) atmospheric conditions. Such conditions aid the backward movement of piled up western pacific waters.

www.iqraias.com

3. Piling of water in the western part of oceans due to rotation of earth (this is a very general point). Kuroshio current

• The north equatorial current turns northward off the Philippines to form the Kuroshio current. Most of it lies in the sub-tropical high pressure belt and its northern part is under the influence of westerlies.

Oyashio Current and Okhotsk current

• There are two more cold currents in the northern Pacific, Oyashio flows across the east coast of Kamchatka Peninsula to merge with the warmer waters of Kuroshio, and the Okhotsk current flows past Sakhlain Islands to merge with the Oyashio current off Hokkaido (Northern Japanese Island).

North-Pacific current

• From the south-east coast of Japan, under the influence of prevailing westerlies, the Kuroshio current turns eastwards and moves as the North-Pacific current, reaches the west coast of North America, and bifurcates into two.

Alaska and Californian current

• The northern branch flows anti-clockwise along the coast of British Columbia and Alaska and is known as the Alaska current. The water of this current is relatively warm as compared to the surrounding waters in this zone.

• The southern branch of the current moves as a cold current along the west coast of USA and is known as the Californian current. The Californian current joins the north equatorial current to complete the circuit.

East Australian current

• Following the pattern in the northern hemisphere, the south equatorial current flows from east to west and turns southwards as the East Australian current. It then meets the South pacific current near Tasmania which flows from west to east.

Peru current or Humboldt Current

• Reaching the south-western coast of South America, it turns northward as the Peru current. It is a cold current, which finally feeds the south equatorial current, thus completing the great circuit.

• And the zone where Peru Cold current meets the warm equatorial ocean waters is an important fishing zone. Indian Ocean Currents

• Indian ocean is half an ocean, hence the behavior of the North Indian Ocean Currents is different from that of Atlantic Ocean Currents or the Pacific Ocean Currents.

• Also, monsoon winds in Northern Indian ocean are peculiar to the region, which directly influence the ocean surface water movement [North Indian Ocean Currents].

Indian Ocean Currents and Monsoons

www.iqraias.com

• The currents in the northern portion of the Indian Ocean change their direction from season to season in response to the seasonal rhythm of the monsoons. The effect of winds is comparatively more pronounced in the Indian Ocean.

Winter Circulation

• Under the influence of prevailing trade winds [easterly trade winds], the north equatorial current and the south equatorial current start from the south of Indonesian islands, moving from east to west.

• This raises the level of western Indian (south-east of horn of Africa) ocean by few centimeters. And this creates a counter-equatorial current which flows between the north equatorial current and the south equatorial current in west-east direction.

• The north-east monsoons drive the water along the coast of Bay of Bengal to circulate in an anti-clockwise direction.

• Similarly, the water along the coast of Arabian Sea also circulate in an anti-clockwise circulation.

Summer Circulation – North Equatorial Current Counter-Equatorial Current are Absent

• In summer, due to the effects of the strong south-west monsoon and the absence of the north-east trades, a strong current flows from west to east, which completely obliterates the north equatorial current. Hence, there is no counter-equatorial current as well.

• Thus, the circulation of water in the northern part of the ocean is clockwise during this season.

Southern Indian Ocean Currents - Agulhas current, Mozambique current, West Australian current

• The general pattern of circulation in southern part of the Indian Ocean is quite similar to that of southern Atlantic and Pacific oceans. It is less marked by the seasonal changes.

• The south equatorial current, partly led by the corresponding current of the Pacific Ocean, flows from east to west.

• It splits into two branches, one flowing to the east of Madagascar known as Agulhas current and the other between Mozambique and Western Madagascar coast known as Mozambique current.

• At the southern tip of Madagascar, these two branches mix and are commonly called as the Agulhas current. It still continues to be a warm current, till it merges with the West Wind Drift.

• The West Wind Drift, flowing across the ocean in the higher latitudes from west to east, reaches the southern tip of the west coast, of Australia.

• One of the branches of this cold current turns northwards along the west coast of Australia. This current, known as the West Australian current, flows northward to feed the south equatorial current.