Lecture 17 & 18 : Geography

Topics Covered in this Lecture

  1. Processes under Hydrological Cycle.
  2. Concept of Humidity
  3. Evaporation:- Factor and Distribution.
  4. Condensation:- Essential Conditions and Forms
  5. Clouds:- Formation and Types.
  6. Precipitation:- Types and Forms
  7. Global Distribution of Precipitation.

Types of Rain/Precipitation

  1. Conventional rain
  2. Orographic
  3. Cyclonic/ frontal rain.

Conventional rain-

Two conditions are necessary-

  1. Intense heating of ground so that the Low-pressure system develops and air vertically threw upwards due to strong convectional current.

  2. Sufficient supply of moisture.


  • When the insolation is high, the ground is sufficiently heated. Vertical Sunrays will heat up the ground in the afternoon between 12 to 3’O clock. After 3’O clock the ground will start transferring the heat to the atmosphere.

  • Thus the Low-pressure system develops making the air rise upwards vertically. Due to low pressure, strong winds from surrounding high-pressure air attracted to the central low pressure.

  • As the air rises upwards, it results in the formation of cumulonimbus clouds. So around 4’ to 5’O clock, visibility reduces and clouds will give very heavy rainfall along with lightning and thunder as well as hail for a short duration.

  • After the heavy rainfall, the temperature decreases because of the cold denser air descends. Such as cold air is known as SQUALL.

  • Convectional rainfall is an everyday phenomenon in equatorial or tropical regions.

  • Therefore, in the equatorial regions, this rainfall supports very densely tropical evergreen forests.

  • But outside the equatorial regions, convectional rainfall is a seasonal phenomenon that is such rain occurs during the summer season but is harmful as it results into soil erosion, destruction of Fertility of the soil, destruction of agriculture crops due to hail storms as well as may result into death of animals and humans.

Orographic rainfall-

  • Rainfall caused due to the mountain’s barriers. The mountains will force the winds to rise upwards on the windward side which will result in the cooling of air and heavy rainfall on the windward side.

  • The air is forced to come down causing drought-like conditions. Hence in the orographic rainfall, there is a maximum difference in the rain between the Leeward side and windward side.

Example- Mumbai – Pune

Mangalore – Bangalore

  • For the orographic rainfall to occurs the mountains should be to wind direction. Therefore, as aravalis are parallel to the direction of southwest monsoon hence the western part of Rajasthan is desert.

  • If the mountains are very nearer to the coast even the low mountains can give very heavy rainfall but the interior mountains should be of great heights to cause heavy rain because the air should be forced to rise up to a greater height.

  • On the leeward side if low mountains the rainfall decreases while on the leeward side of high mountains. There are very little rainfall forming deserts. This effect of mountains is known as the rain shadow effect.

Example 1 – A narrow strip running parallel to western Ghats on its leeward side is drought prove due to the Rainshadow effect of Western Ghats.

2 – Leh- Ladakh is deserts due to rain shadow effect of Himalayas.

3 – Patagonia desert due to rain shadow effect of Andes.

  • Orographic rainfall can occur in any season because it requires onshore winds. Therefore the northwest part of the USA receives rainfall throughout the year due to the onshore nature of Westerlies and obstruction caused by rockees.

Note- Cyclonic Rain is covered under the topic “Cyclone”.

Topics Covered in this Lecture

  1. Movement of Air
  2. 3-Dimensional Circulation in Atmosphere.
  3. Global Pressure belts
  4. Global Wind System

Air Pressure and Winds

Air pressure is the height of the column of air on a substance. It is also known as barometric pressure. Air pressure is maximum at the sea level (0km). In climate, pressure is inversely related to temperature.

There are two warm air rises causing low pressure while cold types of pressure conditions:-

  1. Low pressure
  2. High pressure

Warm air rises causing low pressure while cold denser air decent causing high pressure.

Winds always flow from high pressure to low  pressure. The greater the difference between high pressure and low pressure greater is the speed of wind.

  • Low pressure means the convergence of air while high pressure results in diversions.


  • The earth is divided into seven (7) different pressure belts.

    They are of two types:-

  • Thermally-induced pressure belt:-  these are caused due to changes in the temperature there are three thermally induced pressure belt-

  • Equatorial low-pressure belt- due to very high temperature and two polar high-pressure belts, due to extremely cold air and permafrost conditions.

  • There are four dynamically induced pressure belts two subtropical high-pressure belts where the air is forced to sink and two subpolar low-pressure belts where the air is forced to rise upwards. These pressure belts are created due to the rotation of the earth and hence are dynamically induced pressure belts.


  • The first pressure belt is an equatorial low-pressure belt or the belt of doldrums or ITCZ.

  • A permanent low belt form due to vertical sun rays throughout the year. Due to high insolation, the air is forced to rise in the form of conventional current to give rise to a low pressure on the ground.

  • As it is a zone of mixing of trade winds from the northern and southern hemisphere, it is known as ITCZ and due to frequent calm conditions, it is also known as belt of doldrums.


  • It is located between 23- degrees 35- degrees north and south.

  • The air which rises from the equator becomes colder and denser and therefore it is forced to settle down between 25-degree to 35-degree north and south giving rise to subtropical high pressure.

  • Due to high pressure, and high insolation deserts are located in the subtropical high-pressure belt. This region as it is a transition zone between westerlies and trade winds has frequent calm conditions and therefore are known as horses latitudes. This is also the zone of origin of trade winds and westerlies.


It is the zone of mixing of two different winds that is cold polar easterlies and warm westerlies and hence, it is known as zone of fronts. The low pressure is created as the winds are forced to rise from these latitudes.


  • Due to permafrost or extremely cold conditions, air descends giving rise to thermally induced high pressure.

  • All the pressure belts of the world except polar hp belt northward and southward along with movement of the sun.


There are three types of winds are as follows:

  1. Permanent winds or primary winds (trade winds, westerlies, and easterlies)

  2. Secondary or seasonal or regional (monsoon)

  3. Territory or local winds(loo, chinnok, harmattan, sirocio)

Causes of the winds/factors influencing the winds

  • Pressure Gradient/ difference

Wind always flow from high pressure to low pressure. Greater the difference between high pressure and low pressure is greater is the speed of wind.

  • Rotation of the earth

As per the coriolis effect, the objects , winds, ocean currents are deflected to the right in northern hemisphere and they are deflected to the left in southern hemisphere Coriolis force is inversely proportional to the rotational speed of the earth.

As the rotational speed of earth is maximum at the equator therefore Coriolis force is 0 at the equator and it is maximum at the poles.

  • Friction

Reduces the wind speed. Since there is no friction in the upper air, they move with great speeds. example: jet streams.

  • Gravity

It is the fourth factor and it is permanent winds or planetary. “The control release from the dams would have release the gravity of the calamity” sugar an industry>loss>no new investment because demand is constant and production sugarcane mills>loss.


They flow from subtropical high pressure to the equatorial low-pressure belt. The coverage at the equatorial low-pressure belt and therefore this belt is ITCZ (Inter Tropical Convergence Zone).

Note: why deserts are located in the western side of the continent?

  1. Sub tropical high pressure belt.
  2. Winds are offshore
  3. Washed by cold currents.

The trade winds are onshore on the eastern sides of the continent and therefore they give rainfall mostly throughout the year. Depending upon the rainfall the trade wind zone is divided into-

  1. monsoon  biome- seasonal rain
  2. maritime or pseudo monsoon- rainfall throughout the year due to onshore nature of trade winds

example: madagascar, brazil, carribian etc.

  • offshore trade winds promote dry climate condition and hence deserts are always located in the narrow subtropical high-pressure belts but on the western side of the continent, where the trade winds are offshore. They are also the regions that are would by the cold ocean current.


  • They flow from subtropical high-pressure belt to subpolar low-pressure belt. Westerlies are onshore on the western coast of the continent. (in Europe).

  • These warm westerlies are responsible for increasing the temperature on the western coast of the continent in higher latitudes.

  • For example north-east European type, or British type of climate. The onshore westerlies also give rainfall throughout the year and bring warm ocean currents on the western coast of continents in the higher latitudes.

  • Due to the dominance of water in the southern hemisphere, westerlies blow with great speed and they have different names such as:

roaring 40s @40-degree South

furious fifties @50- degree South

shrieking sixties @60-degree South

  • westerlies enter India during the winter season and are responsible for winter rainfall in north India, as western disturbances.


Extremely cold, dusty(show) winds which are lethal are polar easterlies.

MONSOON (Seasonal wind)

(It refers in Indian geography)- see the topic in Indian geography

The word monsoon means a complete reversal in wind direction. Monsoon type of climate is present In south Asia, mainland Southeast Asia which includes- Myanmar, Thailand, and French East India-

  1. Laos
  2. Cambodia
  3. Vietnam


When the sun is in the northern hemisphere, the sun is vertically overhead over the tropic of cancer. This results in heating of south and southeast Asia, developing low pressure over this region. After 21st June, ITCZ shifts and becomes parallel to the foothills of the Himalayas because low pressure or heating is intensified. On the other hand, in the southern hemisphere, there is winter season and high pressure. This causes the winds to originate over the Australian region. These winds after crossing the equator are deflected to the right due to Coriolis force and approach India as the southwest monsoon as these winds are onshore they give rainfall.


With the shifting of the

Sun to the southern hemisphere, ITCZ shifts to the south, and hence the southern hemisphere starts experiencing the summer season. on the other hand, due to extremely cold temperatures or climate, high pressure develops over India. This causes the winds to originate over the high-pressure center as the northeast monsoon. Since these originate except some winds which pick up moisture from the Bay of Bengal and give rainfall to the Coromandel coast (TN COAST).



These are diurnal or daily winds. During the day sea breeze as the land is more heated compared to the sea. During the night time, land losses the heat more quickly develops high pressure and therefore land breeze flows from land to the sea.


  1. Moderation of climate therefore coastal regions have low daily range of temperature.

  2. Removes pollution (air pollution).


  1. Silk and cotton textile industry dominates in Tamil Nadu
  2. Has highest productivity of sugar (14%)

During the day the upper slopes of mountains are heated, developing low pressure hence winds flow from valley to mountains top as valley breeze. On other hand during the might time, winds flows from the slopes into the valley, because the mountains slopes will loose the heat quickly. While the valley floors are comparatively warmer. Mountain breeze responsible for temperature.

Local winds (world level)

Hot/warm local winds

  1. Chinook
  2. sirocco
  3. fohn
  4. harmattan

cold local winds

  1. mistral
  2. bora
  3. blizzard – northern (the USA and Canada), burran (Siberia)


It is a local wind which flows along the eastern slopes of the rockees, while descending it gets heated and raises the temperature so during the winter season arrival of chinook causes melting of snow and hence the name is snow eater. Similar mind which blows on the northern slopes of the alps is known as fohn.


Agriculture is possible throughout the year in the narrow slopes along the foothills of rockees.


Because of fohn Switzerland is climatic oasis of Europe.

Harmattan/doctor wind

Extremely hot and hyper arid winds which originates in Sahara desert and flow towards the western coast of Africa is known as harmattan. With the arrival of harmattan excessive humidity is removed from this region making life easier and hence they are doctor winds.

Sirocco (Blood Rain)

Warm and dusty winds that originates from the Sahara Desert and flow toward the Mediterranean Sea are known as Sirocco. These winds pick up moisture from the Mediterranean Sea and give rainfall to the southern part of Spain, France, and

Italy. Along with the rain, red-colored sands fall and hence people call it blood rain. Sirocco has also various names:

Khasmin in Egypt

Ghibil in Libya

Chilli in Tunasia

Laventer in Spain

Simoon in Arabia

Sirocco us harmful for agriculture.

Cold winds

  1. Mistral

Very high velocity winds which originate from the Alps and flows toward the Mediterranean Sea is Mistral. Mistral after entering a narrow valley of River Rhone attains a strong force causing disruptions in the communication network in Southern Europe. Hence, North facing houses are avoided in Europe.

  • Bora

A cold wind which blows along the Adriatic Sea. It picks up moisture from Adriatic Sea and give precipitation in Italy.

  • Blizzard

Extremely cold, dusty (full of snow), lethal wind which is an extension of polar easterlies during the winter season is known as Blizzard. It is known as Norther in USA and Canada. Burran in Siberia.

Local winds in India-

  1. Mango Shower
  2. Blossom Shower
  3. Loo
  4. Kalbaisakhi

Mango shower and blossom shower              

Monsoon shower along the western coast of India due to localised pressure differences is either known as Mango shower because they help in early ripening of mangoes or blossom showers because they help in blossoming of coffee plantation in Karnataka and Kerala.     

Kal Baisakhi are local thunderstorms associated with violent winds, torrential downpours, often accompanied by hail. Loo These are strong, gusty, hot, dry winds blowing during the day over the North and North-Western India. Sometimes they even continue until late in the evening.


  1. Mediterranean Type of Climate      

The pressure belts occupy their original position only during the equinox while they shift with the northward and southward movement of the sun. Mediterranean type of climate is an outcome of the shifting of pressure belts. The climate is located in-

  • Countries surrounding Mediterranean Sea
  • Southern USA in California, where the largest city is San Francisco
  • Southern Chile
  • Southern Australia

This region comes under the influence of offshore trade winds during the summers and hence experiences dry climatic conditions. But, during the winters, as all the pressure belts move to the South, this area comes under the influence of onshore westerlies giving rainfall. Hence, Mediterranean type of climate is famous for dry summers, but moist winters.

Here, Sclerophyll vegetation dominates. In this, plants have natural insulating layer which doesn’t allow air to penetrate.

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