CHAPTER-2

IN SEARCH OF THE SOURCE OF WIND

📚 ATMOSPERIC PRESSURE

The weight exerted by air on to the earth's surface is called atmospheric

pressure. 

👉 The air exerts an average weight of 1034mg per square cm.

👉 Mercury Barometer is the instrument used to measure atmospheric pressure.

👉 It is recorded in units like Millibar (mb) & hectopascal (hpa). 

👉 The average at pressure at the sea level is 1013.2 hpa which is known as standard pressure. 

📚 THE FACTORS INFLUENCING ATMOSPHERIC PRESSURE:

1. Altitude 

2. Temperature

3. Humidity

1. Altitude

◻️As altitude of a place increases at: pressure decreases.

◻️ There is an inverse relationship between them.

◻️The pressure decreases at the rate of 1 millibar per an altitude of 10 meters.

Eg: places like ooty & Kodaikanal experience low pressure.

2, Temperature

◻️As temperature of a place increases at: pressure decreases & vice versa.

◻️The reason is when air gets heated loses it density & expands high. This will lead to lowering of pressure.

Eg: Equatorial region experience low pressure. 

3.   Humidity

The content of water vapour in air is known as Humidity.

As humidity of air increases at: pressure decreases.

Eg: coastal regions experience lower pressure

◻️HIGH PRESSURE AND LOW PRESSURE

◻️If the at: pressure of an area is higher than of the surroundings, it is known as high pressure (H) & if it is lower than its surroundings is known as low pressure (L)

◻️Isobars are the imaginary lines drawn on maps connecting same atmospheric pressure regions.

📚 GLOBAL PRESSURE BELTS 

Generally the earth experiences the same atmospheric pressure between certain latitudes. These latitudinal zones are known as global pressure belts. They

are.....

1.  Equatorial LpB

0 ° (5°N 5°S) 

2.  Subtropical HpB

30°  N&S

3.  Subpolar -LpB

60° N & S

4.  Polar HpB

90° N & S

1. Equatorial LpB (0°)

◻️This is the zone where sun's rays fall vertically throughout the year.

◻️Hence temperature will be high in this zone.

◻️The air expands due to sun's heat & rises up on a large scale.

◻️ This is the reason for the low pressure experienced throughout this zone.

◻️This pressure extended 5° N & S latitudes. 

◻️ Also known as 'Doldrum' (The zone with no Winds)

2. Subtropical High

 Pressure Belt (30o N & S)

◻️The hot air coming from the equatorial LPB cools gradually and subsides at the subtropical zone due to the rotation of the earth.

◻️ This will lead to the occurrence of high pressure here.

➤ Also known as 'Horse Latitude' by ancient mariners.

3. Subpolar Low Pressure Belt (60°) N & S

◻️This zone is very close to the pole, the air is colder here.

◻️This cold air is thrown up due to the rotation of earth. As a result low pressure is experienced here.

4 . Polar High Pressure Belt (90°) N & S. 

◻️Sun's rays is not directly reach here throughout year.

◻️ Hence this zone experiences severe cold.

◻️As a result, low temperature of the polar air leads to high pressure there.

📚 WINDS

👉 The horizontal movements of air from high pressure regions to low pressure regions is known as wind. 

👉 Variations in at: pressure leads to the formation of wind. 

📚 FACTORS INFLUENCING SPEED & DIRECTION OF WIND

1. Pressure gradient force

2. Coriolis force

3. Friction

1. Pressure Gradient Force

The distance of various pressure lines in atmosphere can influence the formation of pressure gradient force.

2.  Coriolis force

Due to the earth's rotation every freely moving bodies get deflected to the right in the Northern hemisphere & left in the southern hemisphere. This hidden force is known as Coriolis force.

◻️ Ferrel's Law 

3.  Friction. 

◻️Friction is the obstacles in front of wind.

◻️Wind blows at high speeds over ocean surface and level lands where is friction is less.

◻️Friction is being more in dense forest regions, irregular surfaces where the speed of wind is less.

📚 PLANETARY WINDS

The wind developed between the global pressure belts can be made generally called as planetary winds.

The different planetary winds are. 

1. Trade winds

2. Westerlies

3. Polar Eosterlies

1. Trade Winds

◻️During ancient times these winds helped travelers who sailed for trade in sail boats. They call these winds as 'trade winds'

◻️Trade winds blowing from subtropical HpB to equatorial LPB. 

◻️As these winds blow from the north east direction in northern hemisphere & south east direction in Southern hemisphere. 

◻️ITCZ is the zone where the trade winds from both the hemisphere converge near equator. (Inter Tropical Converge Zone)

2. Westerlies:

◻️These winds blowing from the western direction in both hemispheres. 

◻️That is why known as Westerlies.

◻️They blows from subtropical HpB to Subploar LPB continuously.

◻️The Westerlies are stronger in southern hemisphere than in the northern

hemisphere. 

3. Polar Easterlies

◻️These winds blowing from East in both hemisphere due to coriolis force. Hence known as polar easterlies.

◻️These winds blowing from the polar HpB to the subpolar LpB

◻️They are colder & Stronger winds.

📚 PERIODIC WINDS

◻️In certain regions winds blow in a particular direction over short periods or throughout particular seasons. These are periodic winds.

➤ Monsoon winds are the finest example of periodic wind. 

📚  Factors responsible for the formation of monsoon winds are.

◻️The apparent movement of the sun. 

◻️ Coriolis force

◻️ Difference in heating of land & Ocean.

📚  SW Monsoon Winds

◻️During summer land regions are heated up faster than ocean.

◻️ So low pressure develops over land these days. 

◻️This pressure difference between land & ocean causes winds to blow from ocean to land. 

◻️Due to Coriolis force this wind changes its direction to south west in Northern hemisphere. These are SW Monsoon winds.

◻️ In India June, July & August - SW Monsoon season.

📚 NE Monsoon Winds

◻️During winter the ocean regions are heated up faster than land.

◻️ So low pressure develops over the ocean. High pressure over the land. 

◻️ This pressure difference between ocean & land causes winds to blow from land to ocean. 

◻️Due to Coriolis force this wind blow from the NE direction in Northern hemisphere. These are NE Monsoon Winds

◻️Months: October & November in India.

📚 LAND BREEZE & SEA BREEZE

👉 Sea Breeze:

During day time low pressure develops over the land attracts air from sea. This air movement from sea to land is known as sea breeze.

👉 Land Breeze

◻️During night low pressure develops over the sea attracts air from land. This air movement from land to sea known as land breeze.

📚 Valley Breeze

◻️During the day time air in the valley gets heated up more than the air on mountain tops. 

◻️When air gets heated loses its density expands & rises up.

◻️ As a result, the wind blows from the valley to mountain is known as valley breeze.

📚 Mountain Breeze

👉 During the night air in the mountain regions cools due to the intense cold in that region.

👉 As cool air is more denser it blows towards the valley from mountains. This is known as mountain breeze.