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Chapter 14

Atmospheric Pressure and Winds

Class 9 - Total Geography Morning Star


Short Answer Questions

Question 1

Name the four main pressure belts of the earth.

Answer

The four main pressure belts of the earth are-

  1. Equatorial Low Pressure Belts
  2. Sub-tropical High Pressure Belts
  3. Circum-polar Low Pressure Belts
  4. Polar High Pressure Areas

Question 2

What is the Circum-polar Low Pressure Belt?

Answer

These belts are located between 60° and 70° in each hemisphere. Due to the earth's rotation, the wind surrounding the Polar region blow towards the equator. Centrifugal forces operating in this region create the low pressure belt appropriately called Circum-polar Low Pressure Belt.

Question 3

How does the Coriolis Force vary latitudinally?

Answer

Due to the rotation of the earth on its tilted axis, the wind starts deflecting to its right in the Northern Hemisphere. In the Southern Hemisphere, it starts deflecting to its left from its original path.

Thus, a wind blowing from the north becomes north-easterly in the northern hemisphere and the wind blowing from south becomes south-easterly in the southern hemisphere.

The Coriolis effects is greatest near the poles and zero at the equator because of the earths spherical shape. It is proportional to the angle of the latitude.

Question 4

Name the three chief types of wind.

Answer

The three chief types of wind are-

  1. Permanent or Planetary winds
  2. Periodic winds
  3. Local winds

Question 5

What are periodic winds?

Answer

Certain winds reverse their direction periodically with season and are called periodic winds. These winds blow at regular intervals or in regular cycles. They are winds that result from localised differences in pressure and temperature.

Question 6

What are local winds? Name any two local winds.

Answer

There are certain winds in different parts of the world which flow in comparatively small area and have special characteristics. These are called local winds.

Two local winds are Loo and Mistral.

Question 7

Name two types of variable winds. Why are they so called?

Answer

Two types of variable winds are-

  1. Cyclones
  2. Anticyclones

They are called variable winds because they do not blow in any definite direction and their direction varies with the movement of the pressure system. Their speed also depends on the intensity of the depression.

Question 8

Why are cyclones frequent in summer in the tropical region?

Answer

Cyclones are frequent in summer in the tropical region because of the movement of the Doldrum belt away from the equator.

Question 9

Mention any two differences between Tropical Cyclones and Temperate Cyclones.

Answer

Tropical CyclonesTemperate Cyclones
These cyclones generally originate in the tropical region between 8° and 20° N and S.These cyclones are active over mid-latitudinal region between 35° and 65° N and S.
Tropical cyclones usually travel from east to west.Temperate cyclones usually travel from west to east.

Question 10

How are cyclones named differently in different parts of the world?

Answer

Cyclones are called tornadoes in North America, hurricanes in south-east Caribbean region, typhoons in eastern China, Japan and Philippines islands and cyclones in the Bay of Bengal and Arabian sea.

Question 11

What are two chief characteristics of anticyclones?

Answer

Two chief characteristics of anticyclones are-

  1. There is high pressure in the core and low pressure around it. Winds blow gently outwards.
  2. These winds are clockwise in northern hemisphere and anticlockwise in the southern hemisphere.

Define the following terms

Question 1

Pressure Gradient

Answer

Pressure gradient is defined as the decrease in pressure per unit distance in the direction in which the pressure decreases most rapidly.

Question 2

Winds

Answer

Contrasts in temperature cause changes in air density, which are responsible for variations in pressure. These variations cause horizontal movements of air called winds.

Question 3

Coriolis force

Answer

Coriolis force is the force generated due to rotation of the Earth on its inclined axis. Due to Coriolis force, moving objects (winds and water currents) tend to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflection is least at the Equator and greatest at the poles.

Question 4

Altitude

Answer

Altitude refers to the height of a place in relation to sea level or ground level.

Question 5

Monsoons

Answer

Monsoons are periodic seasonal winds blowing in the regions of South East Asia and Northern Australia. They develop because of differences in heating conditions of the continent and the oceans.

Distinguish between the following

Question 1

Cyclones and Anticyclones

Answer

CyclonesAnticyclones
There is low pressure in the centre and high pressure around it.There is high pressure at the core and low pressure around it.
Winds blow inwards.Winds blow gently outwards.
These winds are anti-clockwise in northern hemisphere and clockwise in the southern hemisphere.These winds are clockwise in northern hemisphere and anticlockwise in the southern hemisphere.

Question 2

Permanent and Periodic Winds

Answer

Permanent WindsPeriodic Winds
Permanent winds blow throughout the year from one latitude to the other in response to the latitudinal differences in the air pressure.Certain winds reverse their direction periodically with season and are called periodic winds. They blow at regular intervals or in regular cycles.
For example- trade winds, the Westerlies, the Polar Easterlies.For example- land and sea breezes and the seasonal winds.

Question 3

Summer and Winter Monsoons

Answer

Summer MonsoonsWinter Monsoons
Summer monsoon usually begins in April and lasts till the month of September.The monsoon usually begins in October and lasts till March.
These are brought by cooler, denser and high-pressure winds over the Indian Ocean, to the low-pressure area over the landmass.These winds blows from the high pressure over the land mass to the oceans which have lower pressure developed over them.
The rainfall can be very intense as these winds travel from water bodies to land and thus, carry a lot of moisture.Winter monsoons are mostly less powerful and less destructive than summer monsoons as they are dry winds and do not contain enough moisture to cause heavy rains.
They are also called South-west monsoon.They are also called receding monsoon.

Structured Questions

Question 1(a)

What is meant by the term 'Atmospheric Pressure'?

Answer

Atmospheric pressure refers to the force per unit area exerted against a surface by the weight of the air above that surface.

Question 1(b)

Explain briefly the factors that affect Atmospheric Pressure.

Answer

The factors that affect Atmospheric Pressure are-

  1. Altitude
  2. Temperature
  3. Water Vapour
  4. Rotation of the Earth

Question 1(c)

Give a geographical reason for each of the following:

(i) The Westerlies in the Southern Hemisphere blow with greater force than those in the Northern Hemisphere.

(ii) There is a seasonal shifting in pressure belts.

(iii) As we go higher, the atmospheric pressure decreases.

Answer

(i) The Westerlies in the Southern Hemisphere blow with greater force than those in the Northern Hemisphere because there is absence of landmass in the southern hemisphere.

(ii) Due to the rotation of the Earth on its axis which is inclined at 23½°, the heating of the continents, oceans and pressure conditions vary greatly in January and in the July season. On 21st June, when the sun is overhead the Tropic of Cancer, the pressure belts shift 5° northwards. When the Sun shines vertically overhead the Tropic of Capricorn on December 22nd, the pressure belts shift 5° southwards from their original position.

(iii) As we go higher, the atmospheric pressure decreases because at higher altitudes, the air is thinner or less dense than the air at the sea level. The fewer number of gas molecules at higher altitudes means fewer molecular collisions and a decrease in air pressure.

Question 1(d)

Draw a well labelled diagram showing the pressure and wind belts of the earth.

Answer

Below labelled diagram shows the pressure and wind belts of the earth:

Draw a well labelled diagram showing the pressure and wind belts of the earth. Atmospheric Pressure and Winds, Total Geography Solutions ICSE Class 9.

Question 2(a)

Briefly explain the three chief types of winds.

Answer

The three chief types of winds are-

  1. Permanent / Prevailing / Planetary Winds — Permanent winds blow throughout the year from one latitude to the other in response to the latitudinal differences in the air pressure. For example, Trade winds, the Westerlies and the Polar Easterlies.

  2. Periodic Winds — Certain winds reverse their direction periodically with season and are called periodic winds. For example, Land breeze and sea breeze, summer monsoon and winter monsoon.

  3. Local winds — There are certain winds in different parts of the world which flow in comparatively small area and have special characteristics. These winds are called local winds. For example, loo, foehn and chinook and mistral.

Question 2(b)

Describe some of the important types of local winds.

Answer

Some of the important types of local winds are-

  1. Loo — In the northern plains of India and Pakistan, a very hot and dry wind blows from the west in summer in the afternoons, known as loo. Its temperature invariably ranges between 45°C and 50°C. It may cause sunstroke to people.

  2. Foehn and Chinook — Foehn is a dry, hot wind which blows in the valley on the northern slopes of Alps. The temperature of the wind is from 15°C to 20°C. The wind is of use for melting snow and it hastens the ripening of grapes.

Similar kind of wind in the USA and Canada move down the west slopes of the Rockies and are known as Chinook.

  1. Mistral — Mistral is a local cold wind that blows from the Alps over France towards the Mediterranean Sea. Mistral brings down the temperature below freezing point.

Question 2(c)

Give a geographical reason for each of the following:

(i) The winds are directed to the right of their flow in the Northern Hemisphere.

(ii) Temperature and pressure are inversely related to one another.

(iii) Humid air is lighter than dry air.

Answer

(i) The rotation of the earth produces another force other than the pressure force which is called the Coriolis force. It causes deflection of winds. Thus, the winds are directed to the right of their flow in the Northern Hemisphere due to Coriolis effect.

(ii) Temperature and pressure are inversely related to one another as atmospheric pressure decreases with increase in temperature. This is because when the temperature rises, air expands. The molecules of air move far apart and hence exert less pressure. On the contrary, with decrease in temperature, the air gets compressed and the space between molecules decreases and exerts more pressure on the region.

(iii) Humid air is lighter than dry air because the molecular weight of water is less than the average molecular weight of air. When water evaporates and enters the atmosphere as a gas, the water vapour molecules take the place of other gas molecules in the air. So, the volume of wet air weighs less than an equal volume of dry air.

Question 2(d)

What is Coriollis Effect? How does it affect the planetary winds?

Answer

Coriolis effect or Coriolis force is the force generated due to rotation of the Earth on its inclined axis. Due to Coriolis effect, planetary winds tend to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflection is least at the Equator and greatest at the poles.

Question 3(a)

Explain the weather conditions associated with tropical and temperate cyclones.

Answer

Tropical cyclones-

  1. Tropical cyclones are mainly developed over the sea.
  2. They are usually limited to a small area and usually travel from east to west.
  3. When there is a tropical cyclone the air is still, but the temperatures and the humidity are high. There is sudden drop in air pressure.
  4. When the front of the vortex arrives, there are strong winds and thick clouds, then the winds become violent with great speed. Dense clouds and heavy rain reduce visibility.
  5. The eye of the cyclone heralds a calm condition.
  6. When the rear of the vortex arrives, there are violent winds, thick clouds and heavy rains. The wind blows in the opposite direction.

Temperate Cyclones-

  1. Temperate cyclones are produced both on land and on sea.
  2. They occupy areas measuring a couple of thousands of kilometres and move from west to east.
  3. In a temperate cyclone, the wind speed is low.
  4. All sectors of the cyclone have different temperatures.
  5. There is not a single place where the winds and rains are inactive.
  6. Direction of winds rapidly change at the front.
  7. The approach of a temperate cyclone is marked by fall in temperature, fall in the mercury level, wind shifts and a halo around the sun and the moon.
  8. A light drizzle follows which turns into a heavy downpour.
  9. They cause light rainfall which continues for many days.

Question 3(b)

What are the Jet Streams? What is the significance of the Jet Streams?

Answer

Jet streams refer to the concentrated bands of rapid air movement found at the tropopause and the stratosphere, located at 10-15 km above the surface of the Earth.

The significance of jet streams is as follows-

  1. Most weather systems are moved forward with the jet streams. The position and strength of the jet stream then helps meteorologists forecast future weather events.
  2. The airline industry consistently uses the jet stream for its flights, which reduces the flight time and aids in reduction in fuel consumption.
  3. Precipitation usually increases in California because the polar jet stream moves farther south and brings more storms with it. In addition, precipitation often increases in Europe because the jet stream is stronger in the Northern Atlantic.
  4. Whatever the position of the jet stream, it has a significant impact on the world's weather patterns and severe weather events like floods and droughts.

Question 3(c)

Give a geographical reason for each of the following:

(i) Doldrums is a low pressure belt.

(ii) Equatorial regions have low atmospheric pressure throughout the year.

(iii) Low atmospheric pressure prevails over the Circum-polar region.

Answer

(i) Doldrums extend from 0 to 5° North and South of Equator. Due to the vertical rays of the sun here, there is intense heating. The air expands and rises as convection current, causing a low pressure to develop here. Hence, Doldrums is a low pressure belt.

(ii) Equatorial regions have low atmospheric pressure throughout the year because atmospheric pressure decreases with increase in temperature. This is because when the temperature rises, air expands. The molecules of air move far apart and hence exert less pressure. On the contrary, with decrease in temperature, the air gets compressed and the space between molecules decreases and exerts more pressure on the region.

(iii) In the sub-tropical region the descending air gets divided into two parts. One part blows towards the Equatorial Low Pressure Belt. The other part blows towards the Circum-Polar Low Pressure Belt. This zone is marked by ascent of warm Sub-Tropical air over cold polar air blowing from poles. Due to earth's rotation, the winds surrounding the Polar region blow towards the Equator. Centrifugal forces operating in this region create the low pressure belt, known as Circum-polar low pressure belt.

Question 3(d)

Draw a well labelled diagram showing a cyclone in the Northern Hemisphere.

Answer

Below labelled diagram shows a cyclone in the Northern Hemisphere:

Draw a well labelled diagram showing a cyclone in the Northern Hemisphere. Atmospheric Pressure and Winds, Total Geography Solutions ICSE Class 9.
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