Seven Science Class Note CBSE NCERT
Wind, Storm and Cyclone
Air: Air is a mixture of gases.
Wind: Moving air is called wind.
Air exerts pressure: The fact that Air exerts
pressure can be understood by many examples.
Tube of bicycle gets inflated when air is filled in it. This happens because air
It is difficult to move the bicycle against the direction of wind. Similarly, a
sailor feels it difficult to sail his boat against the direction of wind. This
happens because air exerts pressure.
When boiled water is filled in a tin can and its mouth is closed with the lid;
followed by cold water being poured over it, the can gets distorted. This
happens because air inside the can gets condensed and creates low pressure. The
higher pressure from outside distorts the shape of the can.
You can fly a kite because air exerts pressure from the rear portion of the kite.
Aeroplane can fly because air exerts pressure.
Birds can fly because air exerts pressure.
High speed wind creates area of low air pressure:
Thatched roof is blown off because of high speed wind. This happens because high
speed wind creates an area of low pressure. Due to this, the thatched roof is
sucked up and finally gets blown away. Thus, increase in wind speed is
associated by decrease in air pressure.
Movement of air:
Air moves from high pressure zone to low pressure zone of air. Air; from high
pressure zone; rushes towards the low pressure zone; to fill the gap. Thus,
greater is the pressure difference greater will be the speed of wind and
of Wind Movement:
Pressure difference in the air is cause by differential heating. Warm air rises
up because it becomes lighter. This creates an area of low pressure below the
rising air. The area of low pressure is quickly filled in by cooler air from
around. That is how the wind movement is created on the earth. Thus differential
heating of air creates pressure difference; which makes the wind movement.
Movement of Air Because of Uneven Heating on Earth:
Movement of Air in Equatorial Region: The
Equatorial region gets the highest amount of sunshine throughout the year. Due
to this, the air near the equatorial region gets heated. The warm air rises up
and creates an area of low pressure. The cooler air; from the higher latitudes;
rushes towards the equator to fill the gap. This results in the wind movement
from tropical zones towards the equator. Similarly, wind movement is caused in
other parts of earth.
Movement of Air Because of Uneven Heating of Land and Water:
Sea Breeze: In coastal areas, the land warms
up faster than the ocean surface. Warm air from the land rises up and creates an
area of low pressure near the ground. The low pressure zone is filled in by
cooler air from the ocean surface. This gives rise to the sea breeze.
Land Breeze: During night, the land cools
down faster than the ocean surface. The warmer air from the ocean surface rises
up and creates an area of low pressure near the surface. This is filled in by
cooler air from the land. This gives rise to the land breeze.
Monsoon: The flow of moist air from ocean
towards land in summer is called Monsoon or Monsoon wind. The word Monsoon came
from Arabic word ‘mausim’, which means weather. The term ‘Monsoon’ had
traditionally been used to describe the typical climate of the Indian Peninsula
during the rainy season. Now-a-days, the term ‘Monsoon’ is being also used to
describe the similar climate in other parts of the world.
Monsoon winds carry water vapour which falls over the land in the form of rain
after cooling down. In India, the harvest depends on the monsoon wind because it
is the major factor towards bringing rainfall in India.
Thunderstorm: Many a times, rain is
associated with lightning. Rain with lightning is called thunderstorm. This
happens mostly in the summer. Thunderstorm occurs frequently in hot and humid
tropical areas, such as India.
In summer, the air rises up along with lot of moisture. After rising up to a
certain height water vapour cools down and starts coming down in the form of
water droplets. The swift drifting of water droplets creates electricity among
clouds. This manifests in the form of lighting and sound. This is known as
CYCLONE: Fast moving air towards a centre;
usually with heavy rain; is known as cyclone. Wind direction, wind speed,
humidity and temperature together create cyclone.
When water vapour in the clouds cools down it releases heat. This heat warms the
air around the clouds. The warm air rises up and creates a low pressure zone.
This low pressure zone is quickly filled in by cooler air from the surrounding.
This creates a system in which the centre is having very low pressure and is
surrounded by wind moving around it. The wind from the surroundings moves in a
circular fashion towards the centre and gives rise to cyclone.
The centre of the cyclone is called ‘eye’. A cyclone may be 10 to 15 km high. It
is a rotating mass of air. The diameter of eye may be vary from 10 to 30 km. Eye
of a cyclone is a calm area, but around the eye air may move with high speed
such as 150 to 250 km/h.
Destruction caused by Cyclone: A strong
cyclone can be very destructive.
Since cyclone is accompanied with high speed wind, it can damage houses,
telephone poles, electric poles and uproot trees.
Strong wind of the cyclone may push the sea water towards the sea shore in the
form of high wave. These high waves may damage houses and push water to low land
which may be accompanied with loss of life and properties.
Being the area of very low pressure, eye of cyclone lifts water in the centre.
This may cause the rise of water up to 3 to 15 metre high. This appears like a
water wall. This water wall; when pushed towards seashore; can inundate a large
area, which can cause the loss of live and property.
In India, the eastern coastal region is more vulnerable to cyclones; in terms of
intensity and frequency of cyclones, compared to the western coast.
Cyclone is known by different names in different countries. Cyclone is known as
HURRICANE in the American continent. It is known as TYPHOON in Philippines and
TORNADOES: Tornadoes are the very fast moving dark cloud in the shape of a
funnel. The speed of violent tornadoes may be up to 300km. Tornadoes are as
destructive as cyclone. Tornadoes may form within cyclones.
ANEMOMETER: An instrument which measures the speed of wind.
Effective Safety Measures:
(1) Safety services towards cyclone and other disaster by Government and other
- Cyclone forecast and warning services: With the advances in technology, cyclone
can be forecast in about 24 to 48 hours in advance. This becomes possible
because of satellites and computers. When a cyclone is nearer to the coastline,
the cyclone warning is forecasted almost at every half an hour.
- Rapid communication to the Government and concerned people: Communication about
the cyclone is given quickly to the Government and the people so that proper
safety measures and rescue operations can be carried out.
- Construction of cyclone shelters in the cyclone prone area.
- Shifting the people quickly to a safer place.
(2) Action on the part of people:
- Pay attention towards warning broadcasts from time to time.
- In case of a cyclone warning, stock necessary food items and medicines.
- If possible, move to a safer place.
- Fishermen should not venture into the sea during a cyclone warning.
- Cooperate with others in the community.
- Help the rescue team.
Test Your Knowledge
NCERT Questions And Answer
- Wind: Moving air is called wind.
- Air exerts pressure.
- Air expands on heating.
- Warm air is lighter and rises up.
- Air moves from high pressure to low pressure.
- Air Current: Movement of air from high pressure zone to low pressure zone
creates air current.
- Uneven heating on land creates air current.
- Air moves from 0-30⁰ north and from 0-30⁰ south towards equator.
- Land gets heated as well as loses heat faster than water.
- Monsoon: The flow of air from ocean towards land in summer is called Monsoon or
- Monsoon winds carry vapour which falls over the land in the form of rain after
- In India harvest is depend on monsoon wind, which brings rain.
- Thunderstorm: Rain with lightning is called thunderstorm. This happens mostly in
- Thunderstorm occurs frequently in hot and humid tropical areas, such as India.
- Cyclone: Fast moving air towards a circle usually with heavy rain is known as
- Wind direction, wind speed, humidity and temperature together creates cyclone.
- Eye: Centre of cyclone is known as eye.
- Tornadoes: Tornadoes are the very fast moving dark cloud in the shape of funnel.
Tornadoes may form within cyclones.
- The speed of violent tornadoes may be up to 300km.
- Tornadoes are as destructive as cyclone.
- Anemometer: An instrument which measures the speed of wind.