The document discusses wind, storms, and cyclones. It explains that air pressure causes wind, and differences in air pressure cause wind to move from high to low pressure areas. Uneven heating of the atmosphere and land/water surfaces creates pressure differences and causes global wind patterns like sea breezes and monsoons. Thunderstorms occur when warm, moist air rises and condenses into rain clouds. Cyclones form when warm air rises in the center of circulating wind patterns, creating low pressure at the eye of the storm. Cyclones can be destructive due to high winds and storm surges. Modern forecasting allows warnings before cyclones make landfall, helping prepare communities through evacuation and shelters.

1. VIDYA BHARTI EDUCATIONAL INSTITUTIONS
SCIENCE NOTES CLASS 7TH
WIND, STROM 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
exerts pressure. 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.
 Airplane 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 vice-versa.
Cause 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.
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2. VIDYA BHARTI EDUCATIONAL INSTITUTIONS
SCIENCE NOTES CLASS 7TH
WIND, STROM AND CYCLONE
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 vapor 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 vapor 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 thunderstorm.
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 vapor 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.
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3. VIDYA BHARTI EDUCATIONAL INSTITUTIONS
SCIENCE NOTES CLASS 7TH
WIND, STROM AND 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 Japan.
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
agencies:
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.
(2)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.
(3) Action on the part of people:
Pay attention towards warning broadcasts from time to time.
CONTACT: 9654089891, 9811083910, 9953898004
HEAD OFFICE: 131/12 KRISHNA COLONY, STREET NO 4, NEAR SHIV MANDIR, GURGAON

4. VIDYA BHARTI EDUCATIONAL INSTITUTIONS
SCIENCE NOTES CLASS 7TH
WIND, STROM AND CYCLONE
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.
CONTACT: 9654089891, 9811083910, 9953898004
HEAD OFFICE: 131/12 KRISHNA COLONY, STREET NO 4, NEAR SHIV MANDIR, GURGAON