Moving air is called wind. A gentle wind is called breeze,
while a very strong wind that damages life and property is called a storm.
There are two kinds of local winds-land and sea breeze.
A sea breeze forms along the side of a large body A of
water. It takes much more heat to warm up water than it does dry land, but it
also takes more time to cool water. During daytime, land gets heated up faster
than the sea. The air above the land heats up, expands, gets lighter and
expands. This creates an area of low pressure. The cooler air from the sea
rushes in to take its place.
At night a reverse process happens because the land will
cool faster. During the night, the land cools down faster while the sea retains
its heat. The air above the water thus acquires a lower pressure than the air
above the land and rises up. Cooler air from the land blows towards the sea.
This is called land breeze. This causes the land breeze effect.
Air Pressure
There is air all around us. We cannot see air but (we can
feel air on our skin when we stand under a fan or when the wind blows.) The
envelope of air around the Earth extends to a height of a few kilometres above
us. This air has weight and presses down on all things present on the Earth's
surface creating a pressure called the atmospheric pressure. 7
How many of us had not had fun bursting balloons after the
birthday party is over? Have you wondered why the bursting of a balloon makes a
loud explosive sound? You might have also heard the sound of a bicycle tyre or
car tyre bursting? That too makes a loud explosive sound.
Air is pumped into a balloon or a tyre. As a result, the air
inside the balloon and the tyre is at a pressure many times that of the air
outside. As soon as there is a tiny hole, the air inside rushes out, tearing
the rubber of the balloon n or the tyre, making a loud d sound) sound
Did You Know?
Why do astronauts wear special suits? The blood inside our
body is at the same pressure as that of air outside us. In space, there is no
air. So, astronauts must wear special suits so that the blood pressure is
maintained. Otherwise, the higher pressure of the blood inside will cause the
blood vessels to burst.
) why why A
Activity 1
EXPERIENTIAL LEARNING, OBSERVATION, ANALYSIS, EVALUATION
To demonstrate that air exerts pressure
Materials needed: A PET bottle, water, hot water
Procedure:
1. Boil about half a cup of water.
2. Pour this hot water into a PET bottle.
3. After the steam starts rising from the mouth of the
bottle, screw its cap on. This will ensure that the cold air which was
previously in the bottle is expelled by the steam.
4. Wait for the water to cool down.
5. You will notice the bottle starts getting crushed and
ends up as a totally crushed mass of plastic.
What happens? When the steam occupying all the volume in the
bottle becomes cold, it condenses back into water. As a result, the air
pressure inside is much lesser than the pressure outside. Due to this unequal
pressure, the bottle is crushed.
Wind and its effects
We know that moving air is called wind. Wind always blows
from a region of higher pressure to a region of lower pressure. That is, wind
always blows from a colder region to a warmer region. The difference in temperature
is one of the factors that decides the speed أحباب
is
Incense sticks
at which wind blows. When air blows genth, the wind is
called a breeze. When it is stronger, it is called a gale Wind blows at great
speeds during storms and can uproot trees and cause damage to crops and
property
Effect of heat on air
The Sun's rays warm up the atmosphere. During summers the
temperature of air is higher than in the winters. The rise in temperature leads
to two results
1 The air molecules gain more energy from the heat and start
moving faster. As a result, they push each other away and the air expands
As the air expands, the air becomes thinner and lighter. It
then rises up. We can see this effect in our daily lives. Smoke from candles,
agarbattis, diyas and factory chimneys-all rise upwards.
These two effects of heat on air are the causes of many of
the weather and climatic changes on Earth.
tivity 2
EXPERIENTIAL LEARNING OBSERVATION, ANALYSIS, EVALUATION
To demonstrate that air expands on heating
terials needed: a glass bottle, a pan, hot water, a loon
cedure:
Attach the open end of the balloon to the mouth of the
bottle.
Pour hot water into a pan.
Place the bottle with the balloon in the water on the pan.
Cold water
You will observe that the balloon will start blowing
up
Wind current patterns
and why does this happen? When the bottle is placed in hot
water, the air inside somes hot and expands. It also becomes lighter and rises
up. It cannot escape as the woon is attached to the neck of the bottle. That is
why the hot, light air enters the balloon
fills it up. TE FOR ADULT: This experiment should be
conducted taking the help of an adult.
Air expands
Hot water
Check Your Progress
Choose the correct answer.
1. On heating, air molecules (gain/lose) energy
2. Gently blowing wind is called a (gale/breeze)
3. Air (expands/contracts) on heating.
Wind blows from a region of (high/low) pressure. (high/low)
pressure to an area of
5. Sea breeze takes place during the takes place during the
(day time/night time) while land breeze (day time/night time).
Wind Current Patterns on Earth
On Earth, the equatorial regions are warmer than the tropics
The warmer air rises up and the cooler air from the tropics blows towards the
equator.
Similarly, the polar regions are colder than the temperate
zones As a result, warmer air from the temperate zone rises up and cooler air
from the poles blows towards the temperate zone.
In this way, wind circulation is set up between the
equatorial, temperate and polar regions.
The given image shows the wind flow patterns on Earth. You
will notice that the arrows are not straight but curved. This is because the
wind blows along a curved path due to the high speed of rotation of the Earth
from west to east.
Monsoons in India
In India, the rainy season, also called the monsoons, is
also a result of this phenomenon During summers, the northern and central parts
of India get extremely hot The air above these regions expands, gets lighter
and rises up. Moisture-bearing winds from the Arabian Sea and Indian Ocean blow
inland, carrying moisture from the water-bodies in the form of dark,
rain-bearing clouds.
During the winter season, the land is colder than the seas
and oceans around it. As a result, wind blows from the land to the sea. As
there are no large water-bodies inside India, the winds that blow in winters
are drier.
INDIA
NOT TO SCALE
INDIAN
OCEAN
Winter
Moving air and lift
Whenever air moves with speed, its pressure becomes less: As
long there is equal pressure inside and outside a body, it maintains its
balance. A balloon bursts because the pressure inside is more than the pressure
outside. So, if the speed of air is different in two areas, the pressure where
the wind speed is slower will be more than where the wind speed is faster.
Let us now discuss some effects of this phenomenon in
real-life situations Arpy
EXPERIENTIAL LEARNING, OBSERVATION, ANALYSIS, EVALUATION
Activity 3
To demonstrate that increase in wind speed leads to fall in
air pressure.
Materials needed: A string, 2 balloons
Procedure:
1. Blow up two balloons.
2. Tie them up side by side on a piece of string so that
they hang next to each other, without touching
3. Now blow strongly through the space between the two
balloons.
AR
A
4. Instead of flying apart, you will see that the balloons
will come closer to each other.
This is because when you blow air between the balloons, the
speed of wind blowing between the balloons is high. As a result, there is a
fall in pressure in the area between them. The higher pressure on either side
of the balloons pushes them closer to each other.
EXPERIENTIAL LEARNING, OBSERVATION, ANALYSIS, EVALUATION
Activity 4
To demonstrate that increased wind speed causes lift
Materials needed: A strip of paper about 20 cm long and 3 cm
wide
Procedure:
1. Hold one end of the paper between your thumb and
forefinger.
2. Bring the fingers holding the paper close to your mouth.
3. Blow over the paper,
4. You will find that the strip of paper will lift upwards
This happens because when you blow over the strip, the speed
of wind over the strip is more than the speed below it. As a result, the air
pressure above the strip will decrease. The higher pressure under the strip
will lift the strip of paper.
D
An aeroplane can fly because the speed of wind on the curved
upper side of its wings is more than on the flat lower side.) This causes a
difference in air pressure. There is an area of higher pressure under the wings
and an area of lower pressure over the wings. This causes the air to lift up
the plane. Look carefully at the wings of a bird as it flies. Wings of the
aeroplane are designed on the wings of birds.
LIFT
LOWER
PRESSUNG
AIRFOIL (WINGE
HIGHER PRESSURE
AIR TRAVELING FURTHER
AIR TRAVELING NOT AS FAR
An aeroplane can fly because of the difference in an
pressure
During storms, when winds are blowing at very high speeds,
an area of low pressure is created over the roofs of huts. The pressure inside
the hut is normal and more than the pressure over the roof. If the roof is
weak, the higher pressure under the roof will blow it away
Measuring wind speed: The instrument that measures the speed
of wind is called an anemometer
An instrument that shows the direction of the wind is called
a
wind vane.
Check Your Progress
Match the following.
1. Rainy season
2. High wind speed
3. Difference in air pressure
4. Direction of wind
5. The reason winds curve as they flow to and from the
equator towards the polar regions
a lift
brotation of the Earth
cmonsoons
d. less air pressure
e. wind vane
Did You Know?
Bowlers in a game of cricket can spin their balls. A bowler
rubs one side of the ball to make. it smoother, When a baller spins the ball,
air moves faster over the smooth surface than on the opposite rougher surface,
As a result there is a difference in air pressure on opposite sides of the
ball. This results in the ball moving away from its straight path!
Thunderstorms
Thunderstorms are a common occurrence in tropical countries
like India. Heavy rains with winds at high speed accompanied
by lightning and thunder constitutes a thunderstorm) During summers, the high
temperatures produce winds that rise up rapidly. The water vapour carried by
these winds condense in the colder upper layers of the atmosphere to form drops
of water or ice. These water droplets fall down as rain. This falling drops of
water along with the rising air create lightning, which in turn produces
thunder.
Precautions to take during thunderstorms
The high-speed winds during a thunderstorm can uproot trees
and electric poles. Lightning carries a huge amount of electric charge and can
be extremely dangerous if it passes through a living body. The following
precautions should be taken while a thunderstorm is
in progress. Stay inside any building while the storm is on.
If you are in the open, do not take shelter under a tree as
the 03 lightning may pass through it. It is better to lie down on the ground.
Do not use an umbrella with a metal rod as the rod might
conduct the lightning
It is safe to stay inside a car or a bus.
If you are in water, climb out immediately and enter a
building
Cyclones
Cyclones can occur anywhere in the world. Cyclones are
called hurricanes in North and South America. They are called typhoons in the
Philippines, China and Japan. They usually develop over seas and oceans and
travel inland causing widespread destruction and havoc in their paths.
SPIRAL RAIN BANDS
EYE
OUTFLOW
DESCENDING
Cyclone formation
How does a cyclone form? The heat from the Sun evaporates
water from seas and oceans into water vapour. When the water vapours reach the
cooler upper parts of the atmosphere, they condense into water droplets and form
clouds. On forming the clouds, they release the heat
which they had absorbed from water-bodies. This heat causes
the air to expand and rise up more in the air. This leads to the formation of
an area of low pressure called a depression.
Cooler air from the surroundings flows in and causes a weak
circulation system. As the cycle is repeated, the depression becomes stronger
and the circulating winds become faster causing a cyclone. Thus, we can say
that the cyclone is a huge spiral storm.
A cyclone has two main parts. The centre of the cyclone is
called the eye of the storm. It is a relatively calm area. Around this calm and
clear eye, there is a region of thick clouds that bear very heavy rains and
high-speed winds. The wind speed and rain intensity gradually decrease away
from this region. The diameter of a cyclone can vary from a few kilometres to a
few hundred kilometres.
Destruction caused by cyclones
As a result of the low pressure inside the cyclone, the
level of sea water rises. This column of water inside the cyclone causes
widespread destruction due to the inflow of sea water accompanied
by heavy rains and winds at very high speeds. Trees are
uprooted and houses are flattened when a strong cyclone passes through the
area. Cyclones can cause loss to life and property on a massive scale.
Communication systems like telephones and electric lines can be damaged. Sea
water enters the low-lying coastal areas and mixes with the soil making it
infertile. It mixes with the inland water bodies and turns them saline.
After a cyclone, typically water-borne diseases such as
cholera and dysentery become rampant. People do not get safe drinking water and
waterlogging causes sanitary problems.
Precautions to take before a cyclone
Thanks to the advancements in technology, satellites and
radars alert us about incoming cyclones and their strengths well in advance.
This helps residents living in coastal regions to shift to safer places. Here
are some precautions that should be taken when a cyclone
warning is given. We should take warnings of cyclones issued
by the government seriously.
We should evacuate our houses and move to safer areas
designated by the government if we are advised to do so.
We should make necessary arrangements to shift the essential
household goods, domestic animals, vehicles, etc., to safer places.
Did You Know?
Approximately 35-40 km above the Earth's surface large bands
of high-speed winds form. These are known as jet streams. These winds can be
between 350 and 700 km per hour! Airplanes can use these highways in the sky to
get to destinations faster.
Think....
What would happen if an air plane flew against a jet stream?
Would it not delay the flight and lead to more fuel consumption since the
airplane is flying against the wind?
ado
ess
Arrange for dry food, torches, medicines and drinking water.
If evacuating, switch off the electrical main supply.
Charge your phones fully in advance. Keep all emergency
numbers handy.
Precautions to take after a cyclone
If you have evacuated, stay till you get the all-clear
signal to shift back home.
Keep on drinking bottled water for some time after the
cyclone has passed.
Do not touch any hanging wire anywhere.
Avoid walking or driving through waterlogged roads.
Tornadoes
A tornado is a rotating, funnel-shaped cloud extending from
the ground to the sky. Though most tornadoes are weak, a few can be very
violent with wind speeds reaching hundreds of kilometres per hour. They are not
very prevalent in our country. Strong tornadoes cause damage only along the
path they take, leaving nearby areas unaffected. Violent ones have the power to
uproot buildings and carry them in the air. They may last from a few seconds to
an hour. However, most tornadoes do not last beyond ten minutes.
Precautions to take during a tornado
Once the warning for a tornado is sounded, one should reach
a tornado shelter, the basement or an interior room on the first floor of a
building as soon as possible.
Shelter should be taken under heavy furniture away from
windows.
People should not stay in vehicles or mobile homes, as they
are most vulnerable to tornadoes.
If a person is caught outdoors, they should curl themselves
up close to the ground, keeping the neck and ears covered.
lowing statements are TRUE or FALSE. Correct the false ones.
ays accompanied by thunder and lightning.
yclone is called its eye.
over the poles.
Quick Recap
AIR
Expands
On
Heating
Wind
Becomes lighter
Blow from areas of high to low pressure
Breeze
Rises up
Exerts
Pressure
Gale
Low
High speed
High
Thunderstorm
Cyclone
Tornado
Less wind speed
More wind speed
EXERCISES
OBJECTIVE-TYPE QUESTIONS
A. Tick() the correct options.
1. When air expands, it becomes
b. lighter
2. Cooler air from the flows into the temperate zones.
a. poles
3. Faster moving air has pressure than slower moving air.
b. less
4. The shape of a tornado resembles a
c. funnel
5. After cyclones, we should be careful to drink only
d. clean
B. State whether the
following statements are TRUE or FALSE. Correct the false ones.
1. During monsoons in India, moisture-laden winds blow
inland from the Bay of Bengal.
- **TRUE**
2. We get rains with thunder and lightning during a
thunderstorm.
- **TRUE**
3. Tornadoes are very common in India.
- **FALSE**.
Correction: Tornadoes are not very prevalent in India.
4. Cyclones are called typhoons in the Americas.
- **FALSE**.
Correction: Cyclones are called hurricanes in North and South America.
5. Ensure that the electrical mains are switched on
before evacuating one's house.
- **FALSE**.
Correction: Ensure that the electrical mains are switched off before evacuating
one's house.
6. We should not use an umbrella with a metal rod during
a thunderstorm.
- **TRUE**
7. A cyclone is a rotating, funnel-shaped cloud extending
from the ground to the sky.
- **FALSE**.
Correction: A tornado is a rotating, funnel-shaped cloud extending from the
ground to the sky.
C. Choose the correct
word to fill in the blanks.
1. An **anemometer** is an instrument that measures the
speed of winds.
2. Cyclones are called **typhoons** in China.
3. The area of low pressure inside a cyclone is called a
**depression**.
4. When we blow air between two balloons hung side by
side, the air pressure in the space between the balloons **decreases**.
5. A **gale** is a strong wind.
SHORT-ANSWER
QUESTIONS
D. Answer each of the
following in brief.
1. **How do we know there is air all around us?**
- We know there
is air all around us because we can feel it on our skin when we stand under a
fan or when the wind blows. Additionally, the envelope of air around the Earth
extends to a height of a few kilometers above us, and this air has weight and
presses down on all things present on the Earth's surface, creating atmospheric
pressure.
2. **How is it possible for an aeroplane to lift itself
above the ground?**
- An aeroplane
can lift itself above the ground because of the difference in air pressure. The
speed of wind on the curved upper side of its wings is more than on the flat
lower side. This causes a difference in air pressure, with higher pressure
under the wings and lower pressure over the wings, which lifts the plane up.
3. **How can we protect ourselves from lightning strikes
if we are caught in a thunderstorm in the open ground?**
- If you are
caught in a thunderstorm in the open ground, do not take shelter under a tree,
as lightning may pass through it. Instead, it is better to lie down on the
ground. Also, do not use an umbrella with a metal rod, as the rod might conduct
the lightning.
4. **How does the air pressure of a place depend on the
wind speed there?**
- The air
pressure of a place depends on the wind speed because faster-moving air has
less pressure than slower-moving air. When the speed of air is different in two
areas, the pressure where the wind speed is slower will be more than where the
wind speed is faster.
5. **In which direction does the wind current flow on
both sides of the Equator?**
- On both sides
of the Equator, the wind current generally flows from the poles towards the
equator. Warmer air rises at the equator, and cooler air from the poles blows
towards the equator, setting up a wind circulation pattern.
E. Answer each of the
following in detail.
1. **Why does a balloon burst with a loud bang?**
- A balloon
bursts with a loud bang because the air inside the balloon is at a much higher
pressure than the air outside. When there is a tiny hole, the air inside rushes
out, tearing the rubber of the balloon and making a loud explosive sound.
2. **Describe the wind flow patterns on Earth.**
- On Earth, wind
flow patterns are set up between the equatorial, temperate, and polar regions.
Warmer air rises at the equator, and cooler air from the tropics blows towards
the equator. Similarly, warmer air from the temperate zones rises, and cooler
air from the poles blows towards the temperate zones. The wind blows along a
curved path due to the high speed of rotation of the Earth from west to east.
3. **Why does India get monsoons?**
- India gets
monsoons because during summers, the northern and central parts of India become
extremely hot. The air above these regions expands, gets lighter, and rises up.
Moisture-bearing winds from the Arabian Sea and Indian Ocean blow inland,
carrying moisture from the water bodies in the form of dark, rain-bearing
clouds. During winters, the land is colder than the seas and oceans around it,
so wind blows from the land to the sea, resulting in drier winds.
4. **How does a thunderstorm develop?**
- A thunderstorm
develops when high temperatures during summers produce winds that rise up
rapidly. The water vapor carried by these winds condenses in the colder upper
layers of the atmosphere to form drops of water or ice. These water droplets
fall down as rain. The falling drops of water, along with the rising air,
create lightning, which in turn produces thunder.
5. **Describe a cyclone along with its parts.**
- A cyclone is a
huge spiral storm that forms over seas and oceans and can cause widespread
destruction when it moves inland. It has two main parts:
- **Eye of the
Storm:** The center of the cyclone, which is a relatively calm and clear area.
-
**Surrounding Region:** Around the eye, there is a region of thick clouds that
bear very heavy rains and high-speed winds. The wind speed and rain intensity
gradually decrease away from this region. The diameter of a cyclone can vary
from a few kilometers to a few hundred kilometers. The low pressure inside the
cyclone causes the sea level to rise, leading to flooding and destruction along
coastal areas.
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