Class 8 | Chapter 6 | Pressure , Wind , Cyclone & Storm

 

What is Pressure?

Pressure is the force acting normally on a unit area of a surface.

Pressure= Force / Area

​

SI Unit

The SI unit of pressure is pascal (Pa).

1 Pa=1 N/m²​

Important relationship

• For the same force, smaller area → greater pressure
• For the same force, larger area → smaller pressure

Everyday Examples of Pressure

A. Broad Straps of a School Bag

School bags have broad straps so that the weight of the bag is distributed over a larger area.

Therefore, pressure on the shoulders is reduced.

Broad straps → larger area → less pressure

B. Broad Handles

A broad handle is more comfortable to hold because the force is spread over a larger area.

Larger area → lower pressure

C. Cloth Under a Heavy Load

When a heavy object is placed on a soft surface, placing a cloth or board underneath can increase the area over which the force acts.

Therefore, pressure on the surface decreases.

D. Pointed Nail

A nail has a very sharp, pointed tip.

The same force acts over a very small area.

Therefore:

Small area → high pressure

This allows the nail to penetrate wood easily.

E. Sharp Knife

A sharp knife has a very thin cutting edge.

The force applied by the hand acts over a very small area, producing high pressure.

Therefore, it cuts objects easily.

Liquid Pressure:-

Liquids exert pressure on the surfaces of their containers.

Liquid pressure has some important properties.

A. Liquid pressure increases with depth

The pressure exerted by a liquid increases as we go deeper into the liquid.

For example, water pressure is greater at the bottom of a deep tank than near its surface.

Why?

A deeper point has a larger column of liquid above it, so the pressure is greater.

B. Liquids exert pressure on the sides

Liquids exert pressure not only downward but also sideways.

For example, water stored in a container pushes against its walls.

C. Why are dams thicker at the bottom?

The pressure of water increases with depth.

Therefore, the lower part of a dam experiences greater pressure than the upper part.

Hence, dams are made thicker at the bottom.

D. Overhead Water Tanks

Water stored at a greater height can produce greater pressure at lower levels.

This is why overhead water tanks are placed at a height: the water can flow through pipes due to the pressure produced by the water column.

Atmospheric Pressure:

The Earth is surrounded by a layer of air called the atmosphere.

Air has mass and therefore exerts pressure.

The pressure exerted by the atmosphere is called atmospheric pressure.

Atmospheric pressure acts in all directions.

It acts:

Downward

Upward

Sideways

Example: Sucker

When a suction cup is pressed against a smooth surface, much of the air beneath it is pushed out.

The atmospheric pressure outside the cup is then greater than the pressure inside.

This difference in pressure holds the sucker against the surface.

Atmospheric Pressure and Our Body

The air around us exerts atmospheric pressure on our body.

Why doesn't our body get crushed?

Because fluids and gases inside our body also exert pressure outward, balancing much of the external atmospheric pressure.

Formation of Wind

Wind is moving air.

Air moves from a region of higher pressure to a region of lower pressure.

Basic idea

High pressure → Low pressure

Differences in temperature can create differences in air pressure, causing air to move.

Sea Breeze

During the day, land heats up faster than water.

Step-by-step

1. Land becomes hot quickly.
2. Air above the land gets heated.
3. Warm air becomes less dense and rises.
4. This creates relatively lower pressure over the land.
5. Cooler, denser air over the sea moves towards the land.

This movement of air from sea to land is called sea breeze.

Day: Sea → Land

Land Breeze

At night, land cools faster than water.

Step-by-step

1. Land becomes cooler.
2. Air above the land becomes cooler and denser.
3. Pressure over land becomes relatively higher.
4. Water remains comparatively warmer.
5. Air above the water rises, producing relatively lower pressure.
6. Air moves from land towards the sea.

This is called land breeze.

Night: Land → Sea

Easy way to remember

Day → Sea breeze → Sea to Land

Night → Land breeze → Land to Sea.

High-Speed Winds Lower Pressure

When air moves very fast, the pressure in that region can become lower than in surrounding regions.

This effect can be demonstrated using balloons.

Activity 6.6 – Balloons

If two inflated balloons are suspended close to each other and air is blown between them:

• Air between the balloons moves rapidly.
• Pressure between them decreases.
• Pressure outside the balloons is relatively greater.
• The balloons are pushed towards each other.

Thus:

Higher air speed → lower pressure

Why Can Strong Winds Blow Off Roofs?

During a storm, air may move rapidly over the roof.

The fast-moving air can produce relatively lower pressure above the roof.

The air pressure inside the building may then be greater.

This pressure difference can produce an upward force on the roof.

Therefore, roofs can sometimes be damaged or blown off during very strong winds.

Why should doors and windows sometimes be kept open during very strong winds?

A dangerous pressure difference can develop between the inside and outside of a building. Properly designed and safely managed openings can help reduce pressure differences.

However, during an actual cyclone or severe storm, follow official local warnings and shelter guidance rather than opening windows simply as a general rule.

Thunderstorms and Lightning

 What is a Thunderstorm?

A thunderstorm is a weather condition in which there are:

Dark clouds

Strong winds

Heavy rain

Lightning

Thunder

Thunderstorms are common during hot and humid weather, especially before or during the rainy season.

How is a Thunderstorm Formed?

The formation takes place in the following steps:

Step 1: Heating of the Earth

The Sun heats the Earth's surface. The air near the surface becomes hot and lighter.

Step 2: Warm air rises

The warm, moist air rises rapidly into the atmosphere.

Step 3: Cooling and condensation

As the air rises, it cools. Water vapour condenses and forms large clouds.

Step 4: Strong upward and downward movements

Inside the cloud, there are powerful upward and downward movements of air.

Step 5: Rain and strong winds

When the water droplets become heavy, they fall as rain, often accompanied by strong winds.

What is Lightning?

Lightning is a huge electric spark that occurs between:

• Different parts of the same cloud,
• Different clouds, or
• A cloud and the Earth.

It produces a very bright flash of light and a large amount of heat.

How is Lightning Produced?

Inside a thundercloud, strong air currents cause water droplets and ice particles to collide.

These collisions cause separation of electric charges.

• The upper part of the cloud generally becomes positively charged.
• The lower part generally becomes negatively charged.
• The Earth's surface below the cloud can become positively charged by induction.

When the difference in charge becomes very large, the air can no longer prevent the charges from moving.

A sudden electric discharge takes place. This is called lightning.

Easy Flow:

Strong air currents → Collision of particles → Separation of charges → Large potential difference → Electric discharge → Lightning

What is Thunder?

The air around a lightning channel becomes extremely hot and expands suddenly.

This rapid expansion produces a powerful sound wave, which we hear as thunder.

Why do we see lightning before hearing thunder?

Light travels much faster than sound.

Therefore:

Lightning is seen first → Thunder is heard later.

Safety During Lightning 

If you are outdoors:

• Take shelter inside a building or closed vehicle.
• Avoid open fields and isolated tall trees.
• Stay away from electric poles, towers and metal fences.
• Do not hold metal objects during a thunderstorm.
• If no shelter is available, crouch down low and make yourself as small as possible.

If you are indoors:

• Stay away from windows, doors and balconies.
• Avoid using wired electrical appliances during the storm.
• Do not touch electrical equipment connected to the mains.
• Avoid bathing or using running water during severe lightning.

Lightning Conductor

A lightning conductor is a metal device installed on tall buildings to protect them from lightning.

It consists of a metal rod fixed at the highest point of the building and connected to the ground by a thick conducting wire.

When lightning strikes, the conductor provides a safe path for electric charge to reach the ground.

Local Names of Thunderstorms

Thunderstorms and related weather phenomena have different local names in India.

Nor'westers

In parts of West Bengal and Assam, severe pre-monsoon thunderstorms are commonly called Nor'westers.

Kal Baisakhi

In West Bengal, these violent pre-monsoon thunderstorms are popularly known as Kal Baisakhi (meaning roughly "calamity of Baisakh").

Mango Showers

In parts of Kerala and Karnataka, pre-monsoon showers are called mango showers because they help in the early ripening of mangoes.

Cyclone

A cyclone is a large system of very strong winds rotating around a low-pressure area. It is usually accompanied by heavy rain, thunderstorms and high-speed winds.

Cyclones generally develop over warm ocean water.

Formation of a Cyclone

Cyclones form mainly over warm oceans when the water temperature is sufficiently high.

Step-by-step formation:

1. Heating of ocean water

The Sun heats the ocean surface. Warm water causes the air above it to become warm and moist.

2. Warm air rises

The warm, moist air rises upward, creating an area of low pressure near the ocean surface.

3. More air moves in

Air from surrounding areas moves towards the low-pressure region.

4. Rotation begins

Because of the Earth's rotation, the moving air starts to spiral around the low-pressure centre.

5. Cyclone becomes stronger

As more warm, moist air rises and releases heat, the cyclone gains energy and becomes stronger.

Eye of a Cyclone 

The eye is the calm, central part of a cyclone.

It is generally characterised by:

• Very low air pressure

• Relatively light winds

• Less cloud and comparatively clear weather

• A roughly circular shape

The strongest winds occur around the eye, in a region called the eyewall.

Remember:

Eye = Calm centre

Eyewall = Strongest winds and heaviest rain

Cyclone Amphan

Cyclone Amphan was a very powerful tropical cyclone that affected India and Bangladesh in May 2020.

Important points:

→It formed over the Bay of Bengal.

→It rapidly intensified over warm ocean water.

→It severely affected parts of West Bengal, especially coastal            areas.

→It brought very strong winds and heavy rainfall.

→It caused extensive damage to houses, roads, trees, electricity      networks and agriculture.

→It also caused coastal flooding and storm surge.

Amphan showed how powerful tropical cyclones can cause widespread destruction in coastal regions.

Destruction Caused by Cyclones

Cyclones can cause serious damage because of high-speed winds, heavy rainfall and storm surges.

Major effects:

 Strong winds

 • Damage or destroy houses.

 • Blow away roofs and signboards.

 • Uproot trees and electric poles.

 Heavy rainfall

 • Causes flooding.

 • Damages crops and roads.

 • Can cause landslides in some areas.

 Storm surge

• Strong winds push seawater towards the coast.

• This can flood coastal areas with seawater.

 Damage to electricity and communication

 • Power lines and communication systems may be damaged.

 Damage to agriculture

 • Crops can be destroyed.

 • Agricultural land may be flooded with salty seawater.

Safety Measures During a Cyclone

Before a Cyclone

• Listen to official weather warnings.
• Keep an emergency kit ready with water, food, medicines, torch and batteries.
• Store important documents safely.
• Secure loose objects around the house.
• Know the location of the nearest cyclone shelter or safe building.

During a Cyclone

• Stay indoors and away from windows and glass.
• Move to a stronger building or designated cyclone shelter if authorities order evacuation.
• Switch off electricity and gas if instructed and safe to do so.
• Do not go outside to watch the storm.
• Avoid flooded roads and flowing water.
• Follow instructions from local authorities.
• After a Cyclone
• Do not return to damaged areas until authorities say it is safe.
• Stay away from fallen electric wires.
• Avoid contaminated floodwater.
• Report emergencies to the appropriate authorities.

After a Cyclone

• Do not return to damaged areas until authorities say it is safe.
• Stay away from fallen electric wires.
• Avoid contaminated floodwater.
• Report emergencies to the appropriate authorities.