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Chapter 5-Pressure Winds Storms and Cyclones Notes

PRESSURE

In our daily life, we apply force in many ways. We push doors, carry school bags, hammer nails into walls, and cut vegetables with knives. The effect of force depends not only on its magnitude but also on the area over which it acts. This gives rise to the concept of pressure. Pressure is the force acting normally (perpendicularly) on a unit area of a surface. In simple words, pressure tells us how concentrated a force is on a particular area.

Formula of pressure :

Pressure = Force ÷ Area Where P = Pressure, F = Force and A = Area. The SI unit of pressure is Pascal (Pa). 1 Pascal = 1 Newton of force acting on 1 square metre area.

Relationship Between Pressure and Area

Pressure is inversely proportional to area. This means smaller area creates Greater pressure while Larger area creates smaller pressure.

Some examples of concept of pressure

Why does a sharp knife cut easily?

A sharp knife has a very small cutting edge. The same force acts on a smaller area, producing a large pressure that cuts objects easily.

Why do school bags have broad straps?

Broad straps increase the contact area on the shoulders. Since pressure decreases when area increases, the bag feels more comfortable to carry.

Why are nails pointed?

The pointed tip has a very small area. Therefore, even a small force produces high pressure, allowing the nail to penetrate wood or walls easily.

PRESSURE IN LIQUIDS

Pressure in the liquids can be understood by the fluid pressure. Liquids also exert pressure because they have weight. Every layer of liquid presses on the layers below it. As a result, pressure exists at every point inside a liquid.

 Characteristics of Liquid Pressure

1. Pressure Increases with Depth

The deeper we go in a liquid, the greater the pressure. This happens because the weight of the liquid above increases with depth.

Example

2. Pressure Acts in All Directions

Unlike solids, liquids exert pressure not only downward but also sideways and upward. This can be observed when water leaks out from holes made in the sides of a container.

3. Pressure Depends on Height of Liquid Column

A taller column of liquid exerts greater pressure at its base. This principle is used in water supply systems.

Practical Applications of pressure

ATMOSPHERIC PRESSURE

Atmosphere: The Earth is surrounded by a thick layer of gases called the atmosphere. The atmosphere consists mainly of Nitrogen (78%), Oxygen (21%) and Carbon dioxide and other gases (1%). These gases have weight and therefore exert pressure. The pressure exerted by the weight of the air surrounding the Earth is called atmospheric pressure. Atmospheric pressure acts on all objects from every direction.

Evidence That Air Exerts Pressure

Why Don't We Feel Atmospheric Pressure?

Atmospheric pressure on our bodies is enormous. However, our body fluids and gases exert an equal pressure outward. Since these pressures balance each other, we do not feel crushed.

FORMATION OF WIND

Wind is the movement of air from one place to another. Wind is caused mainly by differences in air pressure.

Basic Principle

Air always moves from a region of high pressure to a region of low pressure. This movement continues until the pressure becomes equal.

How Pressure Differences Develop

The Sun heats different parts of the Earth unevenly.

When air becomes warm:

This creates a low-pressure region. Cooler and heavier air from nearby high-pressure regions moves in to occupy the vacant space. This movement of air is called wind.

Importance of Wind

SEA BREEZE AND LAND BREEZE

These are local winds caused by unequal heating of land and sea. Sea Breeze Occurs During Daytime. During the day, the land heats faster than sea water. Air over land becomes warm and rises. A low-pressure area develops over land causing cooler air from the sea moves toward land. This movement of air is called sea breeze. Sea breeze cools coastal regions during hot afternoons. Land Breeze occurs During Night. At night the land cools faster than water therefore water remains relatively warmer. Air above the sea rises causing cooler air from land moves toward the sea. This movement of air is called land breeze. Land breeze helps maintain temperature balance between land and sea.

HIGH-SPEED WINDS AND AIR PRESSURE

Air pressure and wind speed are closely related. Scientists have observed that when air moves very fast, the pressure exerted by that air decreases. This principle explains many natural phenomena, including the lifting of roofs during storms and the movement of aircraft.

Principle of wind speed

Higher Wind Speed → Lower Air Pressure. Lower Wind Speed → Higher Air Pressure. This means that rapidly moving air creates a region of lower pressure.

Balloon Experiment

Procedure

  1. Suspend two balloons side by side.
  2. Blow air between them.

Observation

The balloons move towards each other instead of moving apart.

Explanation

This demonstrates shows that high-speed air lowers pressure.

Roofs Blown Off During Storms

During strong storms, high-speed winds pass over roofs. Pressure above the roof decreases while pressure inside the house remains relatively high. This pressure difference creates an upward force on the roof. As a result, weak roofs may be lifted or blown away.

Safety Measure: Keeping doors and windows slightly open allows pressure to equalize and reduces the chances of roof damage.

STORMS

A storm is a violent weather condition characterized by strong winds and often accompanied by rain, thunder, lightning, or hail. Storms occur when large pressure differences develop in the atmosphere.

Conditions Required for Storm Formation

  1. High Temperature- The Sun heats the Earth's surface strongly.
  2. Moisture- Water bodies such as oceans, seas, lakes, and rivers provide moisture.
  3. Rising Warm Air- Warm air becomes lighter and rises.
  4. Low Pressure- The rising air leaves behind a low-pressure area.
  5. Air Movement-Cool air rushes toward the low-pressure region, creating strong winds.

Importance of Storms

Though destructive, storms help to distribute heat across the Earth. It maintain atmospheric balance and bring rainfall to many regions.

THUNDERSTORMS

A thunderstorm is a storm accompanied by thunder, lightning, heavy rain, and strong winds. Thunderstorms are common in tropical countries such as India, especially during summer.

Formation of Thunderstorms

Step 1: Heating of Earth's Surface- The Sun heats the land strongly.

Step 2: Warm Air Rises- Warm moist air becomes lighter and rises rapidly.

Step 3: Cooling- As the air rises, it cools.

Step 4: Condensation- Water vapour condenses into tiny droplets.

Step 5: Cloud Formation- Large towering clouds called cumulonimbus clouds develop.

Step 6: Strong Air Currents- Powerful upward and downward air currents form inside the clouds.

Step 7: Charge Formation- Collisions between ice particles and water droplets create electric charges.

Step 8: Lightning and Thunder-Electrical discharge occurs, producing lightning and thunder.

Characteristics of Thunderstorms

 

LIGHTNING

Lightning is a sudden discharge of electricity in the atmosphere. It is one of the most powerful natural electrical phenomena. Temperatures during lightning may reach about 30,000°C, which is hotter than the surface of the Sun.

Formation of Lightning

Charge Separation

Inside thunderclouds:

As charges continue to accumulate, the difference between them becomes very large.

Electrical Discharge

When the charge difference becomes strong enough to overcome the insulating property of air, electricity suddenly flows. This sudden flow of charge produces a bright flash called lightning.

Types of Lightning

  1. Intra-cloud Lightning- Occurs within the same cloud.
  2. Cloud-to-Cloud Lightning- Occurs between two clouds.
  3. Cloud-to-Ground Lightning- Occurs between a cloud and the Earth's surface. This type is the most dangerous.

THUNDER

The loud sound produced by lightning is called thunder. Lightning heats the surrounding air extremely rapidly.

As a result:

  1. Air expands suddenly.
  2. A shock wave is produced.
  3. The shock wave reaches our ears as thunder.

Why Is Lightning Seen Before Thunder Is Heard?

Light travels much faster than sound. Speed of Light is ≈ 300,000 km/s while the speed of Sound is
≈ 340 m/s. Therefore, we see the flash first and hear thunder later.

SAFETY MEASURES DURING LIGHTNING

Lightning can cause severe injuries and loss of life. Therefore, safety precautions are extremely important. Some safety precautions are;

Don'ts during lightening

CYCLONES

A cyclone is a huge rotating storm system that develops around a low-pressure centre and is accompanied by strong winds, heavy rainfall, and thunderstorms. Cyclones usually form over warm tropical oceans.

Formation of Cyclones

Step 1: Heating of Ocean Water- The Sun heats the ocean surface. Warm water evaporates rapidly.

Step 2: Warm Air Rises- The warm moist air above the ocean rises. This creates a low-pressure region.

Step 3: Condensation- As the air rises higher, it cools. Water vapour condenses into clouds.

Step 4: Release of Heat-Condensation releases latent heat. This heat warms the surrounding air. The air rises even faster.

Step 5: More Air Rushes In- More air moves toward the low-pressure centre. The cycle continues and intensifies.

Step 6: Rotation-Due to the Earth's rotation, incoming air begins to spiral. A rotating storm system develops.

Step 7: Cyclone Formation

The system grows into a powerful cyclone with:

Structure of a Cyclone

Effects of Cyclones

Cyclone Preparedness

Before a Cyclone

During a Cyclone

After a Cyclone

Role of Technology in Cyclone Warning

Modern technology helps in:

Important Tools

Early warnings have greatly reduced loss of life during cyclones.

 

  1. Throwing it harder: Even if we throw the ball harder, it will still fall back down. Throwing it harder simply gives it a greater initial upward velocity, allowing it to reach a higher point before gravity eventually overcomes that upward momentum and pulls it back down.

Different Situations
Many objects around us demonstrate this principle of gravity:

A. In-text Questions and Answers

1. Why do fallen leaves rise, and trees bend when a strong wind blows?

Moving air exerts both force and pressure. During strong winds, this force lifts lightweight fallen leaves and pushes against tree branches and trunks, causing them to bend or sway.

2. Does the width of a bag’s straps make a difference when two bags have the same weight?

Yes. Pressure is force per unit area. A narrow strap transfers the bag’s weight to a smaller area of the shoulder, resulting in greater pressure. In contrast, a broad strap distributes the same weight over a larger area, reducing pressure and increasing comfort.

3. Answer the following.

(a) Why are overhead water tanks placed at a height?
An overhead tank is positioned at a height to create a tall column of water. A taller water column generates greater pressure at the taps, ensuring a steady flow of water.
(b) Who will receive a stronger stream of water: a person on the first floor or a person on the second floor? Give a reason.
The person on the first floor will receive a stronger stream of water. The greater vertical distance between the tank and the first-floor tap results in a taller water column above the tap, which produces higher pressure.

4. Why does water spurt from a hole or leaking joint in a water pipe?

Water inside a pipe is under pressure and exerts force in all directions. When a hole or loose joint creates an opening, the internal pressure forces water outward, causing it to spurt.

5. What happens when the mouth of an inflated balloon is left open?

The air pressure inside an inflated balloon exceeds the external pressure. When the mouth is left open, air moves from the high-pressure interior to the lower-pressure surroundings, causing the balloon to deflate.

6. How does a difference in air pressure produce wind?

Air moves from regions of higher pressure to regions of lower pressure. This movement is termed wind. Thus, differences in atmospheric pressure produce winds.

B. Exercise Questions and Answers

1. Choose the correct option.

(i) Vessel R is connected to vessels P and Q and is filled with water. After the water settles, its level will be:
(d) Equal in all three vessels. In interconnected vessels, water settles at the same horizontal level.
(ii) Rubber sucker M is pressed onto a smooth surface and identical sucker N onto a rough surface. What will happen?
© M will stick, but N will not. M forms a nearly airtight seal on the smooth surface, enabling atmospheric pressure to hold it in place. In contrast, air enters through gaps in the rough surface, preventing N from forming a proper seal.
(iii) A water tank is placed at height H. How can the water pressure on the ground floor be increased?
(a) Increase height H. Raising the height of the water column increases the pressure at the ground-floor outlet.
(iv) Vessels A and B contain water to the same height. If the bottom area of B is greater than that of A, compare the pressures and forces at their bases.
Answer: (b) (P_A=P_B), but (F_A<F_B). Liquid pressure at a given depth is the same in both vessels. However, force equals pressure multiplied by area; therefore, the wider base of B experiences a greater total force.

2. State whether each statement is True or False. Correct the false statements.

(i) Air flows from a region of higher pressure to a region of lower pressure.
Answer: True.
(ii) Liquids exert pressure only at the bottom of a container.
False. Liquids exert pressure not only on the bottom but also on the walls of their container.
(iii) The weather is stormy at the eye of a cyclone.
False. Conditions at the eye of a cyclone are generally calm, while extremely strong winds occur in the surrounding regions.
(iv) During a thunderstorm, it is safer to remain inside a closed car.
True. A closed, hard-topped vehicle offers greater protection during a thunderstorm than remaining outdoors.

3. A boy lies horizontally on loose sand and then stands upright. In which position will he sink more? Explain.

He will sink more while standing. Although his weight remains constant, standing concentrates the force on the small area of his feet, resulting in greater pressure (?Pressure = Force / Area). When lying down, his weight is distributed over a larger area, reducing the pressure.

4. An elephant weighs 20,000 N. Each of its four feet covers an area of 0.25 m². Calculate the pressure exerted on the ground.

Answer:
Given:
Weight (Force) = 20,000 N
Area of one foot = 0.25 m²
Number of feet = 4
Formula:
Pressure = Force ÷ Area
Total area of four feet:
Area = 4 × 0.25
Area = 1 m²
Pressure exerted:
Pressure = 20,000 ÷ 1
Pressure = 20,000 Pa
Therefore, the elephant exerts a pressure of 20,000 Pa on the ground.

5. Boat A has a base area of 7 m² and carries five people. Boat B has a base area of 3.5 m² and carries three people. Each person weighs 700 N. Which boat produces greater pressure, and by how much?

Answer:
Formula:
Pressure = Force ÷ Area
For Boat A:
Total force = 5 × 700
Total force = 3,500 N
Pressure = 3,500 ÷ 7
Pressure = 500 Pa
For Boat B:
Total force = 3 × 700
Total force = 2,100 N
Pressure = 2,100 ÷ 3.5
Pressure = 600 Pa
Difference in pressure:
Difference = 600 − 500
Difference = 100 Pa
Therefore, Boat B exerts 100 Pa more pressure on its base than Boat A.

6. Would lightning occur if air and clouds were good conductors of electricity? Explain.

Answer: Lightning would be unlikely to occur. If air and clouds conducted electricity easily, electric charges would flow away continuously and would not accumulate. Lightning requires a large charge separation followed by a sudden electrical discharge.

7. Two identical balloons are attached to openings at the same height on a bottle. What happens when water is poured above the openings?

Both balloons will bulge. Since their openings are at the same depth below the water surface, the water pressure acting on each is equal. If the balloons are identical and equally elastic, they will bulge by approximately the same amount.

8. Explain how a storm develops into a cyclone.

A cyclone typically develops over warm ocean water. Warm, moisture-laden air above the sea rises, creating a low-pressure region. As this air cools, water vapour condenses to form clouds and releases heat. The released heat warms the surrounding air, causing it to rise further and lowering the pressure even more. Air from nearby high-pressure regions moves inward. Due to Earth's rotation, this incoming air begins to spiral around the low-pressure center. Repeated cycles of rising, cooling, and condensation strengthen the rotating system until it becomes a cyclone characterized by clouds, heavy rain, and high-speed winds.

9. How can the direction of a sea breeze help identify the landward side in the given figure?

On a summer afternoon, land heats more rapidly than the sea. Warm air over the land rises, creating a low-pressure region, while cooler air over the sea remains at higher pressure. As a result, air moves from the sea toward the land as a sea breeze. In the figure, the direction of wind and the bending of trees indicate movement from side B to side A; therefore, A is the landward side and B is the seaward side.

10. Describe an activity showing that air flows from high pressure to low pressure.

Answer:
Materials: Two similar thin rubber balloons, a drinking straw and thread.
Procedure: Attach one deflated balloon tightly to one end of the straw. Inflate the second balloon and attach it to the other end without allowing air to escape. Ensure that both joints are airtight.
Observation: Air passes from the inflated balloon into the deflated balloon. The first balloon becomes smaller while the second becomes larger. The flow gradually stops when the pressure difference becomes very small.
Conclusion: The inflated balloon initially contains air at higher pressure, while the deflated balloon contains air at lower pressure. Consequently, air moves from the region of higher pressure to the region of lower pressure.

11. What is a thunderstorm? Explain how it forms.

A thunderstorm is a storm characterized by lightning and thunder, typically accompanied by strong winds and heavy rainfall.
In hot and humid conditions, the land heats the moist air above it. This warm air becomes less dense and rises, creating a low-pressure region near the surface. Cooler surrounding air moves in, becomes warm, and also rises. At higher altitudes, the rising air expands and cools, causing its water vapour to condense into droplets and form tall clouds. Strong upward and downward air currents develop, resulting in wind and heavy precipitation.
Within the cloud, collisions between water droplets and ice particles separate electric charges. When the charge difference becomes sufficiently large, a sudden electrical discharge occurs, resulting in lightning. The intense heat from lightning rapidly expands the surrounding air, producing the sound known as thunder.

12. Explain the formation of lightning.

Powerful upward and downward air currents within a storm cloud cause collisions between water droplets and ice particles, separating electric charges. Positive charges typically accumulate near the top of the cloud, while negative charges gather near the base. The negative charge at the cloud base induces a positive charge on the ground below. Under normal conditions, air acts as an insulator, but when the charge difference becomes sufficiently large, the air becomes conductive. A sudden discharge then occurs within a cloud, between clouds, or between a cloud and the ground. This bright electrical discharge is called lightning. The intense heat from lightning rapidly expands the surrounding air, resulting in thunder.

13. Why are holes made in large banners and hoardings?

Strong winds generate significant force and pressure differences across a solid banner or hoarding. Holes allow some air to pass through, reducing the pressure difference and the total force on the material. This makes the banner or hoarding less likely to tear or be blown away.