Wind, storms, and cyclones are powerful natural phenomena driven by the movement of air. Cyclones, in particular, can cause immense destruction to life and property, as seen in historical events like the Orissa cyclones of 1999.

Understanding how winds are formed and why storms become destructive requires examining some basic properties of air. Moving air is called **wind**.

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Air Exerts Pressure

One of the fundamental properties of air is that it **exerts pressure**. We experience this in various everyday situations.

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Activity 8.1

This activity demonstrates that air exerts pressure. A tin can with a lid is heated with water inside until the water boils, producing steam. The lid is then immediately sealed tightly, and cold water is poured over the can. The steam inside condenses, reducing the amount of gas (air and steam) inside the can, leading to a decrease in internal pressure. The higher pressure of the air outside the can then pushes inwards, causing the can to crumple or distort. This visual effect clearly shows that the air outside the can is exerting pressure.

Other experiences that show air pressure include:

• Wind helping to fly a kite (air pushing from behind).
• It being easier to row a boat with wind coming from behind.
• Difficulty in cycling against the wind (air pushing from the front).
• Air inside a bicycle tube keeping it inflated and tight due to the pressure it exerts on the inner walls.

High Speed Winds Are Accompanied By Reduced Air Pressure

Another key property is that **when the speed of air increases, the air pressure decreases**.

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Activity 8.2

This activity demonstrates reduced pressure at increased speed. Crumple a small paper ball and place it just inside the mouth of an empty bottle held horizontally. Blowing hard directly into the bottle's mouth causes the air speed near the mouth to increase. This increased speed results in reduced air pressure near the mouth. The air pressure inside the bottle, which is higher than the reduced pressure at the mouth, pushes the paper ball outwards, making it difficult to blow the ball into the bottle.

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Activity 8.3

Hang two inflated balloons side-by-side, a few centimeters apart. When you blow air into the space between the balloons, the air speed in this gap increases. This higher speed leads to reduced air pressure between the balloons. The higher air pressure on the outer sides of the balloons then pushes them inwards, towards each other. This unexpected movement demonstrates that increased air speed causes decreased pressure.

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Activity 8.4

Hold a strip of paper between your thumb and forefinger, letting it hang down. When you blow air over the top surface of the paper strip, the air speed above the strip increases. This causes the air pressure above the strip to decrease. The higher air pressure below the strip then pushes it upwards, causing the paper to lift. This visually demonstrates the effect of reduced pressure at higher air speed.

The observation that high-speed winds are associated with reduced air pressure has real-world implications. For instance, if very high-speed winds (like in a storm) blow over a weakly constructed roof, the increased air speed above the roof can cause the pressure above to drop significantly. The higher pressure of the slower-moving air inside the building can then exert an upward force, potentially lifting and blowing away the roof.

Wind moves from a region of **high air pressure** to a region of **low air pressure**. The greater the pressure difference between two regions, the faster the wind moves. These pressure differences in nature are often created by differences in temperature.

Air Expands On Heating

Temperature plays a crucial role in generating pressure differences because air changes its properties when heated or cooled.

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Activity 8.5

Tightly stretch a balloon over the neck of a boiling tube. Place the boiling tube in a beaker of hot water. Observe the balloon inflate slightly. This happens because the air inside the tube gets heated by the hot water, causing it to **expand** and occupy more space, pushing against the balloon. When the tube is removed from hot water and cooled to room temperature, the balloon deflates slightly. Placing the tube in ice-cold water causes the air inside to cool down and **contract**, reducing its volume and causing the balloon to deflate further or even get sucked inwards. This demonstrates that air expands when heated and contracts when cooled.

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Activity 8.6

Suspend two paper bags of equal size upside down on the ends of a stick balanced at its middle (like a simple scale). Place a burning candle below the opening of one of the bags. The bag above the candle is lifted upwards, disturbing the balance. This occurs because the air above the candle gets heated, becomes warm, and rises up. As warm air rises, it pushes the paper bag upwards. Since the same amount of air occupies more space when warm (expands), it becomes **lighter** than the surrounding cold air. This activity shows that **warm air is lighter than cold air** and **rises up**.

This phenomenon is fundamental to wind generation. When warm air rises at a place, it creates a region of **lower air pressure**. Cooler, denser air from surrounding areas then moves in to fill this space, creating wind.

Wind Currents Are Generated Due To Uneven Heating On The Earth

Differences in temperature across different regions of the Earth cause uneven heating of the atmosphere, leading to pressure differences and the generation of **wind currents** (global air circulation patterns).

Two major situations cause uneven heating:

1. **Uneven heating between the equator and the poles:** Regions near the equator receive maximum sunlight and heat. The air here gets warm, becomes lighter, and rises, creating a low-pressure zone. Cooler air from the regions in the 0-30 degree latitude belts towards the poles then moves towards the equator to fill this gap. At the poles, the air is very cold and dense. Warm air at higher latitudes (around 60 degrees) rises, and cold, dense air from the polar regions rushes towards these warmer latitudes to take its place. This sets up a large-scale wind circulation pattern from the poles towards the equator and vice versa. The Earth's rotation also influences the direction of these winds, causing them to deflect rather than blowing strictly north-south.

2. **Uneven heating of land and water:** Land heats up and cools down faster than water (as seen in Chapter 4). This difference in heating rates causes pressure differences near coastal areas.

• In summer, near the equator, land heats up more than the ocean. Hot air over land rises (low pressure), and cooler, moist air from the ocean moves towards the land. These winds are called **monsoon winds** and bring significant rainfall.
• In winter, land cools down faster than the ocean. Cool, dense air over the land flows towards the warmer ocean (low pressure over water). This reverses the wind direction. These winds from land to ocean often carry less moisture and result in less rainfall.

Monsoon winds carrying water vapour from oceans are a major source of rain in many parts of the world, including India, playing a crucial role in agriculture and water supply (part of the water cycle, as discussed in Chapter 14).

Thunderstorms And Cyclones

While winds and rains are essential, intense weather phenomena like thunderstorms and cyclones can be destructive.

**Thunderstorms** are severe local storms that develop in hot, humid, tropical areas. Strong upward rising winds are produced by rising temperatures. These winds carry water droplets to high altitudes where they freeze. The frozen droplets then fall down. The rapid upward movement of air and downward movement of falling water droplets (or ice) create friction, leading to the discharge of electrical energy (**lightning**) and rapid expansion of air, producing sound (**thunder**).

Thunderstorms can sometimes develop into cyclones. This happens when the water vapour in the rising air condenses into liquid droplets to form clouds. The process of condensation releases a large amount of heat into the atmosphere (latent heat). This heat warms the surrounding air, causing it to rise further and creating a stronger low-pressure area. More air rushes in towards the centre, and the cycle intensifies. This continuous cycle of rising air, condensation, heat release, and inward rush of air leads to the formation of a very **low-pressure system** with strong winds rapidly revolving around the centre.

This intense weather system is called a **cyclone**. The development of a cyclone is influenced by factors like wind speed, wind direction, temperature, and humidity.

Safety during Thunderstorms:

• Avoid isolated tall objects (like single trees).
• Do not take shelter under metallic structures or umbrellas with metallic ends.
• Stay away from windows.
• Safe places include closed buildings, cars, or buses.
• Avoid being in water during a thunderstorm.

Destruction Caused By Cyclones

Cyclones are highly destructive due to several factors:

• **Strong Winds:** High-speed winds can cause severe damage to buildings, communication systems (telephone poles, mobile towers), trees, and other structures.
• **Storm Surge:** The very low pressure in the eye of the cyclone causes the water surface in the centre to rise significantly (3 to 12 metres high). Strong winds push this elevated water towards the shore. This wall of rising water, called a storm surge, inundates low-lying coastal areas, causing extensive loss of life, property, and reducing the fertility of the soil due to salination.

The **structure of a cyclone** includes a calm area at the centre called the **eye** (diameter 10-30 km) with clear skies and light winds. Around the eye is a cloud region (up to 150 km across) with very high-speed winds (150-250 km/h), thick clouds, and heavy rain. Wind speed decreases further away from the eye.

Heavy rainfall associated with cyclones can worsen flooding in affected areas.

Cyclones are known by different names globally: **Hurricanes** in the American continent and **Typhoons** in the Philippines and Japan.

**Tornadoes** are related phenomena, less frequent in India. They are dark, funnel-shaped clouds extending from the sky to the ground, formed by violently rotating air. While most are weak, strong tornadoes can have wind speeds up to 300 km/h. Tornadoes can sometimes form within cyclones. They cause destruction by sucking up debris and objects due to extremely low pressure at the base.

Effective Safety Measures

To minimise the devastating impact of cyclones, effective safety measures are crucial, both at the governmental and individual levels.

Measures by government agencies include:

• A comprehensive cyclone forecast and warning service.
• Rapid communication networks to disseminate warnings to affected areas, ports, fishermen, ships, and the public.
• Construction of cyclone shelters in vulnerable coastal areas.
• Administrative arrangements for prompt evacuation of people to safer locations.

Actions people should take:

• Pay attention to warnings from the meteorological department via TV, radio, or newspapers.
• Prepare to shift essential household goods, livestock, and vehicles to safer places.
• Avoid driving through floodwaters as roads may be damaged.
• Keep emergency contact numbers (police, fire brigade, medical services) readily available.
• Store safe drinking water, as supplies may become contaminated.
• Avoid touching wet electrical switches or fallen power lines.
• Do not venture out unnecessarily during or immediately after a cyclone.
• Cooperate with and assist neighbours and rescue efforts.

Advanced Technology Has Helped

Advancements in technology have significantly improved our ability to predict and monitor cyclones, leading to better preparedness and reduced loss of life compared to the past.

Modern tools like **satellites** and **radars** provide crucial data for tracking the formation and movement of cyclones. This enables meteorological departments to issue timely warnings:

• A **Cyclone Alert** or **Cyclone Watch** is issued 48 hours in advance of an expected storm.
• A **Cyclone Warning** is issued 24 hours in advance.

These warnings are broadcast frequently, especially as the cyclone approaches the coast, giving people more time to prepare or evacuate. Various national and international organisations collaborate to monitor cyclone-related risks.

The speed of wind, a crucial factor in storm formation, is measured using an instrument called an **anemometer**.

Understanding the interconnectedness of temperature differences, air pressure, wind speed, and moisture is key to comprehending how winds, storms, and cyclones form and behave.

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