Let’s start the chapter with a tragic article.
“Orissa was hit by a cyclone with wind speed of 200 km/h on 18 October 1999. The cyclone smashed 45,000 houses making 7,00,000 people homeless. On 29 October the same year, a second cyclone with wind speed of 260 km/h hit Orissa again. It was accompanied by water waves about 9 m high. Thousands of people lost their lives. Property worth crores of rupees was destroyed. The cyclone affected agriculture, transport, communication, and electricity supply.”
In this article, apart from the tragedy, there is something on which we have to ponder upon…
1. What are cyclones?
2. How are they formed?
3. Why are they so destructive?
In this chapter we shall seek answers to some of these questions. Let’s begin by understanding some basic concepts.
2. AIR PRESSURE
The pressure exerted by air on all bodies at all times in all directions is called air pressure.
When air moves at high speeds, it creates a low pressure area. The air inside a balloon exerts pressure in all directions, and makes it blow up. Air opposes the motion of a moving object. This is called air resistance. That is why you have to exert yourself and pedal hard when you ride a bicycle.
Another example in which air pressure can be observed:
Take a glass and fill only 1/3 of it with water. Cover the mouth of the glass with an index card. Now hold the card in place and invert the glass over a sink and remove your hand from the card. The card sticks to the glass. This is due to air pressure. The air outside exerts an upward pressure on the index card. This air pressure is more than the weight of the water in the glass.
Winds are caused by variations in air pressure. A wind blows from a region of high pressure to a region of low pressure. The speed of the wind mainly depends on the difference between the pressures of the air in the two regions.
High speed winds are known to blow away thatched and tiled roofs in rural and semi-urban areas. Strong winds can uproot trees and electric poles, and even snap cables.
The instrument used to measure the speed of wind is called the anemometer. It is usually fixed on the top of a building.
I. Effect of heat on air
Air expands on heating, and the expanded air pushes for more space. Let’s observe few examples to understand the same.
1. Take two deep pans – one filled with hot water and the other with cold water. To prove that air expands
• Stretch a balloon across the mouth of a milk bottle and seal it with tape.
• Keep this bottle in the hot water pan
• The balloon is slowly inflated. This is because the heat from the hot water in the pan heats up the air inside the bottle, which makes the air inside the bottle expand.
Now place the bottle in the pan with cold water. The balloon deflates and shrinks. This is because the air inside the bottle gets cooled and so contracts.
Observe a pressure cooker where the steam comes out of the nozzle and escapes upwards. This is because steam escaping from a pressure cooker is lighter than the relative cool air surrounding it. Thus, hot air rises.
II. Speed of Wind Vs Air Pressure
Let’s understand with the help of following activities.
1. Crumple a small piece of paper into a ball of size smaller than the mouth of an empty bottle.
2. Hold the empty bottle on its side and place the paper ball just inside its mouth.
3. Now try to blow on the ball to force it into the bottle.
4. Try if you can force the paper ball in by blowing into the bottle.
You will be surprised to find that it is difficult to force the paper ball.
Why is it difficult to force the paper ball into the bottle?
1. Take two balloons of approximately equal size.
2. Put a little water into the balloons.
3. Blow up both the balloons and tie each one to a string.
4. Hang the balloons 8–10 cm apart on a cycle spoke or a stick.
5. Blow in the space between the balloons.
What did you expect? What happens?
1. Hold a strip of paper, 20 cm long and 3 cm wide, between your thumb and forefinger.
2. Now blow over the paper.
What do you think will happen to the paper? Will the paper rises up or moves down.
Understanding the activities
1. In Activity-1, when we blow into the mouth of the bottle, the air near the mouth has higher speed. This decreases the pressure there. The air pressure inside the bottle is higher than near the mouth. The air inside the bottle pushes the ball out.
2. In Activity-2, we observe that when we blew between the balloons, they moved towards each other. How could this happen? This could happen if the pressure of air between the balloons were somehow reduced. The pressure outside the balloons would then push them towards each other.
3. In Activity-3, we observe that when we blew over the paper strip, it goes upwards. Again, this could happen if blowing over the paper reduced the air pressure above the strip.
We see that the increased wind speed is, indeed, accompanied by a reduced air pressure.
III. Circulation of Wind
The equatorial and tropical regions get hotter than the polar regions. The warm air at the equator rises, and the cold air moves in from the polar regions. The air moves due to uneven heating of the earth’s surface between the equator and poles, which results in circulation of wind across the globe.
IV. Direction of Wind
Winds are also formed due to uneven heating of land and water in coastal areas. As the earth rotates on its axis from West to East, these wind currents are not exact. There is a difference in the temperatures of the air over sea and over land. Cold winds from the South Pole move north towards the equator, and whereas hot winds from the North Pole move south towards the equator.
IV. Monsoon Winds
The word monsoon is derived from the Arabic word “mausam,” which means season. The winds from the ocean carry water in the form of vapours, resulting rains over land. These are called monsoon winds.
4. THUNDERSTORMS AND CYCLONES
A storm with lightning and thunder is called a thunderstorm. It occurs due to the convection of air in hot and humid areas.
A natural calamity caused by difference in air pressure in the atmosphere is called a cyclone. A cyclone is a violently rotating windstorm. Cyclones are dangerous. Cyclones begin as thunderstorms.
In tropical regions like India, thunderstorms are common, but very few thunderstorms convert into cyclones. In India, the eastern coast is more vulnerable to cyclones than the western coast. In different parts of the world, cyclones are known differently. For example, in America, a cyclone is referred to as a hurricane, while the Filipinos and Japanese called it a typhoon. The largest tropical cyclone recorded was Typhoon Tip that struck Japan in 1979. At its peak strength, the diameter of its eye wall was 2220 kilometres. The wind speeds were recorded at 305 kilometres per hour.
III. Formation of Cyclones
The formation of a cyclone depends on the speed and direction of the wind, temperature and humidity.
A cyclone also arises:
1. Due to the difference in the temperatures of the two regions.
2. A low pressure is created as the air in the high temperature region becomes warm and rises.
3. The gap in the low pressure area is filled by cold air rushing in from the surrounding areas.
4. When the warm air arises, it cools, condenses and forms clouds.
5. When the water vapour in the clouds turns into raindrops, the heat possessed by the water vapour is released into the atmosphere.
6. The process repeats and the release of heat from the water vapour continues.
During a thunderstorm, move away from open garages, metal sheds and water bodies. During a thunderstorm, sit inside a car, a bus or a closed vehicle, or inside a building.
During a thunderstorm, do not:
• Rest under a tree
• Take shelter under an umbrella with a metallic end
• Lie down flat, if in an open place
• Sit near a window
Humidity is the amount of water vapour in the atmosphere. As the humidity in a region increases, the difference in temperature also increases. This results in the formation of a cyclone.
Winds moving at high speeds are known as gales.
VI. Eye of a Cyclone
The calm and clear area at the centre of a cyclone is called the eye of the cyclone.
The following flow chart will help you to understand the phenomena that lead to the formation of clouds and falling of rain and creation of storms and cyclones:
A rotating column of high speed winds that appears as a dark funnel-shaped cloud reaching from the ground to the sky is called a tornado. A tornado may form within a cyclone. A tornado forms due to the effect of low pressure in the eye of a cyclone. Objects near the base of a tornado, such as cars, dust, debris and even houses, are sucked into its funnel and thrown out at the top.
More than a thousand tornadoes occur every year across the world. Most occur in the United States. Tornadoes occur regularly in the regions around the Atlantic Ocean. Weak tornadoes travel with wind speeds of 50 to 60 km/hr, while a violent tornado can travel at a speed of about 400 km/hr. A tornado is a natural disaster, but not as dangerous as a cyclone.
6. DESTRUCTION CAUSED BY CYCLONES
Cyclones cause a lot of damage to land, people and property, and create a lot of havoc in the lives of the people. When a cyclone hits an agricultural field, it makes the land infertile and useless for agriculture. A cyclone spares nothing. It worsens the flood situation. There are indirect impacts of a cyclone as well, such as flooding in coastal areas. Flooding not only damages land, but also pollutes drinking water sources. This can cause epidemics. High waves several kilometres away from the shore indicate an upcoming storm. As a cyclone strikes the coast, the intense winds raise the water into a gigantic wave that is pushed towards the shore. High-speed winds during a cyclone can cause major damage to houses, human beings, animals, trees, power supply, and even communication systems such as telephones.
A high wall of water moving towards the shore from the ocean is called a storm surge.
The combination of a storm surge and a tide is known as a “storm tide”. The most dangerous storms are the ones where the storm surge arrives on the top of a high tide, and then the storm reaches an area that might otherwise have been safe.
A cyclone causes destruction through:
1. High-rise waves
2. High-speed winds that uproot trees, houses and other property
3. Contamination of drinking water
4. Heavy rainfall that worsens the flood situation
5. Flooding of agricultural land by sea water making it infertile.
7. METEOROLOGICAL DEPARTMENT
The department that carries out a scientific study of the atmosphere and focuses on weather forecasting and processes. Tropical cyclones are given names. The names are selected from a list decided by the national meteorological organisation of a country, or by a committee of the World Meteorological Organisation. The names of tropical cyclones that cause major death or destruction are not used again, as a tribute to the people who lose their lives in the disaster.
Some noteworthy points
1. Cyclone Alert: A warning issued 48 hours before the expected time of a cyclone.
2. Cyclone Warning: A warning issued 24 hours before the expected time of a cyclone.
8. EFFECTIVE SAFETY MEASURES
To keep people safe and secure during a cyclone, the government takes certain measures, such as:
1. Constructing cyclone warning centres
2. Making arrangements to relocate people during cyclones
3. Providing information about a cyclone, and warning people, fishermen, ships, ports, airlines and various government agencies
4. Delivering accurate and effective cyclone forecasts and warnings
Safety measures to follow before and during a cyclone:
1. Do not move out unless and until required.
2. Keep a note of all the warnings given by the Meteorological Department.
3. Keep emergency phone numbers of police, ambulance and fire brigade handy.
4. Move people and valuable items to a safe place.
5. Store hygienic drinking water.
6. Help neighbours.
7. Avoid contact with wet electric lines and switches