Class 7 Science Winds Storms and Cyclones Chapter Notes

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Winds, Storms and Cyclones

2.1 Introduction
Moving air is called Wind. If the speed of wind is low, it is pleasant. If the speed is high, it becomes destructive.

Teacher's Note:
a) Wind is simply air in motion.
b) High speed winds create low pressure regions.
c) Wind direction and speed determine weather patterns.

2.2 Air Exerts Pressure
Take a tin can with a lid. Fill it approximately half with water. Heat the can on a candle flame till the water boils. Blow out the candle. Immediately put the lid tightly on the can. Put the can carefully in a shallow metallic vessel or a washbasin. Pour fresh water over the can. As water is poured over the can, some steam in the can condenses into water, reducing the amount of air inside. The pressure of air inside the can decreases than the pressure exerted by the air from outside the can. As a result the can gets compressed.

Teacher's Note:
a) This activity proves that air exerts pressure on surfaces.
b) Condensing steam inside the sealed can creates a partial vacuum.
c) Greater outside atmospheric pressure crushes the thin metal can.
d) Always handle hot cans and boiling water with extreme care.

Activity 1
Crumple a small piece of paper into a ball of size smaller than the mouth of an empty bottle. Hold the empty bottle on its side and place the paper ball just inside its mouth. Now try to blow on the ball to force it into the bottle. 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.

Teacher's Note:
a) High speed air is always accompanied by reduced air pressure.
b) Air moves from a region of higher pressure to a region of lower pressure.
c) This principle explains why blowing into the bottle pushes the paper ball out instead of pushing it in.

Activity 2
When you blow between the balloons, they moved towards each other. This happens because pressure of air between the balloons were reduced. The pressure outside the balloons push them towards each other. [Figure: Two inflated balloons suspended close to each other with a person blowing air in the space between them]

Teacher's Note:
a) Blowing air increases wind speed in the narrow gap between the balloons.
b) Increased speed lowers the pressure in that specific region.
c) Higher atmospheric pressure on the outer sides pushes the balloons inward.

Activity 3
Blowing over a strip of paper [Figure: A person holding a strip of paper just below their lower lip and blowing over the paper strip, causing it to lift upwards]

You saw that when you blew over the paper strip, it went upwards. This could happen if blowing over the paper reduced the air pressure above the strip.

We see that the increased wind speed is, accompanied by a reduced air pressure.

Teacher's Note:
a) Blowing air over the paper reduces pressure above it.
b) Higher pressure beneath the paper pushes it upward.

2.3 Movement of Air

When air moves, it is called wind. Air moves because of difference in the air pressure between two region. Air moves from the region where the air pressure is high to the region where the pressure is low. The greater the difference in pressure, the faster the air moves.
On heating, the air expands and occupies more space. When the same thing occupies more space, it becomes less denser. The warm air is, therefore, lighter than the cold air.
In nature there are several situations, where warm air rises at a place. The air pressure at that place is lowered. The cold air from the surrounding areas rushes in to fill its place. This sets up convection in air.

Answer:
1. Moving air is referred to as wind, which is driven by air pressure differences between regions.
2. High pressure areas push air towards low pressure regions, with greater pressure gradients resulting in higher wind speeds.
3. Heating causes air to expand, become less dense, and rise, creating a drop in air pressure that draws in surrounding cooler air to establish convection currents.

Teacher's Note:
a) Warm air is lighter than cold air because it expands and occupies a larger volume.
b) Convection in air is driven by continuous heating and cooling cycles in nature.
c) Always remember that air always flows from high pressure to low pressure.

2.4 Wind Currents

The uneven heating between the equator and the poles generate wind currents. The air near equator regions gets warm. The warm air rises up, and the cooler air from the regions in the 0 - 30 degrees latitude belt on either side of the equator moves in. In this way, wind circulation is set up from the poles to the warmer latitudes.
The uneven heating of land and water also generate wind currents. In summer, near the equator the land warms up faster and most of the time the temperature of the land is higher than that of water in the oceans. The air over the land gets heated and rises. This causes the winds to flow from the oceans towards the land. These are monsoon winds.
In winter, the direction of the wind flow gets reversed, it flows from the land to the ocean.

[Figure: Earth showing wind flow pattern because of uneven heating between the equator and poles, with cloud cover and atmospheric circulation bands.]

Answer:
1. Uneven heating between the equator and poles sets up global wind currents as warm equatorial air rises and cooler polar or subtropical air moves in.
2. Monsoon winds are caused by the differential heating of land and water, where land warms faster in summer, causing winds to blow from oceans to land.
3. During winter, the land cools faster than the ocean, reversing the wind direction so that it flows from the land to the ocean.

Teacher's Note:
a) Global wind currents are primarily driven by temperature differences between equatorial and polar regions.
b) The word monsoon is derived from the Arabic word mausim, meaning season.
c) Uneven heating of land and water plays a vital role in seasonal wind reversals in tropical regions.

2.5 Thunderstorms and Cyclones

Thunderstorms develop in hot, humid tropical areas like India. The rising temperature produces strong upward rising winds. The swift movement of the falling water droplets along with the rising air create lightning and sound. This is called thunderstorm.
When water vapour changes back to liquid form as raindrops, heat is released to the atmosphere which warms the air around. The air tends to rise and causes a drop in pressure. More air rushes to the centre of the storm. This cycle is repeated. It is this weather condition that we call a cyclone.
Factors like wind speed, wind direction, temperature and humidity contribute to the development of cyclones.

Answer:
1. Thunderstorms occur in hot and humid regions due to strong rising warm currents accompanied by falling water droplets producing lightning and sound.
2. Cyclones develop when latent heat released during the condensation of water vapour warms the surrounding air, causing it to rise and drop the local pressure.
3. Continuous inflow of surrounding high speed air into the low pressure centre creates a rotating storm system.
4. Key meteorological factors contributing to cyclone formation include wind speed, wind direction, temperature, and humidity.

Teacher's Note:
a) Latent heat of condensation is the primary energy source that drives and sustains a tropical cyclone.
b) Students must clearly distinguish between the localized nature of thunderstorms and the large scale destructive winds of a cyclone.
c) Humidity and high ambient temperatures over tropical seas create ideal breeding grounds for cyclonic storms.

2.6 Structure of Cyclone
The centre of a cyclone is a calm area. It is called the eye of the storm. A large cyclone is a violently rotating mass of air in the atmosphere, 10 to 15 km high. The diameter of the eye varies from 10 to 30 km.
A cyclone is known by different names in different parts of the world. It is called a 'hurricane' in the American continent. In Philippines and Japan it is called a 'typhoon'.

[Figure: Diagram of the formation of a cyclone showing a violently rotating air mass 10 to 15 km high, an eye at the centre with a diameter of 10 to 30 km, rising air, sinking air, outer flow, stream flow, and zones of gales gradually increasing and hurricane winds]

Answer:
1. The eye of the storm is the calm central area of a cyclone.
2. A large cyclone is 10 to 15 km high, and the diameter of its eye ranges between 10 to 30 km.
3. Cyclones are called hurricanes in the American continent and typhoons in the Philippines and Japan.

Teacher's Note:
a) The centre is calm because wind speeds are low there.
b) International naming conventions depend entirely on geographic regions.
c) A cyclone rotates violently around this calm low-pressure centre.

2.7 Destruction Caused by Cyclones
Some of the harmful effects of cyclones are as follows:
i) Sea water entered in low coastal areas can damage human and animal life and property.
ii) Cyclones can reduce the fertility of soil.
iii) Frequent rains caused by cyclones can result in floods.
iv) High speed winds caused by cyclones can damage houses, telephone lines and other communication systems.

Answer:
1. Ingress of saline sea water ruins coastal agriculture and threatens lives.
2. Torrential rains trigger widespread riverine and urban flooding.
3. High-velocity gales cause structural collapse of infrastructure and communication networks.

Teacher's Note:
a) Cyclones combine wind, rain and storm surges to cause severe devastation.
b) Salinity from sea water makes agricultural soils infertile for subsequent seasons.
c) Early warning systems help minimize loss of life during such disasters.

2.8 Tornadoes
A tornado is a dark funnel shaped cloud that reaches from the sky to the ground. Most of the tornadoes are weak. A violent tornado can travel at a speed of about 300 km/h. Tornadoes may form within cyclones.

Answer:
1. A tornado is characterized as a dark, funnel-shaped cloud extending from the sky to the ground.
2. While most tornadoes are weak, extreme ones can attain speeds up to 300 km/h.
3. Tornadoes can occasionally originate inside powerful cyclones.

Teacher's Note:
a) Tornadoes represent extremely localized low-pressure systems with violent rotational winds.
b) They span much smaller spatial scales compared to massive tropical cyclones.

2.9 Effective Safety Measures [3 Marks]
Some of the safety measures in case of cyclones are as follows:
- A cyclone forecast and warning service.
- Rapid communication of warnings to the Government agencies, the ports, fishermen, ships and to the general public.
- Construction of cyclone shelters in the cyclone prone areas, and Administrative arrangements for moving people fast to safer places.
Action on the part of the people
- We should not ignore the warnings issued by the meteorological department through TV, radio, or newspapers.
- We should make necessary arrangements to shift the essential household goods, domestic animals and vehicles, etc., to safer place.
- We should keep ready the phone numbers of all emergency services like police, fire brigade, and medical centres.
- Do not drink water that could be contaminated. Always store drinking water for emergencies.
- Do not touch wet switches and fallen power lines.
- Do not pressurise the rescue force by making undue demands.
- Cooperate and help your neighbours and friends.

Answer:
1. Safety measures during cyclones involve both governmental action and public preparedness to minimize loss of life and property.
2. Key steps include heeding meteorological forecasts, securing emergency supplies like clean drinking water, and avoiding hazards such as fallen power lines.
3. Post-warning actions require moving to cyclone shelters and cooperating with rescue teams without panic.

Teacher's Note:
a) Emphasizes the importance of disaster management and timely communication during natural calamities.
b) Students must remember not to touch wet switches to prevent fatal electric shocks during heavy rains.
c) Connects theoretical weather concepts to practical safety drills taught in environmental science.