LESSON NOTES – GRADE 7
CHAPTER 8-WINDS, STORMS AND CYCLONES
I.
Answer the following Questions:
1. How are winds formed?
Wind is moving or flowing air. When the sun warms the ground, the air above
it expands, becomes light and rises up. This results in the drop of air pressure
in a particular region. The cooler and heavier air then starts moving towards
this low pressure region. This flowing of air from the high pressure to low
pressure region is called wind.
2. What causes the formation of wind currents on the surface of earth?
There are two cases:
i. Uneven heating between the equator and the poles.
ii. Uneven heating of land and water.
2. How how are wind currents formed due to the uneven heating between the
equator and the poles?
i. Wind currents are formed due to the uneven heating on the earth surface.
The regions close to the equator get maximum heat from the sun.
ii. The warm air from these regions rise, and the cooler air from the 0-30
degrees latitude belt on either side of the equator moves in. These winds
blow from the north and the south towards the equator.
iii. At the poles, the air is colder than that at latitudes about 60 degrees. The
warm air at these latitudes rises up and the cold wind from the polar-regions
rushes in, to take its place.
iv. In this way, wind circulation is set up from the poles to the warmer
latitudes. A change in direction of wind is caused by the rotation of earth
also.
4. How are wind currents formed due uneven heating of land or water OR
how are monsoon winds caused?
i. During summer, the land masses near the equator warms up faster and
rises up, which creates a low pressure region over the land.
ii. On the other hand the ocean water (Indian Ocean) remains relatively cold
and a region of high pressure exist over it.
iii. Wind blows from a region of high pressure to a region of low pressure ie,
from ocean to land.
iv. These winds carry a huge amount of moisture and create monsoon clouds
and cause monsoon rains.
5. How are thunderstorms formed?
i. The rising temperatures produce strong upward rising winds. These winds
carry water droplets upwards, where they freeze, and fall down again.
ii. The swift movement of the falling water droplets along with rising air
creates charges on the cloud.
iii. When these charges discharge through the air they produce huge sparks
which we see as lightning.
iv. The lightning heats up the air in its path, which cause the air to expand
fast with booming sound which we hear as thunder. It is this weather
condition we call it as thunderstorm.
5. How does thunderstorm becomes a cyclone?
i. When water from the sea evaporates, it takes up heat (latent heat) from
the atmosphere to change into vapour.
ii. When water vapour changes back to rain, this heat (latent heat ) is
released to the atmosphere, which warms up the air around. The air tends to
rise and causes a drop in pressure.
iii. More air rushes to the centre of the storm. This cycle is repeated.
iv.The chain of events ends with the formation of a very low-pressure system
with high speed winds revolving around it. This weather condition is called
cyclone.
6. What factors contribute to the development of cyclones?
The factors which contribute to the development of a cyclone are
a) Wind speed
b) Wind direction
c) Temperature
d) Humidity
7. Explain the structure of a cyclone?
i. The centre of a cyclone is a calm and cloudless area called the eye of a
storm. The diameter of the eye varies from 10 to 30 km.
ii. Around this calm and clear eye there is a cloud region of about 150 km in
size which has high speed winds (150-250 km/h) and thick clouds with heavy
rain.
iii. Away from this region the wind speed gradually decreases.
8. What are the damages caused by the cyclones?
The damages caused by the cyclones are:
a) The sea water enters the low-lying coastal areas, causing severe loss of life
and property.
b) It reduces the fertility of the soil.
c) Continuous heavy rainfall may further worsen the flood situation.
d) High-speed winds accompanying a cyclone can damage houses, telephones
and other communication systems, trees, etc causing tremendous loss of life
and property.
9. What are the effective safety measures to be taken for a cyclone?
a) A cyclone forecast and warning service.
b) Rapid communication of warnings to the government agencies, the ports,
the fisherman, ships and to the general public.
c)Construction of cyclone shelters in the cyclone prone areas, and administrative
arrangements for moving people fast to safer places.
10.Explain why holes are made in hanging banners and hoardings?
Holes are made on banners and hoardings to allow strong wind to pass through which may
otherwise it would tear the banners due to the air pressure. Air pressure will be more on the
banners if there are no holes.
11. What are the following?
a. Thunderstorm: The swift movement of falling droplets along with the
rising air can raise a lot of dust, uproot trees, and blow roofs of houses
and cause large scale destruction to life and property. Such strong winds
accompanied by lightning and sound are thunderstorms.
b. Cyclone: A weather condition which leads to the formation of a very low
pressure system surrounded by high wind speed system revolving around
it is called a cyclone.
c. Tornadoes: A tornado is a dark funnel shaped cloud with very high speed
wind that reaches from the sky with the narrow end on the ground,
formed suddenly on the land.
d. Latent heat: The amount of heat released or absorbed when a body changes
its state is called latent heat. It is 540cal/gm for water.
12. What are the precautions to be taken during lightning?
Ans. On page 87 ( the 5 points given in the blue box)
II.
Draw a neat diagram showing the wind flow pattern due to the uneven heating
of earth.
Fig 8.8, Pg no 85 in the text book.