The instrument used to measure atmospheric
pressure – mercury barometer
Units used to record atmospheric pressure – millibar
(mb) and hectopascal (hPa)
What are the factors affecting the atmospheric
pressure? Explain.
Altitude
Temperature
Humidity
Altitude:- The atmospheric pressure decreases with altitude. The pressure decreases at
the rate of
1 millibar (mb) per an altitude of 10 meters. As one goes
up, there is a decrease in atmospheric pressure due to the rarefication of air
with altitude.
The atmospheric pressure and the altitude are inversely
proportional.
Temperature:- As the temperature
increases the air expands and goes up as a result the atmospheric pressure
lowers. On cooling, the air becomes dense and descends as a result the
atmospheric pressure increases.
The atmospheric pressure decreases as the
temperature increases and vice versa.
Humidity:- Humidity refers to the quantity of water present in the atmosphere. Water
vapour is lighter than air and hence it ascends. If the quantity of water
vapour is more in a unit volume of air, then naturally the atmospheric pressure
will be less.
Humidity and atmospheric pressure are inversely proportional.
Why do mountaineers carry oxygen cylinders?
Ans:
As they go up the availability of oxygen for
breathing decreases as a result of the rarefication of air with altitude. So
the mountaineers carry oxygen cylinders.
Compared to the colder regions, the tropical
regions experience low atmospheric pressure. Why?
Ans:
Tropical regions experience high temperature and so low
atmospheric pressure
What do you mean by high pressure regions and low
pressure regions?
Ans:
If the atmospheric pressure of an area is higher than
that of the surrounding regions, it can be designated as ‘high pressure
region’.
If the atmospheric pressure of an area is lower than that of the surrounding regions, it can be designated as ‘low pressure region’.
What are isobars?
Ans:
Isobars are the imaginary lines joining places having the
same atmospheric pressure.
Global pressure belts
Equatorial low pressure belt
This is the zone where the sun's rays fall
vertically throughout the year. Hence the temperature will be high in this zone
all through the year. The air expands due to sun's heat and rises up on a
massive scale. This is the reason for the low pressure experienced throughout
this zone. The equatorial low pressure belt is situated between 5° North and
South latitudes. As the air in this zone ascends on a large scale, winds are
very feable here. This pressure belt is also known as 'doldrum', meaning 'the
zone with no winds'. The region was a nightmare for the ancient mariners.
Sub tropical high pressure belt
The hot air ascending from the equatorial low pressure
belt cools gradually and subsides at the sub tropical zone due to the rotation
of the Earth. As a result high pressure is experienced all along this zone.
Sub polar low pressure belt
As this zone is close to the Pole, the air is
colder here. Though the cold air remains close to the Earth, the air is thrown
away due to the rotation of the earth. As a result, low pressure is experienced
all along the sub polar region.
Polar high pressure belt
This zone experiences severe cold throughout the
year. As a result, the air remains chilled under the extreme cold that prevails
over the Poles, and this contributes to the steady high pressure experienced
here.
Pressure belt and latitudinal extend
Atmospheric pressure and winds
What is wind?
The horizontal movement of air from a high pressure zone
to a low pressure zone is called wind. Winds are named on the basis of the
direction from which they blow.
What are the factors on which the speed and
direction of wind depend? Briefly explain each.
The speed and the direction of wind are based on
Pressure gradient
Coriolis force
Friction
Pressure gradient
The change in pressure with horizontal distance is termed
as pressure gradient. The pressure gradient is said to be steeper when the
pressure difference is more.
Coriolis force
Coriolis force is the force generated as a result
of the Earth’s rotation. This force is responsible for freely moving bodies to
get deflected to the right in the northern hemisphere and to the left in the
southern hemisphere.
This force increases as it moves towards the Poles
from the Equator. Admiral Ferrel found out that the winds in the Northern
Hemisphere deflect towards their right and those in the Southern Hemisphere
deflect towards their left due to the Coriolis Effect. The law put forward by
him on the basis of this is known as Ferrel's law.
Friction
The speed of wind will be high over ocean surfaces
and plains as the friction is less. On the other hand, the friction being more
along difficult terrains and places with dense forest cover, the speed of wind
will be less in those places.
Pressure Belts and Winds
What are planetary winds?
The winds developed between the global pressure belts can be generally
called as planetary winds. The different planetary winds are
listed below.
• Trade winds
• Westerlies
• Polar easterlies
Briefly describe the different planetary winds?
Different planetary winds are trade wind, westerlies,
and polar winds.
Trade Winds
The winds blow continuously from the subtropical
high pressure belts in both the hemispheres towards the equatorial low pressure
belt. These are known as trade winds
As these winds blow from the north east in the
northern hemisphere, they are known as north east trade winds. In southern
hemisphere these wind blow from south east and so they are known as south east
trade winds. The equatorial low pressure zone where the trade winds from both
the hemispheres converge is known as the Inter Tropical Convergence Zone(ITCZ).
Westerlies
Winds blow continuously from the subtropical high
pressure zones to sub polar low pressure belt. As the direction of these wind
is mostly from the west they are known as westerlies. The westerlies are
stronger in the southern hemisphere than in the northern hemisphere. The
ancient mariners had given different names to the rough westerlies in the
Southern Hemisphere, such as 'Roaring Forties‘ (along 40° latitudes), 'Furious
Fifties' (along 50° latitudes) and 'Shrieking Sixties' (60° latitudes).
Polar
Easterlies
The polar winds are the cold winds that blow from
the polar high pressure areas towards the sub polar low pressure belts. These
winds blow from the East in both the hemispheres due to the Coriolis Force.
Hence these are known as polar easterlies.
The westerlies are stronger in the Southern
Hemisphere than in the Northern Hemisphere. Why?
The westerlies are stronger in the Southern Hemisphere
than in the Northern Hemisphere. This is due to the vast expanse of oceans in
the Southern Hemisphere.
PERIODIC WINDS
What are periodic winds?
Periodic winds are winds that repeat at regular
intervals of time and can be seasonal or diurnal.
Example: monsoon winds
What is monsoon? What are the factors responsible
for the formation of monsoon winds?
The term 'monsoon' is derived from the Arab word 'mousom'.It
means 'winds that change direction in accordance with season'. Monsoon is the
seasonal reversal of wind in a year.
Many factors are responsible for the formation of the
monsoon wind like
• The apparent movement of the sun
• Coriolis force
• Differences in heating
How is the south west monsoon wind formed?
Sun's rays fall vertically to the North of the
Equator during certain months due to the tilt of the Earth's axis. This leads
to an increase in temperature along the region through which Tropic of Cancer
passes. The pressure belts also shift slightly northwards in accordance with this.
The southeast trade winds also cross the equator and moves towards the north as
the Inter Tropical Convergence Zone (ITCZ) moves northwards during the summer in the northern
hemisphere. As the trade winds cross the Equator they get deflected and are transformed
into southwest monsoon winds due to the Coriolis Effect.
The low pressure formed over the land due to the intense day temperature
attracts these sea winds and further contributes to the formation of the
southwest monsoon winds.
North East Monsoon Winds
As a result of the formation of high pressure zones over
the Asian landmass and low pressure zones over the Indian Ocean during winter,
the northeast trade winds get strengthened. These are the North East monsoon
winds.
Write short note on land and sea breeze.
Sea breeze
The land heats up and cools down quickly, whereas
the sea gets heated up slowly and can retain the heat for a longer
period of time. The air in contact with the land
also gets heated up and ascends as the land heats up quickly during the
daytime. This leads to the formation of low pressure over the land which causes
the comparatively cooler air to blow from the sea. This is known as
sea breeze.
Land breeze
As the land cools faster than the sea during the
night it would be high pressure over the land and low pressure over the sea.
This results in the movement of air from the land to sea. This is the land
breeze. The land breeze which starts blowing at night becomes active early in
the morning and ceases by sunrise.
Valley breeze
During the day time the air above the mountains
gets heated and rises up. As a result, the wind blows upslope from the valley
with relatively lower temperature. This is known as valley breeze.
Mountain breeze
During night the air in the mountainous regions
cools due to the intense cold conditions in that region. As cool air is dense,
it blows towards the valley. This is known as mountain breeze.
Local winds
Local winds are winds whose effects are limited to
a relatively smaller area. Formed as a result of the local pressure
differences, these winds are weak.
Loo, Mangoshowers, and Kalbaisakhi are the local winds
experienced in India. Chinook, Harmattan and Foehn are
some of the local winds in other parts of the world.
Chinook
Chinook is a hot local wind that blows down the
eastern slope of the Rockie Mountains in North America. As a result of these
winds, the snow along the eastern slopes of the Rockies melts down. The term
Chinook means 'snow eater', an apt term that describes its peculiarity. Since
this wind reduces the severity of the cold, it is helpful for wheat cultivation
in the Canadian lowlands.
Foehn
Foehn is the wind that blows down the northern
slopes of the Alps mountain. As the air heats up due to pressure from the
descent, it helps in reducing the severity of cold in that region.
Harmattan
Harmattan is a dry wind which blows from the
Sahara desert towards West Africa. On the arrival of these winds, the humid and
sultry conditions of West Africa improve significantly. Hence, people call
these winds as the doctor.
Loo
Loo is a hot wind blowing in the North Indian
plain. These winds blowing from the Rajasthan desert raise the summer
temperature of the North Indian plains. The winds that blow in South India
during this season are called Mango showers. These wind cause the ripening and
fall of mangoes and hence the name.
Variable winds
Variable winds are winds with entirely different
characteristics formed during certain atmospheric situations. Cyclones and
anticyclones are variable winds.
Cyclones
Cyclones are caused by the formation of low
atmospheric pressure at the centre surrounded by high pressure regions. Strong
whirl winds blow towards such low pressure centres from the surrounding high
pressure areas. Due to Coriolis effect winds flow in the anti-clock wise
direction in the Northern Hemisphere and in the clock wise direction in the
southern Hemisphere. Based on the climatic region of their formation, cyclones
can be classified as tropical cyclones and temperate cyclone. Cyclones often
cause extensive damage and destruction wherever they occur.
Anti cyclones
Anti cyclones are phenomenon where strong whirl
winds blow from the high pressure centres to the surrounding low pressure
areas. Due to Coriolis effect the pattern of winds in anti cyclones is clock
wise in the Northern Hemisphere and anti clockwise in the Southern Hemisphere.
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