Air
We
Te know that the Earth is surrounded by a thin layer of air which contains
life-supporting gases such as oxygen, nitrogen, and carbon dioxide, dust, and
water vapour. This layer is called the atmosphere) The word atmosphere is
derived from Greek words atmos, meaning 'vapour' and sphaira, meaning 'sphere'.
The atmosphere is held in place by the gravitational pull of the Earth. Its
density is the highest near the surface of the Earth and goes on decreasing
with height) The atmosphere extends up to a height of 1,600 km above the mean
sea level.
Importance of the atmosphere
It
is this layer of atmosphere that keeps the Sun's harmful rays from reaching us.
It protects Earth from the heat of Sun during the day and keeps it from
freezing at night. In other words, it helps to regulate the temperature on
Earth. Without the atmosphere, Earth would have been burning hot in the day
time and freezing cold at night. This is also the layer where weather-related
phenomena, like rain, wind, snow, and storm takes place.
COMPOSITION OF
ATMOSPHERE
Air
is a mixture of gases like nitrogen, oxygen, and carbon dioxide. Nitrogen makes
up the bulk of air, constituting 78% of air volume. Oxygen constitutes 21% of
air volume. Gases such as carbon dioxide, argon, helium, ozone, hydrogen, etc.,
make up just 1% of the total volume of air. Among the gases, nitrogen, oxygen,
carbon dioxide, and ozone are especially important.
The
composition of air
Nitrogen
78.084%
Oxygen
20.947%
Carbon
dioxide 0.0350%
Argon-0.934%
Others
0.04%
Nitrogen
helps in the growth of living organisms. Humans and animals get nitrogen from
the food they eat. In other words they get nitrogen from the plants. Even
plants cannot directly utilise the atmospheric nitrogen. They get nitrogen from
the bacteria that live in soil and their roots. These bacteria convert
atmospheric nitrogen into a form that plants can use.
The
second most abundant atmospheric gas, oxygen, is the breath of life. Humans and
animals take in oxygen from the air they breathe. Green plants release oxygen
into the air during photosynthesis. Oxygen is constantly being used up by
humans and animals, and replenished by plants. So the quantity of oxygen in the
air remains almost the same. This natural balance can get disturbed if we cut
down plants and trees. Oxygen is also important for combustion.
Carbon
dioxide plays an important role, although it is present in a small quantity
compared to nitrogen and oxygen. It traps heat from the Sun and keeps the Earth
warm enough to sustain life. Without it, the Earth would have been too hot or
too cold to support life. It is CO, gas that green plants use to make food and
release oxygen. In the presence of sunlight, plants take in carbon dioxide and
convert it into starch by the process of photosynthesis. While plants absorb
the gas, humans and animals release carbon dioxide into the air when they
breathe. Therefore, the amount of this gas in air also remains nearly constant.
However, excessive burning of fuels can disturb this balance. Burning of fuels
like coal, wood, petroleum, etc., increases the volume of carbon dioxide in the
air. This is leading to a rise in temperature on the Earth. This increase in
temperature can further lead to melting of snow in cold areas, which can cause
flooding of rivers or rise in levels of sea water.
Carbon
dioxide is also called a greenhouse gas because it traps heat from the Sun.
When the level of carbon dioxide increases, the amount of the Sun's heat
retained by it also increases. This leads to a rise in temperature of the
Earth, which is known as global warming.
Ozone
is also an important gas as it protects us from the harmful ultraviolet rays of
the Sun.
Besides
gases, air also contains water vapour and dust particles. The amount of water
vapour and dust particles in the air varies with time and place. Water vapour
plays a very important role in determining the weather conditions. There would
be no rain or snowfall in the absence of water vapours. Dust particles help
condense water vapour into water droplets to form clouds, which eventually
cause rainfall. Besides dust particles, air also contains other solid matter
like pollen grains, volcanic ashes, and smoke.
STRUCTURE OF
ATMOSPHERE
(The
atmosphere is divided into five layers troposphere, stratosphere, mesosphere,
thermosphere, and exosphere.
Troposphere
This
is the lowest and therefore the densest layer of the atmosphere, containing the
bulk of the atmosphere's total mass. It contains the air we breathe. It is this
layer that contains dust particles and water vapour. Due to their presence in
the troposphere, all weather phenomena, like clouds, rainfall, snow, lightning,
thunder, etc., takes place in this layer. It is also this layer that protects
the Earth from the heat of the Sun during the day and keeps it from freezing at
night. The troposphere extends up to an average height of 13 km above the
Earth's surface. It extends up to a height of 8 km near the Poles and 18 km at
the Equator. The height of the troposphere varies with the seasons. It
decreases in winters and increases in summers. The temperature of air in this
layer decreases with altitude. The temperature falls by 1° C for every 165
metres gain in height. This is known as the normal lapse rate. The upper limit
of the troposphere is called tropopause.
Stratosphere
This
layer lies above the troposphere. It extends up to a height of 50 km. This
layer is considered the most stable layer of the atmosphere as it does not have
any clouds, winds, or other weather phenomena. Therefore, it is suitable for
flying jet planes In the stratosphere, air temperatures remain constant up to a
height of 20 km and thereafter, increase with height. The upper limit of the
stratosphere is known as stratopause) From here on, air temperatures once again
fall with height.
The
lower part of the stratosphere is known as the ozonosphere (15-35 km)) It
contains ozone gas. Ozone absorbs harmful ultraviolet rays of the Sun. The
release of chlorofluorocarbons (CFCs) into the atmosphere by electronic
appliances such as refrigerators, food containers, and jet planes, is slowly
depleting the ozone layer.
Ozone
hole-Scientists have found that the layer of ozone gas over Antarctica is
thinning. This has become a cause for concern because the absence of ozone
layer will lead to harmful ultraviolet rays from the Sun reaching the Earth.
Exposure to such rays can cause diseases such as skin cancer.
Mesosphere
The
layer above the stratosphere is called mesosphere. At extends up to a height of
50 km to 80 km above the Earth's surface. It is the coldest layer of the
atmosphere Air temperatures in this layer decrease with height, touching -100°
C in its upper reaches. Meteorites entering the Earth from space usually burn
up on reaching this layer. Meteorites are small pieces of matter that fall on
the Earth from outer space. The upper limit of the mesosphere is known as
mesopause. Beyond this point, air temperatures increase with height.
Thermosphere
This
layer lies above the mesosphere, extending between 80 km and 400 km above the
Earth's surface. The temperature in this layer increases rapidly with height.
The lower part of this layer is known as ionosphere) The ionosphere contains
electrically charged particles called ions that help in radio transmission. It
reflects radio waves transmitted from the Earth. Air temperatures reach more
than 1500° C in this layer.
Exosphere
This
is the upper most layer of the atmosphere, which contains light gases like
hydrogen and helium. The density of air is very low in this layer. Temperatures
can reach up to 1500° C in this layer. This layer merges into outer space.
WEATHER AND CLIMATE
Weather
and climate play an important role in our lives. They determine our lifestyles,
eating habits, clothes, occupations, the kind of houses we build, etc. Weather
refers to the day to day changes in the atmosphere at a particular place. It
includes changes in temperature, rainfall, sunshine, wind speed, direction,
etc., at a given time and place. So we can say that weather refers to a
short-term variations in the atmosphere. Weather affects our day-to-day life.
Climate refers to general weather conditions over a region over a long period
of time, usually 30-35 years. The climate does not vary from day to day. It
remains unchanged for long periods of time. So we can say that climate refers
to a long-term pattern of weather in a region. It refers to the average
temperature, average rainfall, average sunshine, average wind speed, etc. Thus,
any change in the state of the atmosphere leads to changes in the weather and climate.
There are many instruments
that can tell us about the weather:
Thermometers tell us the
temperature.
Barometers tell us the air
pressure.
Wind vanes tells us the
direction of wind.
Rain gauges tell us the
amount of rainfall in a place.
We
generally use a number of terms to describe weather and climate, such as hot,
cold, windy, cloudy, dry, rainy, etc. These terms describe atmospheric
conditions like temperature, wind, atmospheric pressure, rainfall, etc. These
are known as elements of weather and climate.
TEMPERATURE
Temperature
can be defined as the degree of hotness or coldness of air. It can vary from
time to time and place to place. The distribution of temperature depends on the
amount of solar energy that enters the Earth, or insolation. For example, the
temperature at the Poles is much lower than at the Equator. This is because the
Sun's rays fall directly on the Equator and so it gets more insolation than the
Poles. Insolation also depends on the season. As days are longer during summer,
the amount of insolation is greater and temperatures are higher. The amount of
insolation decreases in winter and therefore, the temperatures are also lower.
The Earth's surface absorbs insolation during the day. It loses heat at night.
This radiation of heat by the Earth is known as terrestrial radiation.
Air
gets heated up not through insolation, but by heat radiation given off by land.
Therefore, the layer of air closest to land gets heated first and the most. It
is for this reason that air temperature falls with altitude. There are four
processes through which the atmosphere gets heated up convection, conduction,
radiation, and advection.
Convection
refers to the transmission of heat from one part of a liquid or gas to another
by the movement of particles. When air comes in contact with heated land
surface, it warms up. Warm air being light, begins to rise. Cool air being
dense rushes in to take its place. At a certain point, the rising warm air
begins to cool down. As it cools, it starts to sink down, where it once again
heats up and the cycle continues. This transfer of heat by the horizontal and
vertical movement of air is called convection.
Conduction
refers to the transfer of heat from one body to another through direct contact.
When air comes in contact with heated land surface, it gets heated up and
transfers some of this heat to the lower levels. As air is a poor conductor of
heat, conduction is restricted to the lower levels of the atmosphere.
Radiation
refers to the transfer of heat energy by a body in the form of waves. This is
how terrestrial radiation from the Earth heats up the air.
Advection
refers to the horizontal transfer of heat by winds. Winds transfer heat from
one place to another. A place gets heated up when winds coming from warm
regions blow over it. It cools down when winds coming from colder regions blow
over it.
Air
temperature also depends on the distance of a place from a water body. Places
near water bodies experience moderate temperatures compared to those away from
water bodies. The uneven heating of the Earth gives rise to planetary winds,
which move from hot regions to cold regions and vice versa. When winds from
warm regions blow over cold areas, they pass on their heat to the cool air in
these areas, thereby raising the air temperature. Similarly, when cold winds
blow over warm places, they cool the air in the areas, thereby lowering the air
temperature.
Ocean
currents are rivers of hot or cold water which flow within oceans. When winds
blow over hot ocean currents, they get heated up and warm the air in the areas
they blow over. Similarly, when winds blow over cold ocean currents, they get
cooled and lower the temperatures in the areas they blow over. You will usually
find that cities are warmer than villages. This is because of higher emission
of carbon dioxide in cities due to burning of fuels by industries, running of
vehicles, etc. The materials used in buildings and roads also trap more heat
during the day. Buildings are also closer together in cities, which also lead
to higher temperatures.
ATMOSPHERIC PRESSURE
The
blanket of air around the Earth has some weight and therefore, exerts pressure
on the Earth's surface. So we can say, atmospheric pressure is the pressure
exerted by the weight of air on the Earth's surface. We do not feel this
pressure because it is exerted equally in all directions. Also, our body exerts
a pressure that balances out air pressure.
Atmospheric
pressure varies from place to place and from time to time due to the following
factors:
Air temperature: The distribution of air pressure
depends on the air temperature. If the temperature is high in a place, the air
gets heated up, expands, and becomes lighter. This warm air rises up to create
a low pressure area. Such areas experience clouds and rainfall. If the
temperature is low, the air becomes heavy and dense. This cold air sinks to
create a high pressure area. Such areas experience clear weather. When hot air
rises, cold air rushes in to take its place. Therefore, air always moves from
an area of high pressure to an area of low pressure.
Air density:
If the density of air is low at a given
place, it exerts less pressure) on the Earth's surface to form a low pressure
area. This usually happens in hot places. In cold places, the air is quite
dense. Therefore, it exerts greater pressure on the Earth's surface to form a
high pressure area.
Moisture: Air
containing moisture is warmer, lighter, and less dense than dry air. Therefore,
it exerts less pressure than air which contains little or no moisture.)
Altitude:
As the atmosphere is held in place by the
gravitational pull of the Earth, its density decreases with height. Thus,
atmospheric pressure falls as we move towards the upper layers of the
atmosphere/It is the highest at sea level and falls as we gain height.
PRESSURE BELTS
Due
to differences in temperature, density, moisture and altitude, atmospheric
pressure varies from place to place. This coupled with the rotation of the
Earth leads to creation of pressure belts. There are four permanent pressure
belts in the world.
1. Equatorial Low Pressure belt: This
belt lies between 5°N and 5°S latitudes. Temperatures remain high all through
the year in this region due to high rate of insolation. Thus, air in this
region heats up, expands, and rises to create a permanent low pressure area.
This region experiences calm weather. As there are no surface winds in this
belt, it is also known as doldrums (meaning 'depression').
2. Sub-tropical High Pressure belts: There
are two Sub-tropical High Pressure belts located between 30° and 35° latitudes
in the Northern and Southern Hemispheres. One lies near the Tropic of Cancer
and the other lies near the Tropic of Capricorn. Areas falling under these
belts experience low temperatures compared to the Equatorial regions. In the
Equatorial regions, the air gets heated up, it begins to rise and move towards
the Poles. When it reaches the Sub-tropical regions, it begins to cool down.
Due to the rotation of the Earth, cool air from the Sub-polar areas also comes
to these regions and meets the warm air from the Equator. This leads to the
formation of high pressure over these belts.
3. Sub-polar
Low Pressure belts: There are two Sub-polar Low Pressure belts located
between 60° and 65° latitudes in the Northern and Southern Hemispheres. One
lies near the Arctic Circle and the other is situated near the Antarctic
Circle. The low temperatures in these regions should have ideally led to the
formation of high pressure belts. However, the rotation of the Earth is
responsible for creating low pressure belts in the regions.
4. Polar
High Pressure belts: There are two Polar High Pressure belts, lying in the
North and the South Poles. As the temperature in these regions is extremely
low, the air becomes very cold and heavy, creating high pressure conditions
here.
WIND
The
horizontal movement of air is called wind. The vertical movement of air is
known as air current. Air moves from an area of high pressure to an area of low
pressure. So we can say that wind is the movement of air from high pressure
areas to low pressure areas.) Winds are named after the direction from which
they blow. For example, winds blowing from the east are called easterly winds.
Due to the rotation of the Earth, winds do not blow directly from high pressure
to low pressure areas. Instead, they get deflected to their right in the
Northern Hemisphere and to their left in the Southern Hemisphere. This is known
as the Coriolis Effect. Winds can be of three types -local, periodic, and
permanent.
PERMANENT WINDS
Permanent
winds blow throughout the year without changing their direction. Trade winds,
westerlies, and easterlies are all permanent winds;
1.
Trade
winds derive their name from the Latin word trado, which
means 'constant'. These winds blow in the Tropical regions, from the
Sub-tropical High Pressure belts to Equatorial Low Pressure belts. In the
Northern Hemisphere, they blow from northeast to southwest and are therefore
called northeast trade winds. In the Southern Hemisphere, they blow from the
southeast to the northwest and are therefore called southeast trade winds.
2.
Westerlies blow from Sub-tropical High Pressure belts to
Sub-polar Low Pressure belts. They are called 'westerlies' as they blow from
the west. They blow from the southwest to the northeast in the Northern
Hemisphere, and from northwest to the southeast in the Southern Hemisphere. The
westerlies are very high speeds winds. They bring rainfall to the western
margins of the continents.
3.
Easterlies
are also called Polar winds. They
blow from Polar High Pressure belts to Sub-polar Low Pressure belts. They blow
from the northeast in the Northern Hemisphere and from the southeast in the
Southern Hemisphere. These winds are very cold and dry.
PERIODIC WINDS
Periodic
winds blow in a particular direction during a specific part of the day or year.
Monsoon winds, land and sea breezes are examples of such winds. Monsoon winds
are seasonal winds which reverse their direction with the change of season) In
summer, they blow from sea to land and are moisture laden. Therefore, summer
monsoons are warm and moist, and bring heavy rainfall. However, as soon as the
season changes from summer to winter, they reverse their direction. They begin
blowing from land to sea. Therefore, winter monsoons are cold and dry.
LOCAL WINDS
Local
winds blow in one particular place for a particular period of time. They affect
local weather conditions prevailing at a place. Local winds are known by
different names such as Loo, Harmattan, Foehn, Sirocco, Mistral, and Chinook.J
Loo is an extremely hot and dry wind that blows in the northern Indian plains
during the months of May and June. Harmattan is a dry, dusty wind which
originates in the Sahara desert and blows over West Africa between November and
March. The term 'Foehn' is used to describe warm, dry, and strong winds that
blow down the slopes of mountains. It was originally used to describe winds
which blew down the Alps in Europe. These Land Sea winds raise the temperature
of places they blow over by as much as 30° C within hours. Chinook is a
foehn-type wind which blows in the eastern parts of the Rockies (mountains) in
North America.
Coastal
areas experience sea breeze during the day and land breeze at night. During the
day, the land gets quite hot, while the waters of the sea remain quite cool.
The air above the land heats up and rises, while the cool air above the sea
rushes in to take its place. This wind is called sea breeze. At night, the sea
is warmer than the land. Air flows from the land to the sea, giving rise to
land breeze.
Some local winds
Bora:
North-easterly winds from eastern Europe to Italy.
Gregale:
North-easterly winds from Greece.
Khamsin:
South-easterly winds from North Africa to the eastern Mediterranean.
Levanter:
Easterly winds through the Strait of Gibraltar.
Mistral:
Northerly winds from central France and the Alps (mountains) to the
Mediterranean.
Sirocco:
Southerly winds from North Africa to southern Europe.
VARIABLE WINDS
Variable
winds are not regular and prevail only for a short duration. Cyclones and
anti-cyclones are examples of such winds. A cyclone is caused when a low
pressure area is surrounded by high pressure on all sides. As a result, winds
begin to blow from all sides towards the low pressure area at the centre,
causing strong spiral winds. These winds move in an anti-clockwise direction in
the Northern Hemisphere and in a clockwise direction in the Southern
Hemisphere. They cause huge damage to life and property. Anti-cyclones happen
when a high pressure area is surrounded by low pressure on all sides.
Anti-cyclones move clockwise in the Northern Hemisphere and anti-clockwise in
the Southern Hemisphere. They are accompanied by calm, fine weather.
HUMIDITY
The
air contains water in the form of vapour. Land and water bodies constantly lose
water through evaporation. The amount of water vapour or moisture present in
air at any time is called humidity) When air holds the maximum water vapour it
can hold and cannot take in any more moisture, it is said to be saturated.
Humidity
is usually expressed in terms of absolute or relative humidity. The amount of
water vapour present in a unit volume of air is called absolute humidity). It
is expressed in kilograms per cubic metre. Relative humidity is the ratio of
the amount of water vapour present in air and the maximum water vapour air can
carry at that temperature and pressure. It is expressed as a percentage Areas
with higher relative humidity have a greater possibility of precipitation.
Clouds
are mainly of three types: cirrus, stratus, and cumulus. Cirrus clouds are thin
clouds which are found high up in the atmosphere. Stratus clouds are low-lying
sheet-like clouds. They are grey in colour. Cumulus clouds are large clouds
which often extend to high altitudes. Sometimes, the word nimbus is put before
or after the name of any rain-bearing cloud for example, cumulonimbus.
On
humid days, the air has a lot of moisture. On such days the clothes do not dry
fast. The amount of moisture in air increases with rise in temperature. Warm
air can hold more water vapour. On warm days, water vapour rises. As it moves
up, it begins to cool down and turn into water droplets. The conversion of
water vapour into water droplets is called condensation. These droplets of
water accumulate in the air in the form of clouds. When clouds become heavy
with water droplets, they shed it in the form of rain, snow, sleet, and hail.
The process through which condensed water falls on the surface of the Earth is
known as precipitation.
Rainfall
is the most common form of precipitation. It is important for the survival of
all forms of life.
Other forms of precipitation-
Snow: When air
temperature falls below freezing point, water vapour present in air condenses
directly into crystals of ice, or snowflakes. When these flakes fall on the
Earth, snowfall is said to occur.
Sleet: Rain containing some ice, or melting snow is called
sleet. When raindrops pass through a zone of freezing temperatures, they freeze
and come down as sleet.
Hail: These are hard pellets of ice which are formed due to
continuous freezing and melting of ice crystals as they fall towards the Earth.
When they become too big to be carried by winds, they come down as hail.
The
jet planes leave a white trail behind them which is visible for some time if
the wind is not blowing. This happens due to condensation of water vapour
released by jet engines.
Rainfall can be of three types:
( i) Convectional rainfall: The upward and downward
movement of air as it warms or cools is called air current. Warm air contains a
lot of moisture. When warm air rises, it cools down. Water vapour present in
the air condenses to form clouds. When clouds become heavy with water droplets,
they shed. it in the form of rain. Such rainfall is usually accompanied by
lightning and thunder. It is common in the Equatorial regions.
(ii) Cyclonic rainfall: We know that winds blow from
high pressure areas to low pressure areas. When a low pressure area is
surrounded by high pressure areas on all sides, winds begin to move in from all
sides to the low pressure centre, forming a spiral. The warm air in the centre
is forced to rise. As the warm air moves up, it cools down and causes rainfall.
This type of rainfall is common in temperate zones.
Cyclonic
rainfall
(iii) Orographic rainfall:
When winds that have a lot of humidity strike a mountain range, they are forced
to rise. As the winds rise, they condense to form clouds. Rainfall caused by
such winds is also called relief rainfall. Such winds cause rainfall on the
side of the mountain they strike, also called the windward side. The other side
of the mountain, which does not receive much rainfall, is called the leeward
side. For example, when monsoon winds strike Western Ghats, its western slopes
that are on the windward side receive rainfall. The eastern slopes do not get
much rainfall.
New Terms
Atmosphere:
Thin layer of air surrounding the Earth
which contains life-supporting gases, dust particles, and water vapour.
Troposphere: The layer
closest to the Earth's surface where all weather phenomena take place; it
extends up to a height of 13 km above the Earth's surface.
Stratosphere: It
extends up to a height of 50 km. It is suitable for flying aeroplanes as it
does not have any clouds or other weather phenomena.
Mesosphere: It lies on
top of the stratosphere. It extends up to a height of 80 km.
Thermosphere: It lies
above the mesosphere. The lower part of this layer is known as ionosphere,
which reflects radio waves. It extends between 80 km and 400 km.
Exosphere: The upper
most layer of the atmosphere. It contains light gases such as hydrogen and
helium.
Meteorites: Small
pieces of matter that fall on the Earth from outer space
Weather: Day to day
condition of the atmosphere in a place.
Climate: Average
weather condition in a place over a long period of time.
Temperature: Degree of
hotness or coldness of air.
Insolation: Amount of
solar energy entering the Earth.
Ions: Electrically
charged particles.
Atmospheric pressure:
Pressure exerted by the weight of air on the Earth's surface.
Wind: Movement of air
from high pressure areas to low pressure areas.
Local winds: These are
winds that blow in one particular place for a particular period of time.
Periodic winds: These
winds blow in a particular direction during a specific part of the day or year.
Permanent winds: These
winds blow throughout the year without changing their direction.
Coriolis effect:
Causes winds to deflect to their right in the Northern and to their left in the
Southern Hemispheres.
Trade winds: Blow in
the Tropical regions, from the Sub-tropical High Pressure belts to the
Equatorial Low Pressure belts.
Westerlies: Blow from
Sub-tropical High Pressure belts to Sub-polar Low Pressure belts.
Easterlies: Also known
as Polar winds; they blow from Polar High Pressure belts to Sub-polar Low
Pressure belts.
Monsoon winds:
Seasonal winds which reverse their direction with the change of season.
Sea breeze: Wind
blowing from sea to land.
Exercise
1. The atmosphere prevents extreme temperature conditions
on Earth mainly by
A. Absorbing insolation
B. Reflecting terrestrial radiation
C. Acting as an insulating blanket
D. Blocking all solar radiation
Answer: C
Explanation: Atmosphere regulates heat balance by preventing extreme
heating and cooling.
2. Which gas constitutes the largest proportion of the
atmosphere by volume?
A. Oxygen
B. Carbon dioxide
C. Nitrogen
D. Argon
Answer: C
Explanation: Nitrogen forms about 78% of air volume.
3. Atmospheric nitrogen cannot be directly used by plants
because
A. It is chemically inert
B. It is present in small quantity
C. It is toxic to plants
D. It is heavier than air
Answer: A
Explanation: Nitrogen must be fixed by bacteria before plant absorption.
4. The balance of oxygen in the atmosphere is maintained
mainly due to
A. Respiration
B. Combustion
C. Photosynthesis
D. Decomposition
Answer: C
Explanation: Photosynthesis replenishes oxygen consumed by living
beings.
5. Which gas is primarily responsible for the greenhouse
effect?
A. Oxygen
B. Nitrogen
C. Carbon dioxide
D. Helium
Answer: C
Explanation: CO₂ traps outgoing terrestrial radiation.
6. Excessive CO₂ in the atmosphere may directly lead to
A. Ozone formation
B. Global warming
C. Acid rain only
D. Increase in nitrogen cycle
Answer: B
Explanation: Increased CO₂ enhances heat retention → global warming.
7. Ozone is important for life on Earth because it
A. Traps heat
B. Absorbs infrared radiation
C. Absorbs ultraviolet radiation
D. Reflects visible light
Answer: C
8. Weather phenomena occur mainly in which atmospheric
layer?
A. Stratosphere
B. Troposphere
C. Mesosphere
D. Thermosphere
Answer: B
9. The temperature in the troposphere generally
A. Increases with altitude
B. Remains constant
C. Decreases with altitude
D. Fluctuates randomly
Answer: C
10. Normal lapse rate refers to
A. Increase of temperature with altitude
B. Decrease of pressure with altitude
C. Decrease of temperature with altitude
D. Increase of humidity with altitude
Answer: C
11. The tropopause is
A. Upper boundary of stratosphere
B. Lower boundary of mesosphere
C. Upper boundary of troposphere
D. Lower boundary of thermosphere
Answer: C
12. Jet aircrafts usually fly in the stratosphere because
A. Air pressure is high
B. Weather disturbances are absent
C. Oxygen concentration is high
D. Temperature is very low
Answer: B
13. The ozone layer is located in the
A. Troposphere
B. Mesosphere
C. Lower stratosphere
D. Thermosphere
Answer: C
14. Ozone depletion is mainly caused by
A. Carbon monoxide
B. Sulphur dioxide
C. Chlorofluorocarbons
D. Methane
Answer: C
15. Meteors generally burn up in the
A. Stratosphere
B. Mesosphere
C. Thermosphere
D. Exosphere
Answer: B
16. The ionosphere is important because it
A. Causes rainfall
B. Reflects radio waves
C. Produces ozone
D. Contains water vapour
Answer: B
17. Weather differs from climate because weather
A. Is uniform
B. Is long-term
C. Is short-term
D. Does not vary
Answer: C
18. Climate of a region is generally determined over a
period of
A. 5–10 years
B. 10–15 years
C. 20 years
D. 30–35 years
Answer: D
19. Which instrument measures atmospheric pressure?
A. Thermometer
B. Barometer
C. Anemometer
D. Hygrometer
Answer: B
20. Insolation refers to
A. Heat radiated by Earth
B. Solar energy received by Earth
C. Heat absorbed by atmosphere
D. Heat lost by atmosphere
Answer: B
21. Air is heated mainly by
A. Direct sunlight
B. Reflection
C. Terrestrial radiation
D. Conduction from upper layers
Answer: C
22. Which process transfers heat by horizontal movement
of air?
A. Conduction
B. Convection
C. Radiation
D. Advection
Answer: D
23. Convection currents are caused because
A. Warm air is heavier
B. Cold air rises
C. Warm air rises
D. Air is a good conductor
Answer: C
24. Cities are warmer than villages mainly due to
A. Higher altitude
B. Proximity to sea
C. Urban heat island effect
D. Higher rainfall
Answer: C
25. Atmospheric pressure is highest at
A. Mountain tops
B. Sea level
C. Upper troposphere
D. Poles
Answer: B
26. Low pressure is associated with
A. Clear skies
B. Descending air
C. Rising air
D. Dry weather
Answer: C
27. Pressure decreases with altitude because
A. Temperature decreases
B. Gravity decreases
C. Air density decreases
D. Moisture increases
Answer: C
28. Doldrums are located near
A. Tropic of Cancer
B. Tropic of Capricorn
C. Equator
D. Poles
Answer: C
29. Sub-tropical high pressure belts are located around
A. 0° latitude
B. 30° latitude
C. 60° latitude
D. 90° latitude
Answer: B
30. Polar regions have high pressure mainly because
A. High insolation
B. Low rotation speed
C. Cold and dense air
D. High humidity
Answer: C
31. Wind is defined as
A. Vertical movement of air
B. Horizontal movement of air
C. Circular movement of air
D. Random movement of air
Answer: B
32. Winds always move from
A. Low to high pressure
B. High to low pressure
C. East to west
D. Poles to equator
Answer: B
33. Coriolis force causes winds to
A. Stop
B. Change speed
C. Deflect direction
D. Rise vertically
Answer: C
34. Trade winds blow from
A. Equator to poles
B. Subtropical highs to equatorial low
C. Poles to equator
D. Subpolar lows to poles
Answer: B
35. Westerlies are important because they
A. Bring rainfall to eastern coasts
B. Bring rainfall to western coasts
C. Cause cyclones
D. Cause monsoons
Answer: B
36. Polar easterlies are
A. Warm and moist
B. Hot and dry
C. Cold and dry
D. Warm and dry
Answer: C
37. Monsoon winds reverse their direction due to
A. Coriolis force
B. Pressure differences
C. Earth’s revolution
D. Ocean currents
Answer: B
38. Sea breeze occurs because
A. Sea heats faster than land
B. Land heats faster than sea
C. Pressure is same
D. Winds are permanent
Answer: B
39. Loo is a
A. Cold moist wind
B. Hot dry wind
C. Cold dry wind
D. Warm moist wind
Answer: B
40. Chinook winds cause
A. Heavy snowfall
B. Sudden rise in temperature
C. Cyclones
D. Drought
Answer: B
41. Cyclones form due to
A. High pressure centre
B. Low pressure centre
C. Uniform pressure
D. Cold air sinking
Answer: B
42. Cyclones rotate anticlockwise in
A. Southern Hemisphere
B. Northern Hemisphere
C. Equator
D. Poles
Answer: B
43. Anticyclones are associated with
A. Stormy weather
B. Rainfall
C. Calm and clear weather
D. Floods
Answer: C
44. Humidity refers to
A. Amount of rainfall
B. Amount of moisture in air
C. Amount of clouds
D. Amount of pressure
Answer: B
45. Relative humidity is expressed in
A. Kg/m³
B. °C
C. Percentage
D. mm
Answer: C
46. Which clouds are associated with heavy rainfall and
thunderstorms?
A. Cirrus
B. Stratus
C. Cumulus
D. Cumulonimbus
Answer: D
47. Conversion of water vapour into water droplets is called
A. Evaporation
B. Condensation
C. Sublimation
D. Transpiration
Answer: B
48. Orographic rainfall occurs due to
A. Cyclonic lifting
B. Convection
C. Mountain barriers
D. Ocean currents
Answer: C
49. Rainfall common in equatorial regions is
A. Cyclonic
B. Orographic
C. Convectional
D. Frontal
Answer: C
50. Leeward side of mountains receives
A. Heavy rainfall
B. Moderate rainfall
C. No rainfall
D. Less rainfall
Answer: D
51. Increase in greenhouse gases will first affect
A. Insolation
B. Terrestrial radiation
C. Atmospheric pressure
D. Wind direction
Answer: B
52. Why does air temperature decrease with altitude in
the troposphere?
A. Less pressure
B. Less dust
C. Heating from Earth’s surface
D. More radiation
Answer: C
53. Which factor does NOT directly affect atmospheric
pressure?
A. Temperature
B. Moisture
C. Latitude
D. Altitude
Answer: C
54. Which rainfall type is associated with temperate
cyclones?
A. Convectional
B. Orographic
C. Cyclonic
D. Relief
Answer: C
55. Which gas helps maintain Earth’s temperature balance
the most?
A. Oxygen
B. Nitrogen
C. Carbon dioxide
D. Argon
Answer: C
56. Which atmospheric layer has the least air density?
A. Troposphere
B. Stratosphere
C. Thermosphere
D. Exosphere
Answer: D
57. Pressure belts shift seasonally due to
A. Earth’s rotation
B. Earth’s revolution
C. Plate tectonics
D. Ocean currents
Answer: B
58. Which wind brings winter rainfall to Mediterranean
regions?
A. Trade winds
B. Easterlies
C. Westerlies
D. Monsoons
Answer: C
59. Which phenomenon explains deflection of winds?
A. Greenhouse effect
B. Coriolis effect
C. El Niño
D. La Niña
Answer: B
60. Which statement is correct?
A. Warm air is denser than cold air
B. Cold air rises faster
C. Moist air is lighter than dry air
D. Dry air causes rainfall
Answer: C
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