Essay on Atmosphere
         Read this essay to learn about atmosphere. After reading this essay you
will learn about: 1. Definition of Atmosphere 2. Characteristics of Atmosphere 3. Physical
Properties 4. Mass 5. Composition 6. Structure 7. Chemical and Photochemical Reactions 8.
Role of Atmospheric Air in Agriculture 9. Solid Particles 10. Atmospheric Pressure.
Essay Contents:
1. Essay on the Definition of Atmosphere
2. Essay on the Characteristics of Atmosphere
3. Essay on the Physical Properties of Atmosphere
4. Essay on the Mass of Atmosphere
5. Essay on the Composition of Atmosphere
6. Essay on the Structure of Atmosphere
7. Essay on Chemical and Photochemical Reactions in Atmosphere
8. Essay on the Role of Atmospheric Air in Agriculture
9. Essay on Solid Particles in Atmosphere
10. Essay on Atmospheric Pressure
Essay # 1. Definition of Atmosphere:
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The envelope of colourless, tasteless and odour-less gases surrounding the earth is called the
atmosphere.
Aircraft fly within the Earth's atmosphere, the gaseous envelope that surrounds the planet. The
atmosphere's weather, temperature, and properties all affect aircraft flight, as does the way in
which air, the fluid of interest to aerodynamicists, moves around and over the parts of an aircraft.
Its existence is felt only when it is in motion. It is derived from two Greek words; atmos means
water vapour and spharia means sphere i.e. sphere of water vapour.
The atmosphere plays vital role in maintaining the heat balance on the earth by absorbing the
radiation received from the sun and remitted to by the earth.
Differences in temperature and pressure within an airflow result in turbulence, or small-scale
motion of the atmosphere. An aircraft experiences turbulence because small currents of wind are
moving in a different direction from the main flow of wind. Turbulence also occurs because of
winds blowing over irregular terrain. In passenger aircraft, turbulence may cause minor problems
such as spilled coffee and in extreme cases, injuries if seat belts are not fastened. Excessive
shaking or vibration may render the pilot unable to read instruments. In cases of precision flying
such as for air-to-air refueling, bombing and gunnery, or aerial photography, turbulence-induced
motions of the aircraft are a nuisance. Turbulence-induced stresses and strains over a long period
may cause fatigue in the airframe and particularly heavy turbulence may cause the loss of control
of an aircraft or even immediate structural failure.
This phenomenon is called greenhouse effect. Which keeps the earth warm enough to sustain life on
the earth, oxygen supports life on earth. Nitrogen is an essential macro nutrient for plants, CO2 is
essential for photosynthetic activity of plants.
A fluid is a continuous and shapeless substance that assumes the shape of its container and whose
molecules move freely past one another. All fluids are compressible to some extent, that is, their
molecules can be compacted and their density increased under increasing pressure, but liquids are,
as a rule, highly incompressible compared with gases. Even gases, like air, though may be treated
as incompressible if their speeds are not great. For subsonic airflow over an airplane traveling
below about 150 meters per second (about 492 feet per second), air may be treated as
incompressible (that is, there is no change in density throughout the flow). At higher speeds, the
behavior of the airflow changes and the effects of compressibility, or change in density, must be
taken into account.
Essay # 2. Characteristics of Atmosphere:
Atmosphere enables life to exist on earth:
1. Without its protective insolation, temperature would swing from unbearable cold at night to
unbearable hot during the day.
Researchers have studied the Earth's atmosphere since the 17th century, documenting variations
across seasons, in different geographic areas and climates, and at varying altitudes. Beginning in
the 1920s, and updated many times since, meteorologists in the United States and around the
world have used a large amount of data to construct what is called the standard atmosphere. The
standard atmosphere is a hypothetical model that lists average (mean) conditions for atmospheric
composition, pressure, temperature, and several other parameters in a motionless, stable
atmosphere from sea level to an altitude of 1,000 kilometers (about 621 miles) and without
considering the variations that occur with the seasons. The standard atmosphere is useful when the
vertical distribution of pressure, temperature, density, and speed of sound is required such as when
calibrating aircraft altimeters and determining aircraft and rocket performance and design.
2. It protects us from meteors and primary cosmic ray particles. It is selective towards the sun’s
electromagnetic radiation, stops harmful ultraviolet and X-radiation, but transmits vital visible
radiation.
Flight path of an aircraft through various forms of turbulence. Relatively stable air exists about
thunderstorms.
Essay # 3. Physical Properties of Atmosphere:
The important physical properties of the atmosphere are:
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i. A given quantity of air occupies equally all space of a container. It exerts equal pressure in all
directions.
Air density is a very important factor in the lift, drag, and engine power output of an aircraft and
depends upon the local temperature and pressure. Since the standard atmosphere does not indicate
true conditions at a particular time and place, it is important for a pilot to contact a local airport for
local atmospheric conditions. From these local temperature and pressure readings, density may be
obtained and, hence, takeoff distance and engine power output may be determined.
ii. The pressure of a given quantity of air is inversely proportional to its volume, keeping the
temperature constant.
iii. The volume of a given quantity of air is directly proportional to its absolute temperature, keeping
the pressure constant.
Since air pressure is proportional to density as well as temperature, it follows that a change in
either temperature or density will cause a corresponding change in the pressure.
iv. A given quantity of air cools when it expands and is heated when it is compressed without
subtracting heat from it.
Essay # 4. Mass of Atmosphere:
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The weight of the atmosphere on one square foot of earth is almost a ton and the whole weight of
the dry air is about 5600 million tons.
Atmospheric pressure is the force per unit area exerted on a surface by the weight of air above that
surface in the atmosphere of Earth (or that of another planet). In most circumstances atmospheric
pressure is closely approximated by the hydrostatic pressure caused by the mass of air above the
measurement point. Low-pressure areas have less atmospheric mass above their location, whereas
high-pressure areas have more atmospheric mass above their location. Likewise, as elevation
increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with
increasing elevation. On average, a column of air one square centimeter in cross-section, measured
from sea level to the top of the atmosphere, has a mass of about 1.03 kg and weight of about 10.1
N (2.28 lbf) (A column one square inch in cross-section would have a weight of about 14.7 lbs, or
about 65.4 N). Over the area of your body, there is about 1,000 kg of air; this is approximately the
same as having a small car press down on you A barometer is a scientific instrument used in
meteorology to measure atmospheric pressure. Pressure tendency can forecast short term changes
in the weather. Numerous measurements of air pressure are used within surface weather analysis
to help find surface troughs, high pressure systems, and frontal boundaries.
In the same unit the weight of the water vapour would be about 146 million tons and the weight of
ozone would be 3300 million tons.
A clear idea about the mass of the atmosphere is obtained by imagining that if the weight of all the
air is replaced by the same weight of ordinary water, this would amount to a layer of water about 10
meter deep covering whole of globe.
The primary ingredients in the Earth's atmosphere are nitrogen (78 percent) and oxygen (21
percent). The remaining one percent consists of argon, carbon dioxide, several trace gases
(extremely small amounts), and water vapor. Above about 56 miles (90 kilometers) from the
Earth's surface, the different gases begin to settle or separate out according to their respective
densities. In ascending order one would find high concentrations of oxygen, helium, and then
hydrogen, which is the lightest of all the gases.
If all the water vapour in the air were condensed to rain and dropped evenly over the globe, a layer
of water about 25 mm deep would form.
Essay # 5. Composition of Atmosphere:
Eight to ten kilometres of air above the earth is essentially a mixture of nitrogen, oxygen, carbon
dioxide, water vapour, dust and several rare gases-argon, neon, helium and methane.
The tropopause is the dividing line between the troposphere and the next region, the stratosphere,
which is found between approximately 10 miles and 30 miles (16 and 48 kilometers) above sea
level. The stratosphere contains a type of oxygen called ozone (O3) that absorbs sunlight, resulting
in temperatures similar to those found near the Earth's surface. The ozone layer absorbs harmful
solar ultraviolet radiation and protects the Earth. The temperature in the stratosphere rises with
altitude, reaching about -40 degrees F at 30 miles (48 kilometers) up. Almost all aircraft flight
occurs in the troposphere and stratosphere.
The approximate percentage of these gases except carbon dioxide and water vapour are practically
the same every where.
The percentage of water vapour is quite variable: from 0.2 per cent to 2.5 per cent. For many
purposes it is sufficient to regard air as composed of nitrogen and oxygen with the molecules of
nitrogen four times as those of oxygen.
The regions of the atmosphere can also be characterized by the distribution of various chemical
processes that happen within them, by their molecular composition, and also by the dynamic and
kinetic processes that occur within each region.
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Composition of the Atmosphere:
Highly variable constituents are water vapours (H2O), ozone (O3), sulphur dioxide (SO2), nitrogen
dioxide (NO2), carbon monoxide (CO) and dust particles.
Pilots have to compensate for the direction and velocity of winds to stay on course. Although
statistical averages of wind speed as a function of altitude have been calculated, real wind velocity
at any particular time and place varies considerably from the statistical average. To avoid drift as a
result of wind, pilots should consult local airports for wind conditions and forecasts along their
intended flight path.
Essay # 6. Structure of Atmosphere:
The basis of division of the atmosphere is based on temperature, composition, ionisation and
chemical reactions. Mainly it can be divided into homosphere and heterosphere.
A high value of air pressure will cause air to be compressed. With lower values of air pressure, air
is less compressed. Compression of air leads decreases…...
i. Homosphere:
It extends from sea level to about 100km. The composition and mixing of gases is uniform in this
region. The main constituents of this region are nitrogen, oxygen, argon and carbon dioxide and
their ratio is constant.
Gas molecules are in a constant state of collision and move about freely. If a gas is kept in a
closed vessel, this motion is restricted by the walls of the container. The gas molecules exert an
outward push because they continuously dash against the walls of the container.
ii. Heterosphere:
This region extends from 100km to the top of the atmosphere. Turbulent mixing is reduced and air is
no longer uniform. The molecules and atoms tend to separate and arrange themselves in layers each
with different composition.
Atmospheric pressure is a force in an area pushed against a surface by the weight of air in Earth's
atmosphere. The earth is covered in a layer of air. However, this layer is not distributed evenly
around the globe. At different times, the layer of air is thicker in some places than in others.
Where the layer of air is thicker, there is more air. Since there is more air, there is a higher
pressure in that spot. Where the layer of air is thinner, there is a lower atmospheric pressure.
We do not exactly know, how far above the earth atmosphere extends but it is probably 1600 km or
more. It may well extend over 1600 km (exosphere) to few thousands of km, where the air is so
rarefied that its density is one millionth of ground level.
The Earth's atmosphere is divided into different levels or regions primarily by temperature. The
lowest region of the atmosphere is the troposphere, which begins at the Earth's surface and extends
to an altitude of approximately 10 miles (16 kilometers), or 55,000 feet—about 10.4 miles (16.8
kilometers), above sea level at the equator. Around the North and South Poles, the troposphere is
only a little more than 5 miles (8 kilometers), or 28,000 feet (8,805 meters), deep. The temperature
of the troposphere decreases about 2 degrees Celsius, or 3.5 degrees Fahrenheit per 1000 feet.
Humans live in the troposphere and most weather occurs here.
Here air particles move freely and some escape into the near vacuum of the space.
Most of the atmosphere is confined to the lower layers only. About 75 per cent weight of the
atmosphere is confined to troposphere with an average height of 12 km. The air is not a uniform
mass but can be divided into different layers with their characteristics.
Perhaps more than any other single measurement, atmospheric pressure is the best indicator of
current and changing weather conditions. This is because the weather characteristics in an area
such as rainfall, cloud cover and sunshine are usually related to the pressure system in the area.
Thus, knowledge of existing and expected atmospheric pressure distribution enables
meteorologists to determine the weather conditions that will be expected in a particular area.
Based upon these properties, the atmosphere is divided into following layers:
(a) Troposphere
(b) Stratosphere
(c) Mesosphere
(d) Thermosphere
(e) Exosphere
Besides these five, there are two other zones:
(f) Ozonosphere
(g) Ionosphere
The top of each sphere is known as pause.
Between altitudes of approximately 30 and 55 miles (48 and 89 kilometers) is the mesosphere. In
this region, the temperature first increases to about 50 degrees F (10 degrees C), then decreases
until about 50 miles altitude (80 kilometers) the mesopause it drops to as low as -130 degrees F (90 degrees C). The thermosphere begins at approximately 50 miles (80 kilometers) above sea
level and extends to 6,000 miles (9,656 kilometers), the outer limit of the Earth's atmosphere.
Temperature in the thermosphere increases to about 2,200 degrees F (1,204 degrees C). The exact
temperature depends on solar activity. The region beyond the Earth's atmosphere is referred to as
space, or outer space.
i. Troposphere:
Troposphere extends from sea level to about 8km at the poles and 16km at the equator. The average
height of the troposphere is 12km. It is the base of the atmosphere. It accounts for the 75 per cent
weight of the atmosphere and contains almost all the moisture and dust particles present in the
atmosphere.
The atmosphere contains moisture in the form of water vapor. Water vapor is less dense than dry
air and consequently, humid air (air containing more water vapor) is less dense than dry air.
Because of this, a plane's takeoff roll will be longer, its rate of climb slower, and its landing speed
higher in humid air than in denser dry air. Further, forms of precipitation such as icing on aircraft
wings, zero visibility in fog or snow, and physical damage caused by hail all affect aircraft
performance.
All types of weather phenomena such as dust storms, clouds, rain, fog, hailstorms and snowfall etc.
take place in this part of the atmosphere. It is characterised by vigorous or strong mixing. Another
important phenomena of this part is the decrease of temperature with height, which is approximately
6.5°C/km in lower layers and 6.7°C/km in upper layers. This phenomenon is known as lapse rate of
temperature.
All these weather conditions affect flight. It is the responsibility of the pilot to obtain as much
information as possible to ensure a safe, efficient flight.
The top of the troposphere is known as tropopause. The average height of tropopause is 12 km.
Tropopause is also known as isothermal layer. Its height varies from equator towards poles. The
higher the temperature of the lower layers, the higher is the tropopause.
Weather conditions that occur within the troposphere affect flight and are carefully studied partly
for that reason. Conditions such as wind, temperature, water in the atmosphere, atmospheric
pressure, and turbulence all affect the way an aircraft flies.
The height also varies with the pressure at sea level. The height of the tropopause at the equator is
about 16 km, whereas at poles it is about 8 km. Its temperature at the top over the equator varies
from -75 to -85°C, whereas at poles it is about -55 to -60°C.
Atmospheric pressure is the pressure exerted on an area by the weight of air above the surface of
the land. On a certain plane, low pressure areas are those with less atmospheric mass above them
while high pressure areas are those with a higher atmospheric mass above them. Atmospheric
pressure decreases with increase in altitude. This is because as altitude increases, there is lesser
and lesser overlying atmospheric mass, hence less atmospheric pressure.
This is the most turbulent part of the atmosphere. Convective currents and strong mixing (latent
mixing) takes place in this zone. Wind speed generally increases with height. It is maximum at the
top of the troposphere.
Like any other material object the air also has weight. The pressure of air at a given place is
defined as a force exerted in all directions in consequence of the weight of all the air above it.
Thus, the mass of a column of air above a given point determines the atmospheric pressure at that
point.
The centre or core of strong wind is known as jet stream. Wind speed of jet stream ranges from 80 to
400 km/hr.
Between the tropical and polar tropopause, there exists a sloping middle latitude tropopause in the
vicinity of the jet stream in both the hemispheres.
This is so because the higher we go, the thinner the atmosphere becomes, and thus the molecules
are more diffused, and there is less pressure because inter-molecular space is greater. At the height
of Mt Everest, the air pressure is about two-thirds less than what it is at the sea-level.
Multiple or overlap tropopause occur in the middle latitudes. The moving weather systems i.e. air
masses and the associated cloud systems are confined to the troposphere.
As these systems change their position, the characteristics of the tropopause vary in space and time.
Why is it that a climber on one mountaintop may have more difficulty breathing than a second
climber at a similar altitude in another part of the world? This illustrated essay from NOVA
explains how atmospheric pressure relates to the amount of oxygen in the air. It also describes the
influence of environmental factors such as altitude and temperature on oxygen levels and
atmospheric pressure.
The height of the tropopause is greater at the equator. Therefore, the lowest temperature in the
atmosphere are found right over the equatorial tropopause.
ii. Stratosphere:
This is the region next to troposphere. Its height extends from about 15 to 50 or 55km.
So, according to the gas law, an increase in either density or temperature will cause an increase in
pressure provided the other variable (density or temperature) remains constant.
There is a layer of uniform temperature from 15-20 km above the top of the troposphere and beyond
that the temperature increases. The top of the stratosphere is known as stratopause.
Stratosphere is the layer of high temperature and the temperature is higher than that at the earth
surface.
Winds are a natural motion of the air parallel to the Earth's surface caused by the uneven heating
and cooling of the Earth and atmosphere. Air that is heated rises because the heat applied to air
decreases the air's density to the point where it is lighter in weight than the surrounding cooler air.
Air at higher altitudes also exerts less atmospheric pressure because fewer air molecules are
present and because of the lesser effect of gravity. (Atmospheric pressure is the force exerted by
the air over a specified area.) When less dense air rises, it displaces the cooler, denser air, which
moves horizontally to fill the lower pressure area created. This horizontal motion is wind. Motion
in a vertical or nearly vertical direction is called a current.
It contains very little dust and moisture except that which is sent into this part by the volcanic
eruptions. The temperature in the lower layers of the stratosphere remains constant up to 15-20 km,
which is about -60°C, sometimes this layer is also called isothermal layer.
Air pressure, therefore, is defined as the force exerted against a surface by continuous collision of
gas molecules. The amount of pressure exerted by air at a particular point is determined by two
factors, namely, temperature and density.
The important characteristics of the stratosphere are:
a. This region is almost free from clouds, dust particles and water vapours. Sometimes clouds are
seen in this layer at night, called pearl clouds or noctilucent clouds.
Looking at Earth From Space: Glossary of Terms. National Aeronautics and Space
Administration. Office of Mission to Planet Earth. August 1994.
b. The lower part of this zone has isothermal conditions.
c. There is a slight increase in temperature with height and it continues up to about 50 km.
Isothermal layer expands up to 25 or 35 km in some cases. After that temperature starts increasing
and maximum temperature of 290°K is found at 50 km height, which is higher than that at the
earth’s surface.
Since air pressure decreases with increasing altitude, at a certain height this balance between
outward pressure exerted by the air inside human body and inward pressure exerted by the
atmosphere outside is disturbed resulting in marked physiological disturbances, such as nose
bleed, ear bleed, etc.
d. It contains most of the ozone of the atmosphere, which acts as a shield for the ultra violet rays of
the sun.
The top of the stratosphere is called stratopause. It is a layer of high temperature where the
temperature is even higher than that at the earth’s surface.
Similarly, our atmosphere may be deemed to be a closed container bounded by the earth's land-sea
surface from below, and from above by the force of gravity that does not allow it to escape to
outer space.
Thus, base of the stratosphere is colder than the top of the stratosphere.
iii. Mesosphere:
Mesosphere means the middle part of the atmosphere. It extends from 50-85 km from the surface of
earth. The temperature ceases to rise and stratopause marks the boundary of the lower limit of
mesosphere. From this level, the temperature starts decreasing and becomes about -80°C at 87 km
height.
This plastic bottle was sealed at approximately 14,000 feet altitude, and was crushed by the increase
in atmospheric pressure (at 9,000 feet and 1,000 feet) as it was brought down towards sea level.
This level is the top of the mesosphere and is called mesopause. The composition of the gases is
homogeneous. Lowest temperature of the atmosphere is found at the top of the mesosphere. The
pressure is very low decreasing from 1 mb at 50 km to 0.01 mb at 80 km.
The lower part of the mesosphere is warmer than the upper part. The temperature decreases with
height from the base of the mesosphere. As a result, convection currents are set up, which carry
traces of water vapours in the upward direction.
In this figure, the pilot is attempting to fly from point A to point B, but winds blowing crosswise
to his intended flight path push him to point C. The pilot should have pointed the aircraft slightly
into the wind, which would have canceled out any drifting off course.
These water vapours condense on the surface of meteoric dust particles to form ice crystals. That’s
why,noctilucent clouds are seen in the mesopause or at the top of the mesosphere over high latitudes
during summer season.
Next to Stratosphere. It has cold temperature and low atmospheric pressure.
The atmosphere exerts a pressure of 1034 grams per square cm (14.7 lb per square inch) at sea
level. This amount of pressure is exerted by the atmosphere at sea level on all animals, plants,
rocks, etc.
It extends up to 85 km. In this region the temperature decreases with height i.e., it exhibits a positive
lapse rate. This is due to relatively low levels of ozone and other species that can absorb U.V.
radiation from the sun. The temperature at the top of the Mesosphere reaches about -92°C. It
contains N2,O2 O2+, NO+.
iv.Thermosphere:
Thermo means hot or warm. Thermosphere is found above the mesopause.
The higher the altitude, the thinner the air is, and the lower the atmospheric pressure is. This is
because high places do not have as much air above them, pushing down.
It extends from 90 km to about 600 km. Initially the temperature increases slowly and later on it
increases rapidly due to the absorption of short wave radiation. The temperature is very high in this
region.
A low pressure system is a region where the atmospheric pressure that is at sea level is below that
of the adjacent regions. A low pressure system is generally associated with cloudy, rainy or snowy
weather. Areas around the world that experience low pressure systems include Tibetan Plateau and
the lee of the Rocky Mountains.
At about 350 km, the temperature is as high as 1600°C. This region is also called chemosphere as
most of the chemical reactions take place in this region.
In this part, there is less tendency of the gases to mix with heavier molecules and atoms settle down
due to gravity.
Air is an admixture of several gases. In order to clearly understand the concept of air pressure, the
behaviour of gases and the principle that governs this behaviour must be grasped.
Height of the thermosphere fluctuates with the conditions prevailing on the surface of the sun. If the
sun is calm, height of thermosphere is about 400 km and if the sun is active, then the height of
thermosphere is 500 km or above.
Another important phenomena occurring in this sphere is the ionisation of the air molecules and the
atoms. Where these phenomena take place, that part is known as ionosphere.
v. Exosphere:
It is the outermost layer of the earth’s atmosphere which lies beyond 600 km above the earth
surface.
A high pressure system is a region where the atmospheric pressure nthat is at sea level is higher
than that of the adjacent regions. High pressure systems are generally associated with plenty of
sunshine, low cloud cover and lower rainfall amounts. An area that usually experiences high
pressure system is Tonsontsengel in Mongolia. In fact, it has the highest pressure ever recorded on
Earth (1085.7 hectopascals).
At this height, density of the atmosphere is extremely low. Hydrogen and helium gases predominate
in this region. It extends to a height of 1600 km. It has high temperature.
All the layers form the total blanket of air in the biosphere, which is that part of lithosphere and
atmosphere in which living organisms live together and interact with one another. Oxygen in the
troposphere plays an important role in the processes taking place on earth’s surface.
vi. Ozonosphere:
The rise in temperature in the stratosphere is also associated with the presence of ozone, which is
found between 12-50 km, but its maximum concentration is at 25 km which is approximately 10
times more than that at sea level ozone (007ppm).
A typical statistical maximum wind speed curve.
This region is also called ozonosphere. The ozone absorbs UV radiations, as a result the temperature
increases.
Thus the base of the stratosphere is colder than the top of the stratosphere. Convective activities are
almost absent in this part. Clouds are also absent. Sometimes noctilucent clouds can be seen
between 20-30 km.
vii. Ionosphere:
This layer is not so important for the purpose of meteorology, but it is important for
radio-communication. It extends from 50-600 km and in some cases the ionised matter is found up
to greater heights about 1000km.
The mountaineers have to face a lot of difficulty because of the much reduced atmospheric
pressure at greater heights.
It covers both mesosphere and thermosphere.
Conditions required for ionisation are:
a. Air should be sufficiently thin for the ionisation of various gases.
b. Air pressure should be very low.
c. Ionisation starts when there are sufficient free electrons.
All these conditions are fulfilled in this zone. In this zone, there are different layers in which radio
waves are reflected back to the earth surface. In the ionosphere air particles are electrically charged
(ionised) by the sun’s ultra-violet radiation and congregate in four layers: D, E, F1 and F2.
The lower most layer is known as D-layer, which extends from 50-80 km. Second layer is the E
layer, which extends from 80-120 km. Third layer is the F1 layer, which extends from 120-180 km
and fourth layer is the F2 layer, which extends from 180-300 km and above.
These layers reflect radio waves back to the ground and are very useful in the communication
network of earth or the globe. The temperature increases rapidly in this layer and may reach as high
as 1600°C at about 350km.
Essay # 7. Chemical and Photochemical Reactions in Atmosphere:
The various chemical and photochemical reactions taking place in the atmosphere depend upon the
temperature, composition, humidity and intensity of sun light. Photochemical reaction take place in
the atmosphere by the absorption of solar radiation in the ultra violet region. Absorption of photons
by chemical species gives rise to electronically excited molecules which can bring about certain
reactions.
The electronically excited molecules may undergo any of the following changes:
(i) Reaction with other molecules on collision
(ii) Polymerization
(iii) Internal arrangement
(iv) Dissociation
(v) De-excitation by fluorescence of deactivation to return to the original state.
Any of the first four changes may serve as an initiating chemical steps in a primary process.
A thunderstorm is the most violent of all turbulences. In a thunderstorm strong updrafts and
downdrafts exist side by side. The severity of the aircraft motion caused by the turbulence will
depend upon the magnitude of the updrafts and downdrafts and their directions. Many private
aircraft have been lost to thunderstorm turbulence because of structural failure or loss of control.
Commercial airliners generally fly around such storms for the comfort and safety of their
passengers.
The three steps involved in an overall photochemical reaction as absorption of radiation.
a. Primary reaction b. Secondary reaction c. Chemical species are NO2, SO2, HNO3, N2, ketones,
H2O2, organic peroxides, aeroles.
Essay # 8. Role of Atmospheric Air in Agriculture:
i.
The reason that man does not feel the weight of atmosphere falling on his shoulders is that the air
inside him exerts an equal amount of outward pressure balancing the inward pressure of
atmosphere.
Air mass near the earth surface contains enormous amount of water vapours which play an
important role in causing rainfall. Rainfed crops are at the mercy of rainfall amount and its
distribution during its life cycle.
ii. Water vapours of the air have the capability to absorb outgoing radiation from the earth, and
provides warmth to the crop plants during night. This phenomenon is more common during winter
season in north India when the night time temperature may decrease to freezing level. Those crops
which are susceptible to low temperature injury can suffer from injuries.
iii. The increased amount of water vapour in the air carries sensible heat and also reflect outgoing
long wave radiation back to the earth. As a result, the air temperature rises providing protection to
the crops. The rise of air temperature is more pronounced during cloudy night in winter season.
Nitrogen (N2):
It is odourless, tasteless and colourless gas. It is relatively inactive chemically, though many of its
compounds are active. Nitrogen is an important component of the organic compounds. Animals can
not directly utilise inorganic nitrogen in producing proteins. For this, they have to depend on the
plants.
On the other hand, plants are not able to use atmospheric nitrogen directly, but can take nitrogen
from nitrates and other nitrogenous compounds. Most of the nitrogen needed by plants comes from
decaying vegetable matter (humus), from nitrogen fixing bacteria and from nitrate containing
fertilisers.
Oxygen (O2):
It is also colourless, tasteless and odourless gas. It is highly reactive chemically and is capable of
combining with all the elements except inert gases. Oxygen is essential for the respiration of the
animals. Combustion is not possible without oxygen.
The atmosphere has an appreciable amount of oxygen. Oxygen which exists in the gaseous form,
represents only a part of the total oxygen stored in the earth-atmosphere system. Animals and plants
store oxygen as a component of organic molecules during lives, while in the rocks of the
lithosphere, it is bound into chemical compounds such as oxides and carbonates.
The level of oxygen in the atmosphere will remain fairly constant as long as the oxygen used by
living beings is returned to the atmosphere in equal amounts by photosynthetic activity of the plants.
However, some imbalance might have caused by anthropogenic factors.
Large scale burning of fossil fuels remove oxygen from the environment, which is not easily
replenished as human beings destroy large amounts of forest cover which easily copes with CO2
increase and oxygen reduction.
Both nitrogen and oxygen are almost transparent to the incoming radiation from the sun and also to
the outgoing radiation from the earth and the atmosphere, but it does absorb short wave ultraviolet
and X -ray radiation at high level in the atmosphere. The absorption of larger ultraviolet radiation
splits two atoms of oxygen associated in the molecule of oxygen into single atoms.
Oxygen occurs throughout the lowest 120 km of the atmosphere. It exists mainly as molecular
oxygen (O2) below 60 km, while above this, atomic oxygen is more prevalent, which is generated
by the effects of cosmic radiation on the oxygen molecules.
Carbon dioxide (CO2):
Although it is present in small amount yet it is very important. It differs slightly in amount from
place to place. Over the sea, it is slightly greater than over vegetation. Over large cities, it may rise
to 0.04% and in a closed room it may rise to 1%, if the ventilation is poor.
It supplies all the carbon for growth of plants and every substance that is obtained from the plants
directly or indirectly contains carbon. This carbon is obtained from the CO2 of the atmosphere.
Carbon dioxide absorbs radiation, therefore it is considered to be of great climatic significance. The
concentration of CO2 is increasing due to air pollution. More CO2 in the atmosphere means more
heat absorption. Increased CO2 is likely to increase crop production but temperature is also
increasing.
Increasing temperature is harmful for the crop production. It has been found that higher
concentration of CO2 may compensate increase in air temperature by 1°C.
Ozone (O3):
Generally ozone is found between 10 to 50 km, however, most of the ozone is concentrated between
15 and 30 km above the earth surface and it has a maximum concentration at 22 km. Its amount is
hardly 0.07 ppm. It has a tremendous importance to human life because of its high absorption to UV
radiation. Without it, the intensity of UV radiation at earth would have increased.
Under such intensity, the eyes of animals would not have developed. On the other hand, if the
amount of ozone is increased by some amount, UV radiation reaching the earth would decrease so
much that the production of vitamin D would cease.
Ozone is produced by irradiation of O2 molecules by UV radiation in the region between 10 to 50
km. This irradiation produces O atoms. These oxygen atoms react with oxygen molecules to
produce ozone.
O2 + UV radiation = O + O
O2 + O = O3
No ozone is produced above 50 km, as there the molecular oxygen is very less for necessary
collision with oxygen atom. Below 10 km, no ozone is produced because necessary amount of UV
radiation is not available for the dissociation of oxygen molecule.
Ozone in the Upper Atmosphere:
Ozone (O3) is found mostly in the stratosphere. It is produced by gaseous chemical reaction in the
stratosphere. An oxygen molecule (O2) absorbs ultraviolet radiation and splits into two oxygen
atoms (O). A free oxygen atom (O) combines with an oxygen molecule to form ozone (O3).
Similarly O3 can be destroyed.
Like O2, ozone absorbs ultraviolet radiation and it splits to form O2 and O. Also two oxygen atoms
(O) can join to form O2. The net effect is that ozone (O3), molecular oxygen (O2) and atomic
oxygen (O) are constantly formed, destroyed and reformed in the ozone layer, absorbing ultraviolet
radiation with each transformation.
The absorption of ultraviolet radiation by the ozone layer protects the earth’s surface from this
damaging form of radiation. If the concentration of ozone is reduced, transformations among O, O2
and O3 are reduced. Therefore, the absorption of ultraviolet radiation is reduced.
Under this situation, ultraviolet radiation would reach the earth’s surface at full intensity. As a
result, all bacteria exposed on the earth surface would be destroyed and animal tissues would be
damaged. Thus, the presence of ozone layer is an essential protection in maintaining a viable
environment for life on the earth.
Threat to the Ozone Layer:
A serious threat to the ozone layer is posed by the release of chlorofluorocarbons (CFC’s) from the
earth surface into the atmosphere. Chlorofluorocarbons are widely used as cooling fluids in the
refrigeration system, when these appliances are disposed off, their CFC’s are released into the air.
Molecules of CFC’s in the atmosphere are very stable close to the earth.
They move upward by diffusion without chemical change until they reach the ozone layer. Chlorine
oxide molecules are formed under the influence of ultraviolet radiation. Chlorine oxide molecules
convert the ozone molecules into oxygen by chain reaction. In this way O3 concentration in the
stratosphere is reduced, therefore, fewer molecules are left to absorb ultraviolet radiation.
There are other gaseous molecules like nitrogen oxides, bromine oxides and hydrogen oxides which
can reduce the concentration of ozone in the stratosphere. Volcanic dust in the stratosphere can also
reduce the ozone concentration. The reduction in ozone concentration can affect the earth. The
reduction of ozone may increase the incidence of skin cancer in human beings. It may reduce the
crop yield.
Ozone Hole:
A hole in the ozone layer was discovered over Antarctica in mid-1980’s. Here, seasonal thinning of
the ozone layer occurs during the early spring of the southern hemisphere and ozone reaches
minimum during the month of October.
Thinning occurs after the formation of a polar vortex in the stratosphere during winter period. This
vast whirlpool of wind traps the air it contains and keeps this air out of the sun during the months of
long polar night. The air in the vortex becomes very cold and clouds containing ice crystals and
other water containing compounds form within it.
The crystals provide the surface where chemical reactions can take place. These reactions can
convert chlorine into stable chlorine oxide (CIO) which is highly reactive in the presence of
sunlight. As the southern hemisphere spring approaches, the polar vortex is illuminated by the sun
and the chlorine oxide reacts with ozone. As a result, ozone concentration is reduced and ozone hole
is formed.
A polar vortex is also formed in the northern hemisphere but it is much weaker as compared to
southern hemisphere and is less stable. As a result, no early-spring ozone hole is observed in the
arctic. The studies have indicated that ozone depletion is less in the mid latitude as compared to the
polar region.
As the global ozone is thinning, the rate of incoming ultraviolet radiation is likely to increase.
Scientists have estimated that with each one per cent decrease in global ozone, ultraviolet radiation
should increase by two per cent.
Essay # 9. Solid Particles in Atmosphere:
There are two types of solid particles in atmosphere: organic and inorganic.
Organic particles:
The number of organic particles in the air is comparatively small. They are spores of plants and
bacteria. In the air over cities, their number is more as compared to the open country. Over the
oceans their number is 1/m3, whereas over the crowded cities their number is 3000/m3.
Inorganic particles:
The number of inorganic particles is more than the organic particles. Inorganic particles are mostly
dust particles.
Their presence in the atmosphere is due to following reasons:
(a) They are raised to the atmosphere from earth surface by strong winds.
(b) They are thrown up into the atmosphere by volcanoes.
(c) They are passed into the atmosphere as smoke by combustion of fuel.
(d) Meteors while passing into the atmosphere get disintegrated adding large quantities of dust
particles in the upper atmosphere.
Dust particles are mostly concentrated in the lower layers of the atmosphere. The amount of dust
particles is more in the sub-tropical and mid latitudes than in the equatorial region. Dust particles
can be hygroscopic particles which include ammonia and salt, others are non-hygroscopic particles
which include sand and mica.
Their number is very small over oceans and on mountains but very large over big cities. Over cities
their number is 1,00,000/m3, whereas over oceans they range between 500 to 2000/m3.
These particles play an important role in various meteorological phenomena:
(a) They are the chief cause of haze in dry & warm air weather.
(b) They form condensation nuclei on which water vapours condense to form fog and cloud.
(c) Red colour of the sky at the time of sun-rise and sun-set is due to these particles which intercept
and reflect radiation.
Water vapours:
The amount of water vapours is very small in the atmosphere. It never exceeds 4 per cent by volume
and even in tropical regions it rarely exceeds 1 per cent. It is mainly confined to the lower layers of
the atmosphere. It is estimated that 90 per cent of the moisture lies below 6 km and less than 1 per
cent above 10 km.
Though its amount is very small, it plays an important role. There would have been no plant or
animal life in the absence of water vapours. All the phenomena of dew, fog, clouds, rain, hail, snow
and frost are due to water vapours. On condensation, they release latent heat of condensation which
is the driving force behind all storms. The water holding capacity of the air is directly proportional
to the air temperature.
Vertical Distribution of Temperature and Pressure:
Atmosphere is not a homogeneous mass but is heterogeneous. If we proceed upwards from the earth
surface, different properties of the atmosphere are encountered. There are large variations in terms
of temperature, density, constituents etc. from the surface of earth to the top of the atmosphere.
Essay # 10. Atmospheric Pressure:
It is the weight of the air column extending from earth’s surface to the top of the atmosphere falling
on a unit area. It is expressed in millibars (mb).
1 mb = 013 dynes/cm2 = 100 pascals or 1 hectapascal
Air is highly compressible. It has greatest density near the surface of the earth because it is
compressed under the weight of upper air columns. Pressure is directly related with the density of
air. Hence, Pressure decreases with increase in altitude and rate of decrease depends upon the
density of air.
The air pressure depends upon the following factors:
i. Altitude of the place (height):
Pressure decreases with increase in altitude but the rate of decrease depends upon the density of air.
Higher the density of air, higher is the rate of decrease of air pressure. Hence, the decrease is rapid
in the lower layers (where the density is high) and very slow in the upper layers (where the density
is low).
ii. Temperature:
Pressure gradient also depends upon the changes in temperature. There are large variations in
temperature in different layers of the atmosphere. In the lower layers i.e. up to about 10 km
(troposphere) the temperature decreases steadily with height.
At about 10 km, the temperature decreases to around -60°C. This level is called tropopause. There
are small variations in the temperature and it remains constant from about 10 km to 20 km.
After that it increases rapidly and continues up to 50 km, where the temperature is slightly more
than that at the surface. After 50 km height, the temperature remains constant. Later on, there is
again a decrease in temperature, it falls to -80°C at about a height of 87 km. From this layer onwards
the temperature again starts increasing (thermosphere) and may reach as high as 1600°C at an
altitude of 350 km.
iii. Presence of water vapours:
Pressure also depends upon the presence of water vapours. The pressure at sea level is about
1000mb. At 50 km height, it is about 1mb. At 100 km, the pressure is about 1/1,000,000 part of the
sea level pressure.
iv. Density:
The decrease in air density along the vertical is closely associated with the pressure curve. Some
idea of density variation is obtained by comparing the mean free path of air molecules. At sea level,
the mean free path is 1/7,500,000 cm. At 65 km, it is about 1/500 cm. At 100 km, it is little over 2.5
cm and at 320 km, it is about 1500 cm.
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