LECTURE 1 : ATMOSPHERE - STRUCTURE AND COMPOSITION
The atmosphere has a direct influence on the climate on the
surface of the earth. It is thus important to start with this
topic to find out which part or parts of the atmosphere that
have greater influence.
The atmosphere is not one continuous homogenous layer with
constant properties. Taking the temperature characteristic
alone, the atmosphere can be divided into several layers
(Figure 1.1), each layer is defined by it temperature gradient
with height.
The lowest layer of the atmosphere is the troposphere within
which temperature declines with altitude. This rate of decline
is termed the environmental lapse rate and its average value is
about 6.5 0 C per km. This layer is capped at about 12 km by the
tropopause. This height is not fixed but goes higher in the
tropics (16 km at the equator) but lower in the polar regions
(6 km at the poles). This inversion layer also acts as a lid
which effectively limits convection. This is the most important
layer as far as its influence on the climate at the earth's
surface is concerned. Here 75 per cent of the total molecular
gases and almost all of the water vapour and storms that affect
daily weather on earth are found.
Above this layer are the stratosphere, mesosphere and
thermosphere or ionosphere each of the lower two is capped by a
shallow transitional layer called stratopause and mesopause
respectively. The stratosphere extends from the tropopause to
about 50 km and contains much of atmospheric ozone (peak
density at 22 km). Here ozone acts as an important shield to
ultraviolet rays which otherwise would reach the earth's
surface direct. (Bear this in mind when we talk about ozone
depletion and climatic change later on). Because of this effect
of ozone this layer shows an inversion with low temperatures
(in winter at - 80 0C) at the equatorial tropopause but warmer
temperatures of over 0 0C are at the upper levels due to
absorption of ultraviolet radiation by ozone.
The mesosphere, however, behaves like the stratosphere with
temperature decreasing with height with a minimum of - 95 0C at
about 80 km. At the mesopause noctilucent clouds are observed
in summer in the mid latitudes.
The thermosphere shows very low densities and temperatures
rises with height due to absorption of ultra violet radiation
by atomic oxygen. Temperatures reach 1200 0 K at 350 km. Above
100 km solar x-rays and ultra violet radiation cause
ionization, or electrical charging, which separates negatively
charged electrons from oxygen atoms and nitrogen molecules. The
penetration of the ionizing particles through the atmosphere
from about 300 km to 80 km, between 20-250 latitude from the
earth's magnetic poles bring about the phenomenon of auroras Aurora Borealis and Aurora Australis. This layer is also
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important from the view of radio transmission as it reflects
radio signals.
each layer.
Atmospheric composition
The atmosphere is a mixture of different gases but two elements
nitrogen and oxygen alone account for 99 % of the volume of
clean dry air but they are of little significance in
influencing weather and climate. The principal gases are shown
in Table 1.1. Apart from these gases, the atmosphere contains
water vapour, ozone and dust particles. Though these are
present in small amounts, climatically they are very important.
Water vapour is most variable but globally the average amount
in the atmosphere would amount to about 2 cm of water. In the
humid tropics it may be about 4 % by volume, but at the poles
it may be less than 1 %.
Water vapour is most important in affecting climate as it
accounts for the precipitation received by the earth's surface.
In cloud form it affects the radiation balance by absorbing
both incoming solar and terrestrial radiation. This affects the
energy balance of the atmosphere as well as that of the earth's
surface. As water exists in three forms, the change in state is
accompanied by either a release or absorption of heat.
The atmosphere contains many dust particles which may have
their origins from sea sprays, volcanic dust, dust from the
ground and smoke from burning (natural causes). These are also
significant as they affect radiation and act as condensation
nuclei for precipitation formation.
Carbon dioxide and ozone, although present in very small
quantities are highly significant meteorological elements.
Carbon dioxide always features in discussion of global warming
and green house effect. These will be discussed in a later
section
on
climatic
change.
Ozone,
however,
which
is
concentrated at about 10-50 km above the earth's surface plays
an important role in absorbing ultraviolet radiation from the
sun. Because of ozone in this layer, the earth is habitable.
Like CO2, ozone features significantly in any discussion of
global climatic change as a result of the thinning of ozone
layer or expansion of ozone hole over the Antarctic.
Human activities, combustion of hydrocarbons, and the growth of
cities generally while on the one hand have added significant
amounts of other gases and pollutants into the atmosphere as
well as CO2, they also, on the other cause rapid depletion of
ozone through the extensive use of chloro-flourocarbons (CFCs)
Maintaining gas balance
In the natural state and over the whole earth, the amounts of
the various gases by volume in the atmosphere must remain
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constant over time. Otherwise, the consequences would be too
frightening to envisage should the gases either increase or
decrease indefinitely over time. Already, an increase of CO2
since the turn of the century has been a cause of concern.
Thus, in order to understand how these gases are maintained in
balance, the following section shows three examples of gases
and their cycles.
The Carbon Cycle
Carbon compounds are basic to all life forms and they are found
in any of these two states - as organic carbon or carbon
dioxide. But as CO2 in the atmosphere it accounts for very small
percentage. The rest of CO2 is in dissolved form in oceans which
account for more than 100 times than that found in the
atmosphere.
All other carbon compounds are in the form of carbonates which
readily interchange in chemical reaction and CO2 is produced.
For a further understanding of this gas balance, reference must
be made to the production and consumption of CO2 as well as its
storage.
Production of CO2 on land comes from volcanic eruption,
respiration of plants, animals, and organic decomposition. In
oceans, there is constant interchange of CO2 gas between the
atmosphere and the oceans and this takes place by molecular
diffusion. Deep ocean layers contain much greater dissolved CO2
because the solubility of CO2 increases with pressure.
Consumption of CO2 takes place by the following activities photosynthesis where CO2 is reduced to organic carbon and the
production of carbonates which will be stored in the oceans or
on land. The organic carbon storage within fossil fuels is
double the total amount of organic carbon contained within
active pools (Table 1.2).
The movements of carbon through the carbon cycle (Figure 1.2)
begins with the fixation and reduction of carbon by plants,
expressed as the net primary productivity. All of this fixed
carbon will ultimately be consumed by soil bacteria and will
eventually return to the atmosphere for recycling as CO2. In the
oceans, the total net primary production is higher, one and a
half times that on land. Most of the organic matter is rapidly
reoxidised to CO2 by heterotroph bacteria and returned to the
surface layers, but some move to the deep ocean floor where it
is oxidised slowly.
Increment of carbon in active pools is brought about by
precipitation and accumulation of sedimentary deposits, which
eventually become solid carbonate rocks (lithification) and
hydrocarbons or fossil fuels. These are naturally returned to
the surface through tectonic activities, dissolved through
chemical weathering and CO2 is released to the atmosphere. Human
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activities through mining and petroleum production brings such
materials to the surface and through combustion releases CO2
into the air.
The Oxygen Cycle
Molecular oxygen (O2), carbon dioxide (CO2) and water (H2O) are
the three inorganic forms of oxygen most important to life
processes. The greatest amount of oxygen is found in the form
of water in the oceans, followed by molecular O2 in the
atmosphere, fresh water, dissolved CO2, and in soil and ground
water. Only a small proportion of oxygen is found in living and
dead organic matter. In storage pools, the largest amount is in
rocks as oxides. Production of O2 is through photosynthesis,
followed by chemical reactions of reduction, and lithification
of sediments. Consumption however, naturally takes place in
respiration and biological oxidation, as well as chemical
oxidation of silicate materials exposed at the surface (Figure
1.3).
Human activities result in reducing O2 in the air through:
a.
b.
removal of oxygen by fossil fuel combustion
clearing and draining land, speeding chemical oxidation of
soils and soil organic matter
reducing photosynthesis by large scale clearing of forest
for agriculture, housing, etc
c.
However, as the oxygen pool (Table 1.3) is so large, the
potential impact of human activities on oxygen amount in the
air is small.
1.3.3
Nitrogen Cycle
Please read up on this cycle.
What lessons can we learn from the above?

The layer that affects climate most is the lowest 10 km from
the earth's surface

There are major gases - N2 and O2

There are active minor ones - H2O, CO2, O3, CH4, N2o, CFCs,
liquid ice H2O, aerosols

There are inactive minor gases - Ar, Ne, He, Kr

Large O2 inventory for life and photosynthesis - unique to
Earth
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
Radiatively active gases control the Earth's temperature

Gases however small in volume are important to climate and
life on Earth. Some gases are naturally produced while
others are produced artificially
Discussion
1. Why is CO2 important to climate? Isn't an increase in CO2
good for plant growth and therefore food crop production?
2. What is the significance of Ozone? How is O3 broken down to
result in ozone depletion?
3. How are CFCs produced and what impact do they exert on the
earth's climate?
Reading materials: Read any of these chapters
1.
2.
3.
4.
Strahler, A.H. and A.N. Strahler (4thed),1992. Modern
Physical Geography. John Wiley, Chapter 2.
McKnight,
T.L.1996.
Physical
Geography:
A
Landscape
Appreciation. Prentice Hall. Chapter 3.
Christopherson,
R.W. (2nd Ed), 1994. Geosystems: An
Introduction to Physical Geography. MacMillan, Chapter 3.
Moran, J.M. and M.D. Morgan, 1995. Essentials of Weather.
Prentice Hall, Chapter 1.
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