Atmosphere is a mixture of gases surrounding
any celestial object for example Earth. It has a gravitational
field strong enough to prevent the gases from escaping to
outer space. The principal constituents of the atmosphere
of the Earth are nitrogen which is 78 per cent and oxygen
which is 21 per cent. The atmospheric gases in the remaining
1 per cent are argon which is 0.9 per cent, carbon dioxide
which is 0.03 per cent, some varying amounts of water vapor
and even trace amounts of hydrogen, ozone, methane, carbon
monoxide, helium, neon, krypton, and xenon.
The mixture of gases in the air today has had over 4.5
billion years in which to evolve. The earliest atmosphere
must have consisted of volcanic emanations mainly. Gases
that erupt from volcanoes today are basically a mixture
of water vapor, carbon dioxide, sulphur dioxide, and nitrogen,
it has almost no oxygen. If this is the same mixture that
existed in the early atmosphere then various processes would
have had to operate to produce the mixture we have today.
One of such processes was condensation. As the air cooled
then much of the volcanic water vapour condensed to fill
the earliest oceans. Chemical reactions would also have
occurred on the earth. Some carbon dioxide would have reacted
with the rocks of the Earth's crust to form carbonate minerals
and even some would have become dissolved in the new oceans.
Later, as primitive life was capable of photosynthesis evolved
in the oceans, new marine organisms began producing oxygen.
Almost all the free oxygen in the air today is believed
to have formed by photosynthetic combination of carbon dioxide
with water. This was the way oxygen was produced. About
570 million years ago the oxygen content of the atmosphere
and oceans became high enough to permit marine life capable
of respiration, hence they prospered. Later, some 400 million
years ago, the atmosphere contained enough oxygen for the
evolution of air-breathing of land animals.
The water-vapour content of the air varies considerably,
depending on the temperature and relative humidity. With
100 per cent relative humidity the water-vapour content
of air varies from 190 parts per million (ppm) at -40°
C (-40° F) to 42,000 ppm at 30° C (86° F). Minute
quantities of other gases, such as ammonia, hydrogen sulphide,
and oxides of sulphur and nitrogen, are temporary constituents
of the atmosphere. Oxides and other pollutants added to
the atmosphere by factories and vehicles have become a major
concern only because of their damaging effects in the form
of acid rain. In addition, the strong possibility exists
that the steady increase in atmospheric carbon dioxide,
mainly as the result of fossil-fuel combustion over the
past century, may affect the Earth's climate through the
process known as the greenhouse effect.
Similar concerns are posed by the sharp increase in atmospheric
methane. Methane levels have risen 11 per cent since 1978.
About 80 per cent of the gas is produced by decomposition
in rice paddies, swamps and even the intestines of grazing
animals. Besides adding to the greenhouse effect, methane
reduces the volume of atmospheric hydroxyl ions, thereby
impairing the atmosphere's ability to cleanse itself of
pollutants. This can be very harmful.
The study of air samples shows that up to at least 88 km
(55 mi) above sea level the composition of the atmosphere
is substantially the same as at ground level; the continuous
stirring produced by atmospheric currents counteracts the
tendency of the heavier gases to settle below the lighter
ones. In the lower atmosphere a gas called ozone which is
a form of oxygen with three atoms in each molecule, is normally
present in extremely low concentrations. The layer of atmosphere
from 19 to 48 km up contains more ozone, produced by the
action of ultraviolet radiation from the Sun. Even in this
layer the percentage of ozone is only 0.001 by volume. Atmospheric
disturbances and downdrafts carry varying amounts of this
ozone to the surface of the Earth. Human activity adds to
ozone in the lower atmosphere, where it becomes a pollutant
that can cause extensive damage to crops .
The ozone layer became a subject of concern in the early
1970s when it was found that chemicals known as chlorofluorocarbons
(CFCs), or chlorofluoromethanes, were rising into the atmosphere
in large quantities because of their use as refrigerants
and as propellants in aerosol dispensers. The concern centred
on the possibility that these compounds, through the action
of sunlight, could photochemically attack and destroy stratospheric
ozone, which protects the Earth's surface from excessive
ultraviolet radiation. As a result industries in industrialized
countries have tried to replace chlorofluorocarbons in all.
The atmosphere may be divided into several layers. In the
lowest one, the troposphere, the temperature as a rule decreases
upwards at the rate of 5.5° C per 1,000 m .This is the
layer in which most clouds occur. The troposphere extends
up to about 16 km in tropical regions (to a temperature
of about -79° C, or -110° F) and to about 9.7 km
in temperate latitudes (to a temperature of about -51°
C, or -60° F). Above the troposphere is the stratosphere.
In the lower stratosphere the temperature is practically
constant or increases slightly with altitude mainly over
tropical regions. Within the ozone layer the temperature
rises more rapidly, and the temperature at the upper boundary
of the stratosphere, almost 50 km above sea level, is about
the same as the temperature at the surface of the Earth.
The layer from 50 to 80 km is called the mesosphere and
is characterized by a marked decrease in temperature as
the altitude increases.
From investigations of the propagation and reflection of
radio waves, it is known that beginning at an altitude of
80 km rays such as ultraviolet radiation, X-rays, and showers
of electrons from the Sun are able to ionize several layers
of the atmosphere which causes them to conduct electricity.
These layers reflect radio waves of certain frequencies
back to Earth because of the relatively high concentration
of ions in the air above 80 km this layer is an extending
to an altitude of 640 km is called the ionosphere. It is
also termed the thermosphere because of the high temperatures
in this layer .The region beyond the ionosphere is called
the exosphere, which extends to about 9,600 km the outer
limit of the atmosphere.
The density of dry air at sea level is about 1/800 the
density of water while at higher altitudes it decreases
rapidly as it is being proportional to the pressure and
inversely proportional to the temperature. Pressure is measured
by a barometer and is expressed in torrs, which are related
to the height of a column of mercury that the air pressure
will support where 1 torr equals 1 mm of mercury. Normal
atmospheric pressure at sea level is 760 torrs, that is,
760 mm of mercury. At about 5.6 km it is 380 torrs , half
of all the air in the atmosphere lies below this level.
The pressure is again approximately halved for each additional
increase of 5.6 km in altitude. At 80 km the pressure is
0.007 torr .
The troposphere and most of the stratosphere
can be easily explored directly by means of sounding balloons
equipped with instruments to measure the pressure and temperature
of the air and with a radio transmitter to send the data
to a receiving station at the ground. Rockets carrying radios
that transmit meteorological-instrument ,readings have explored
the atmosphere to altitudes above 400 km . Study of the
form and spectrum of the aurora gives information to a height
possibly as great as 800 km.
Atmospheric Pressure is also called Barometric
Pressure, force per unit area exerted by an atmospheric
column (that is, the entire body of air above the specified
area). Atmospheric pressure usually is measured with a mercury
barometer (hence the commonly used synonym barometric pressure),
which indicates the height of a column of mercury that exactly
balances the weight of the column of atmosphere the base
of which coincides with that of the mercury column. Also,
it may be measured using an aneroid barometer, in which
the sensing element is one or more hollow, partially evacuated
corrugated-metal disks supported against collapse by an
inside or outside spring; the change in the shape of the
disk with changing pressure can be recorded using a pen
arm and a clock-driven revolving drum.
Atmospheric pressure is expressed in several different
systems of units: inches (or millimetres) of mercury, pounds
per square inch (psi), dynes per square centimetre, millibars
(mb), atmospheres, or kilopascals. Standard sea-level pressure,
by definition, equals 29.92 inches (760 mm) of mercury,
14.70 pounds per square inch, 1013.25 ´ 103 dynes
per square cm, 1013.25 millibars, one atmosphere, or 101.35
kilopascals. Variations about these values are quite small;
for example, the highest and lowest sea-level pressures
ever recorded are 32.01 inches (in the middle of Siberia)
and 25.90 inches (in a typhoon in the South Pacific). The
small variations in pressure that do exist largely determine
the wind and storm patterns of the Earth.
Near the Earth's surface the pressure decreases with height
at a rate of about 3.5 mb for every 30 m (100 feet). The
pressure at 270,000 m (10-6 mb) is comparable to that in
the best man-made vacuum attainable. At heights above 1,500
to 3,000 m (5,000 to 10,000 feet), the pressure is low enough
to produce mountain sickness and severe physiological problems
unless careful acclimatization is undertaken.