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:
- Essay on the Definition of Atmosphere
- Essay on the Characteristics of Atmosphere
- Essay on the Physical Properties of Atmosphere
- Essay on the Mass of Atmosphere
- Essay on the Composition of Atmosphere
- Essay on the Structure of Atmosphere
- Essay on Chemical and Photochemical Reactions in Atmosphere
- Essay on the Role of Atmospheric Air in Agriculture
- Essay on Solid Particles in Atmosphere
- Essay on Atmospheric Pressure
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Essay # 1. Definition of Atmosphere:
The envelope of colourless, tasteless and odour-less gases surrounding the earth is called the atmosphere. 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. 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.
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.
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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.
Essay # 3. Physical Properties of Atmosphere:
The important physical properties of the atmosphere are:
i. A given quantity of air occupies equally all space of a container. It exerts equal pressure in all directions.
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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.
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. 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. 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 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.
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.
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.
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.
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.
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. 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.
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.
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.
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.
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.
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.
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. 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. 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. 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. 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. 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.
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.
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.
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. 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 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.
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. 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. 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. 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. 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. 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 (0.07ppm). 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. 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.
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. 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.
