Atmosphere of The Earth

The Atmosphere of The Earth

The Earth's atmosphere consists of a mixture of different

gases, generally, we call it air. The main components of

the atmosphere are nitrogen, oxygen, and argon. It also

contains water vapour and other gases. The approximate

amounts of the gases in the atmosphere are given below.

Nitrogen (N2): 78.08%

Oxygen (O2): 20.95%

Argon (Ar): 0.93%

Carbon Dioxide (CO2): 0.0423% (21st July 2024)

Neon (Ne): 0.0018%

Helium (He): 0.0005%

Methane (CH4): 0.000193% (March 2024)

Krypton (Kr): Approximately 0.000114%

Hydrogen (H2): Approximately 0.00005%

There are other gases and water vapour (0-4%) present 

in the atmosphere. There are five atmospheric layers of

the earth, starting from the earth's surface the layers are

the troposphere, stratosphere, mesosphere, thermosphere,

and exosphere. 

Troposphere

It is the lowest layer of the earth's atmosphere. It extends 

from the earth's surface to, on average, about 12 kilometres 

in height. But the height is lower at the pole (about 9

kilometres) and higher at the equator (about 17 kilometres).

It is the densest of all atmospheric layers and is

compressed by the weight of the other atmospheric layers

above it. It contains 80% of the total mass of the planetaty

atmosphere. The important features of this layer are given

below.

1. The weather activities happen here. Mostly all clouds

 forms here except the cumulonimbus thunderclouds,

whose top portion can extend up to the lower part of the

stratosphere. 

2. Aviation takes place here including the transition region

(tropopause) of the troposphere and stratosphere.

3. This region holds most of the oxygen that is needed to

breathe and the carbon dioxide that is required for

photosynthesis. 

4. This layer contains 99% of all water vapour and aerosols

(minute solid and liquid particles suspended in the

atmosphere).

5. The troposphere's temperature is the warmest closer to the

earth's surface due to the transfer of energy from the earth's

surface and decreases with altitude. The temperature drops,

on average, 6.5 °C or 11.7 °F per kilometer. At the equator

the average temperature of 20 °C (68 °F) at sea level changes

to approximately -70 to -75 °C(−94 to −103 °F) at the

tropopause and at the poles the average temperature of

0°C (32 °F) at sea level changes to −45 °C(−49 °F) at the

tropopause.

6. Due to the vertical change in temperature and change in

water vapour, this layer experiences turbulence.

7. The pressure of the atmosphere is maximum at sea level

(average 101.3 kPa) and decreases with height.

Tropopause is the boundary between the troposphere and

stratosphere. A jet stream of air has been seen in this part.

There is an abrupt change in environmental lapse rate

(ELR, air temperature changes with height) visible in this

region, because at its bottom part, the temperature

decreases with altitude in the troposphere, and at its top,

the temperature increases with height in the stratosphere.

Tropopause is considered an inversion layer (temperature

rises with height).

Stratosphere

The stratosphere is the next atmospheric layer of the

troposphere. It is the second layer from the surface of the

earth. It is located between about 12 kilometres to 50

kilometres above the earth's surface. The important features

of this layer are given below.

1. The most important feature of this layer is that the earth's

ozone layer is located here.

2. Ozone layer absorbs sun's ultraviolet (UV) radiation and

ozone is exothermically (a thermodynamic process where

reaction releases energy from the system to surroundings)

photolysed (chemical compound broken down by absorption

of light or photon) into oxygen in a cyclical fashion. Due to

this absorption of ultraviolet radiation, this layer is warmer

than the troposphere. Temperature inversion is visible here,

which means the temperature increases with the increase in

altitude. The temperature near the tropopause is  −51 °C

(−60 °F; 220 K) on average and near the mesosphere, it is

−15 °C (5.0 °F; 260 K) on average. The temperature changes

with the seasons. It decreases in polar nights (winter).

3. The pressure at the bottom of the stratosphere is 10%

of the pressure of the sea level. At the middle, the pressure

is 2.5% of the sea level pressure and at the top, it is 0.1%

of the sea level pressure.

4. The stratosphere is dry. It is weather and cloud free, only

polar stratospheric clouds are visible in the lowest and

coldest altitudes.

5. Bacteria can be found here and some bird species

sometimes fly above the troposphere. Jet-powered aircraft

can fly in this layer. Weather balloons also fly at this height.

Stratopause is the boundary between the stratosphere

and mesosphere and is located approximately 50 kilometres

above sea level. The temperature here is −2.5 °C (27.5 °F)

and the pressure here is 1/1000 pressure of sea level.

Mesosphere

The mesosphere is the third atmospheric layer of the earth.

It is located between 50-80 kilometres above the earth's

surface. The layer is separated from the stratosphere by

the boundary stratopause. The important features of this

layer are given below.

1. In the mesosphere the temperature decreases with altitude.

The top layer of the mesosphere is called mesopause, it is

the coldest place on earth. The average temperature in

mesopause is  -85 °C (120°F; 190 K).

2. The pressure here is very low, it is 0.1% sea level pressure

at the bottom and 0.0001% sea level pressure at the top of

this layer.

3. Noctilucent clouds or night shining clouds are visible here.

These clouds are made up of tiny crystals of water ice and

are visible from the ground in astronomical twilight. 

4. Most meteors burn up in this atmospheric layer.

5. The sounding rockets and the rocket-powered aircraft can

reach this layer.

6. A 5 kilometres deep sodium layer is found at a height

between 80-105 kilometres. This layer is made of unbound,

non ionized sodium atoms which radiate weakly to contribute

to the airglow (faint emission of light from the planetary

atmosphere).

Mesopause is the layer between the mesosphere and

the thermosphere. There are two mesopauses, one at 85

kilometres and another one at 100 kilometres above sea

level. Both of the layers are separated by slightly warmer

layers of air. The summer mesopause is cooler than winter

due to the upwelling at the summer pole and downwelling

at the winter pole.

Thermosphere

This layer is located above the mesosphere and below the

exosphere. It ranges from 80 kilometres to 600 kilometres

above sea level. The lower part of this layer contains the

ionosphere.

The ionosphere is an important part of the atmosphere.

The electrons, charged atoms, and molecules are found

here. The absorption of UV radiation causes these

ionizations and hence causes the rise in temperature. The

ionosphere is located from 48 kilometres to 965 kilometres.

It includes the uppermost part of the mesosphere,

thermosphere and exosphere.

The important features of the thermosphere are given below.

1. The temperature rises with altitude due to the absorption

of highly energetic solar radiation. The daytime temperature

ranges from 500 (900°F) to 2,000 °C (3,630 °F) or more.

2. Despite the high temperature, the density of the gas is so

low that the molecules can't conduct heat. So, the temperature

can't be felt by our skin. Pressure is also very low in this layer

due to the low density.

3. Aurora borealis and aurora australis are occasionally seen

at this layer. 

Thermopause is the boundary layer between the

thermosphere and the exosphere. The temperature varies

from absolute zero (-273.15 °C or −459.67 °F) to

987.547 °C (1,810 °F). The altitude of the thermopause

can vary, but approximately located around 500-1000

kilometres above sea level. The pressure and density of

the gases are extremely low at this point.

Exosphere

The exosphere is the outermost layer of the earth's

atmosphere. The lower boundary of this layer or exobase

is not well defined, it isconsidered to start around 500-

1000 kilometres andextends to about 10,000 kilometres

above the earth and merges with outer space. The important

features of this layer are given below.

1. Molecules found here are of extremely low density, so

they don't behave like a gas. Hydrogen is present throughout

this layer, and other gases like helium, carbon dioxide, and

atomic oxygen are also found here. The atoms and molecules

are far apart from each other so they can move a hundred

kilometres without colliding with each other. The particles

here escape into space.

2. The temperature of the exosphere can range from 

500-2000 °C (932°F to 3,632°F) depending on the solar

activities. However, the temperature doesn't feel to the

skin due to the very low density of the gas molecules.

3. There is no weather here at all. Aurora borealis and aurora

australis are occasionally seen at the lower part of this layer.

4. Earth's artificial satellites orbit in the exosphere around the

earth. 

Q&A

1Q. How high can a passenger aeroplane fly?

1A. A passenger aeroplane can fly up to (approximately

12 km)

the troposphere.

2Q. How high can a jet plane fly?

2A. Some military fighter aircraft can fly at an altitude of 

15- 19 km.

3Q. Where does Aurora occur?

3A. It occurs in the thermosphere and sometimes in the

exosphere.

4Q. Where do meteors burn?

4A. Meteors burn in the mesosphere.

5Q. Where does the satellite/ space station orbit around

the earth? 

5A. The satellites/ space station orbits around the earth in

the exosphere.

6Q. Is the earth's gravity the same in the earth's atmosphere?

6A. Earth's gravity decreases with altitude. The gravity is

9.8m/s2 at the earth's surface. It is 9.58m/s2 (approximately)

at the boundary of the mesosphere (80km) and 8.20m/s2

(approximately) at the boundary of the thermosphere and

exosphere (600km). At the upper boundary of the exosphere

(10,000km), the earth's gravity is negligible and it is

approximately 1.49m/s2.