Atmosphere

1 Atmosphere – Introduction 2 Significance of Earth’s Atmosphere 3 Composition of Atmosphere 4 Structure of Atmosphere

1 Atmosphere – Introduction

An atmosphere is a layer or a set of layers of gases surrounding a planet or other material body, that is held in place by the gravity of that body.
An atmosphere is more likely to be retained if the gravity it is subject to is high and the temperature of the atmosphere is low.
Atmosphere is a mixture of various gases containing huge amount of solid and liquid particles collectively known as aerosols.

From the perspective of a planetary geologist, the atmosphere acts to shape a planetary surface. Wind picks up dust and other particles which, when they collide with the terrain, erode the relief and leave deposits (eolian processes). Frost and precipitations, which depend on the atmospheric composition, also influence the relief. Climate changes can influence a planet’s geological history. Conversely, studying the surface of the Earth leads to an understanding of the atmosphere and climate of other planets. For a meteorologist, the composition of the Earth’s atmosphere is a factor affecting the climate and its variations. For a biologist or paleontologist, the Earth’s atmospheric composition is closely dependent on the appearance of the life and its evolution.

2 Significance of Earth’s Atmosphere

3 Composition of Atmosphere

The atmosphere is made up of different gases, water vapour and dust particles. The composition of the atmosphere is not static and it changes according to the time and place.
The portion of the gases changes in the higher layers of the atmosphere in such a way that oxygen will be almost negligible quantity at the heights of 120 km.
Similarly, carbon dioxide (and water vapour) is found only up to 90 km from the surface of the earth.

Air mixture behaves in many ways as if it were a single gas. As a result of this phenomenon, the following generalizations describe important relationships between temperature, pressure, density and volume, that relate to the Earth’s atmosphere.

(1) When temperature is held constant, the density of a gas is proportional to pressure, and volume is inversely proportional to pressure. Accordingly, an increase in pressure will cause an increase in density of the gas and a decrease in its volume.

(2) If volume is kept constant, the pressure of a unit mass of gas is proportional to temperature. If temperature increase so will pressure, assuming no change in the volume of the gas.

(3) Holding pressure constant, causes the temperature of a gas to be proportional to volume, and inversely proportional to density. Thus, increasing temperature of a unit mass of gas causes its volume to expand and its density to decrease as long as there is no change in pressure.

Major constituents of dry air, by volume
Gas		Volume(A)
Name	Formula	in ppmv(B)	in %
Nitrogen	N2	780,840	78.084
Oxygen	O2	209,460	20.946
Argon	Ar	9,340	0.9340
Carbon dioxide	CO2	400	0.04
Neon	Ne	18.18	0.001818
Helium	He	5.24	0.000524
Methane	CH4	1.79	0.000179
Not included in above dry atmosphere:
Water vapor(C)	H2O	10–50,000(D)	0.001%–5%(D)

notes:

(A) volume fraction is equal to mole fraction for ideal gas only,

also see volume (thermodynamics)

(B) ppmv: parts per million by volume

(D) Water vapor strongly varies locally

Nitrogen

- The atmosphere is composed of 78% nitrogen.
- Nitrogen cannot be used directly from the air.
- Biotic things need nitrogen to make proteins.
- The Nitrogen Cycle is the way of supplying required nitrogen for living things.

Oxygen

- The atmosphere is composed of 21% oxygen.
- It is used by all living things and is essential for respiration.
- It is obligatory for burning.

Argon

The atmosphere is composed of 0.9% argon.
They are mainly used in light bulbs.

Carbon Dioxide

- The atmosphere is composed of 0.03% carbon dioxide.
- Plants use it to make oxygen.
- It is significant as it is opaque to outgoing terrestrial radiation and transparent to incoming solar radiation.
- CO2 increases the temperature of lower atmosphere and the phenomena is called as the GHG effect.

Ozone gas

- Present around 10-50 km above earth surface and acts as a sieve, absorbing UV from the sun.
- Ozone averts the harmful rays from reaching the surface of the earth.

Water vapour

- Water vapour is a variable gas, declines with altitude.
- It also drops towards the poles from the equator.
- It acts like blanket letting the earth to neither to become too hot nor too cold.
- It also contributes to the stability and instability in the air.

Dust particles

Dust particles are in higher concentration in temperate and subtropical regions due to dry winds in contrast to the Polar Regions and equatorial regions.
They act as hygroscopic nuclei over which water vapour of atmosphere condenses to create clouds.

4. Structure of the atmosphere

The atmosphere can be divided into five layers according to the diversity of temperature and density. They are:

Troposphere
Stratosphere Homosphere
Mesosphere
Thermosphere (Ionosphere)
Exosphere

Heterosphere

Troposphere

It is considered as the lowest layer of Earth’s atmosphere.
Height – 8 at the Poles to 18 km at Equator.
All weather occurs within this layer.
This layer has water vapour and mature particles.
Temperature decreases at the rate of 1 degree Celsius for every 165 m of height.
Tropopause separates Troposphere and Stratosphere.
All Weather Phenomena take place in this layer.
Upper limit of this layer is known as Tropopause.

Stratosphere

It lies above the troposphere is spread up to the height of 50 km from the earth’s surface which has an extent of 40 km.
In this layer the temperature remains almost same upto the height of 20 kms above the earth’s surface.
This layer is very dry as it contains little water vapour. Due to presence of ozone (15-35 kms) in lower stratosphere a bacteria can survive in this layer.
This layer is considered ideal for flying of aircrafts mostly due to low temperature, which results in optimum fuel burn and low density of air, reduces parasitic drag on airframe. This layer even provides smooth movement as this layer has very less weather turbulence.
Stratopause separates Stratosphere and Mesosphere.

Mesosphere

The Mesosphere is found above the stratosphere.
It is the coldest of the atmospheric layers.
The mesosphere starts at 50 km above the surface of Earth and goes up to 85 km.
The temperature drops with altitude in this layer.
By 80 km it reaches -100 degrees Celsius.
Meteors burn up in this layer.
The upper limit is called Mesopause which separate Mesosphere and Thermosphere.

Thermosphere

This layer is found above Mesopause from 80 to 400 km.
Radio waves which are transmitted from the earth are reflected back by this layer.
The temperature increases with height.
Aurora and satellites occur in this layer.

Ionosphere

- The lower Thermosphere is called the Ionosphere.
- Ionosphere consists of electrically charged particles known as ions.
- This layer is defined as the layer of the atmosphere of Earth that is ionized by cosmic and solar radiation.
- It is positioned between 80 and 400 km above the Mesopause.

Ionosphere spread between 80 km to 400 km above the surface of the earth. There are number of ionic layers in the sphere, e.g. D layer, E layer, F layer and G layer.

D layer disappears with sunset as it is associated with solar radiation.
E layer is known as Kennelly-Heaviside which is confined to the height between 99 km – 130 kms. It reflects the medium and high frequency radio waves.
F2 layer is called as appleton layer, lying between 150 km-380 km.
F1 and F2 area the two sub layers of E2 layer combined known as Appleton Layer.
G layer is beyond 400 km.

Exosphere

Extends beyond 400 kms.
It is the outermost layer of the atmosphere.
The zone where molecules and atoms escape into space is mentioned as the exosphere.
It extends from the top of the thermosphere up to 10,000 km.

Pidwirny, M. (2006). “Atmospheric Effects on Incoming Solar Radiation”. Fundamentals of Physical Geography, 2nd Edition. Date Viewed. http://www.physicalgeography.net/fundamentals/7f.html