Ch. 08�Composition and Structure of Atmosphere

Composition of Atmosphere

The atmosphere is composed of gases, water vapour and dust particles.

The proportion of gases changes in the higher layers of the atmosphere in such a way that oxygen will be almost in negligible quantity at the height of 120 km.

Similarly, carbon dioxide and water vapour are found only up to 90 km from the surface of the earth.

​

Composition of Atmosphere

Composition of Atmosphere Permanent Gases of the Atmosphere Constituent % by Volume

• Nitrogen (N2) 78.08

• Oxygen (O2) 20.95

• Argon (Ar) 0.93

• Carbon dioxide (CO2) 0.0325

• Neon (Ne) 0.002

​

Composition of Atmosphere

Composition of Atmosphere (Cont.) Constituent % by Volume

• Helium (He) 0.0005

• Krypto (Kr) 0.001

• Xenon (Xe) 0.00009

• Hydrogen (H2) 0.00005

​

Gases

Carbon dioxide is meteorologically a very important gas as it is transparent to the incoming solar radiation but opaque to the outgoing terrestrial radiation.

It absorbs a part of terrestrial radiation and reflects back some part of it towards the earth’s surface. It is largely responsible for the green house effect.

​

Composition of Atmosphere

The volume of other gases is constant but the volume of carbon dioxide has been rising in the past few decades mainly because of the burning of fossil fuels. This has also increased the temperature of the air.

Composition of Atmosphere

Ozone is another important component of the atmosphere found between 10 and 50 km above the earth’s surface and acts as a filter and absorbs the ultra-violet rays radiating from the sun and prevents them from reaching the surface of the earth.

Water Vapour

Water vapour is also a variable gas in the atmosphere, which decreases with altitude. In the warm and wet tropics, it may account for four per cent of the air by volume, while in the dry and cold areas of desert and polar regions, it may be less than one per cent of the air. Water vapour also decreases from the equator towards the poles.

Water Vapour

It also absorbs parts of the insolation from the sun and preserves the earth’s radiated heat.

It thus, acts like a blanket allowing the earth neither to become too cold nor too hot.

Water vapour also contributes to the stability and instability in the air.

Dust Particles

Atmosphere has a sufficient capacity to keep small solid particles, which may originate from different sources and include sea salts, fine soil, smoke- soot, ash, pollen, dust and disintegrated particles of meteors.

Dust Particles

Dust particles are generally concentrated in the lower layers of the atmosphere; yet, convectional air currents may transport them to great heights.

Dust Particles

The higher concentration of dust particles is found in subtropical and temperate regions due to dry winds in comparison to equatorial and polar regions.

Dust and salt particles act as hygroscopic nuclei around which water vapour condenses to produce clouds.

�TROPOSPHERE �.

This is the layer of the atmosphere closest to the Earth's surface, extending up to about 10-15 km above the Earth's surface.

It contains 75% of the atmosphere's mass. The troposphere is wider at the equator than at the poles. Temperature and pressure drops as you go higher up the troposphere.

Tropopause

The Tropopause: At the very top of the troposphere is the tropopause where the temperature reaches a (stable) minimum. Some scientists call the tropopause a "cold trap" because this is a point where rising water vapor cannot go higher because it changes into ice and is trapped.

​

Tropopause

If there is no cold trap, Earth would loose all its water! The uneven heating of the regions of the troposphere by the Sun causes convection currents and winds. Warm air from Earth's surface rises and cold air above it rushes in to replace it

Tropopouse

The zone separating the tropsophere from stratosphere is known as the tropopause.

The air temperature at the tropopause is about minus 800C over the equator and about minus 45o C over the poles.

The temperature here is nearly constant, and hence, it is called the tropopause.

STRATOSPHERE

This layer lies directly above the troposphere and is about 35 km deep.

It extends from about 15 to 50 km above the Earth's surface.

The lower portion of the stratosphere has a nearly constant temperature with height but in the upper portion the temperature increases with altitude because of absorption of sunlight by ozone.

STRATOSPHERE

This temperature increase with altitude is the opposite of the situation in the troposphere.

• The Ozone Layer The stratosphere contains a thin layer of ozone which absorbs most of the harmful ultraviolet radiation from the Sun.

The ozone layer is being depleted, and is getting thinner over Europe, Asia, North American and Antarctica --- "holes" are appearing in the ozone layer.

1

​

Statopause

The stratopause (formerly Mesopeak) is the level of the atmosphere which is the boundary between two layers: the stratosphere and the mesosphere. In the stratosphere the temperature increases with altitude, and the stratopause is the region where a maximum in the temperature occurs.

​

Mesosphere

The mesosphere lies above the stratosphere, which extends up to a height of 80 km.

In this layer, once again, temperature starts decreasing with the increase in altitude and reaches up to minus 100° C at the height of 80 km.

​

THERMOSPHERE

The thermosphere extends from 80 km above the Earth's surface.

The thermosphere is the fourth layer of the Earth's atmosphere.

THERMOSPHERE

It is found above the mesosphere. The air is really thin that high up. The temperature changes with the solar activity.

If the sun is active, temperatures in the thermosphere can get up to 1,500°C or higher!

Inosphere

The ionosphere is located between 80 and 400 km above the mesopause.

It contains electrically charged particles known as ions, and hence, it is known as ionosphere.

​

Inosphere

Radio waves transmitted from the earth are reflected back to the earth by this layer. Temperature here starts increasing with height.

​

The Exosphere

The exosphere is the uppermost layer of Earth's atmosphere.

In the exosphere, an upward travelling molecule moving fast enough to attain escape velocity can escape to space with a low chance of collisions.

​

The Exosphere

If it is moving below escape velocity it will be prevented from escaping from the celestial body by gravity. In either case, such a molecule is unlikely to collide with another molecule due to the exosphere's low density.

​

Elements of Weather and Climate

The main elements of atmosphere which are subject to change and which influence human life on earth are temperature, pressure, winds, humidity, clouds and precipitation.

Thank� You�