ATMOSPHERE The envelope of gases and

water vapour surrounding the planet earth is called atmosphere.

Atmosphere is divided into several layers.

Each layer has its own altitude and temperature.

Layers of the atmosphere:There are four layers of the atmosphere. Trposphere Stratospher

e Mesophere Thermosphe

re• Ionosphere• Exosphere

TROPOSHPERE The atmospheric layer closest to the

Earth’s surface is called troposphere. Tropo- means “turning” or “changing”. Conditions in the troposphere are more

variable than in the other layers. It extends to about 12km above Earth’s

surface. Troposphere contains 75-80% of the

atmosphere. It is the layer of atmosphere in which we live.

Nearly all dust particles and water vapours are in the troposphere.

Weather occurs in this layer. Most of the clouds are formed in the

troposphere. Air crafts fly in this region. As altitude increases in the troposphere,

the temperature decreases and varies from 17 degree centigrade to -55 degree centigrade.

On average, for every 1km increase in altitude, the air gets about 6.5 degree centigrade cooler.

STRATOPHERE The second layer as one moves upward from the

Earth’s surface is called stratosphere. The stratosphere extends from top of the

troposphere to about 50km above Earth’s surface.

Strato- means “layer” or “spread out”. The lower stratosphere is cold about -55 degree

centigrade but the upper layer is stratosphere is warmer than the lower stratosphere (-55 to 5C).

It contains little water vapours and the miximum amount of ozone layer.

What is ozone layer ?

OZONE LAYER The Ozone layer or ozone shield is an

allotropic form of oxygen. It is present in the Earth’s stratosphere

and absorbs most of the sun’s ultraviolet (UV) radiations.

It protects the living things on the Earth from dangerous ultraviolet radiations from the sun.

Human activity releases many compounds in the atmosphere. Such compounds threaten the stability of ozone layer.

The region in which the amount of ozone has been reduced is called as ozone hole.

MESOPHERE The mesosphere extends from the top of

stratosphere to about 80km from the Earth’s surface.

Meso- means middle, so the mesosphere is the middle layer of the atmosphere.

In the outer mesosphere, temperature approaches -93 degree centigrade.

This layer protect the Earth’s surface from being hit by most meteroids.

THERMOSPHERE The outermost layer of the atmosphere is

the thermosphere. It extends from 80km above Earth’s

surface outward into space. Thermo means heat. This layer is very hot up to 1800 degree

centigrade this is because sunlight strikes it first.

It is further divided into two layers.

The lower layer of thermosphere is called the Ionosphere.

It extends from 80 to 400km above the Earth’s surface.

The outer layer of thermosphere is called the Exosphere.

It extends from 400km to thousands of kilometers from the Earth’s surface.

In the northern hemisphere, brilliant light displays, occur in the ionosphere. These light displays are called Aurora borealis. Auroras are caused by particles from the sun that enter the ionosphere near the poles. These particles strike the atoms in ionosphere, causing them to glow.

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