CH 7 Composition and Structure of the Atmosphere PDF

CLASS XI / BOOK 1/ FUNDAMENTALS OF PHYSICAL GEOGRAPHY CH 7 / COMPOSITION AND STRUCTURE OF THE ATMOSPHERE --------------------------------------------------------------------------------------------------------------------------------------- The Atmosphere- Atmosp...

CLASS XI / BOOK 1/ FUNDAMENTALS OF PHYSICAL GEOGRAPHY CH 7 / COMPOSITION AND STRUCTURE OF THE ATMOSPHERE --------------------------------------------------------------------------------------------------------------------------------------- The Atmosphere- Atmosphere is a mixture of different gases and it envelopes the earth all round. It contains life giving gases like oxygen for humans and animals and carbon dioxide for plants. The air is an integral part of the earth's mass and 99 per cent of the total mass of the atmosphere is confined to the height of 32 km from the earth‘s surface. The air is colourless and odourless and can be felt only when it blows as wind. Composition Of The 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 to90 km from the surface of the earth. → Carbon Dioxide- Carbon dioxide is meteorologically a very important gas. It is transparent to the incoming solar radiation (insolation) 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. Carbon dioxide is largely responsible for the greenhouse effect. When the volume of other gases remains constant in the atmosphere, the volume of the carbon dioxide has been rising in the past few decades mainly because of the burning of fossil fuels. This rising volume of carbon dioxide is the main reason for global warming. → Ozone Gas: Ozone is another important component of the atmosphere found mainly between 10 and 50 km above the earth’s surface. It acts as a filter and absorbs the ultra-violet rays radiating from the sun and prevents them from reaching the surface of the earth. The amount of ozone gas in the atmosphere is very little and is limited to the ozone layer found in the stratosphere. → Water Vapour Gases form of water present in the atmosphere is called water vapour. It is the source of all kinds of precipitation. The amount of water vapour decreases with altitude. It also decreases from the equator (or from the low latitudes) towards the poles (or towards the high latitudes). Its maximum amount in the atmosphere could be up to 4% which is found in the warm and wet regions. Water vapour reaches in the atmosphere through evaporation and transpiration. Evaporation takes place in the oceans, seas, rivers, ponds and lakes while transpiration takes place from the plants, trees and living beings. Water vapour absorbs part of the incoming solar radiation (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 Dust particles are generally found in the lower layers of the atmosphere. These particles are found in the form of sand, smoke-soot, oceanic salt, ash, pollen, etc. Higher concentration of dust particles is found in subtropical and temperate regions due to dry winds in comparison to equatorial and polar regions. These dust particles help in the condensation of water vapour. During the condensation, water vapour gets condensed in the form of droplets around these dust particles and thus clouds are formed. Structure Of The Atmosphere → Troposphere It is considered as the lowest layer of Earth’s atmosphere. The troposphere starts at the surface of the earth and goes up to a height of 7 to 20 km. All-weather phenomenon occurs within this layer. This layer has water vapor and dust particles. Temperature decreases at the rate of 1 degree Celsius for every 165 m of height.(Lapse rate) Tropopause separates Troposphere and Stratosphere. → Stratosphere It is the second layer of the atmosphere found above the troposphere. It extends up to 50 km of height. This layer is very dry as it contains little water vapour. This layer provides some advantages for flight because it is above stormy weather and has steady, strong, horizontal winds. The ozone layer is found in this layer. The ozone layer absorbs UV rays and safeguards earth from harmful radiation. 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 separates Mesosphere and Thermosphere. → Thermosphere This layer is found above Mesopause from 80 to 400 km. Radio waves that are transmitted from the earth are reflected by this layer. The temperature increases with height. Aurora and satellites occur in this layer.  → Ionosphere The lower Thermosphere is called the Ionosphere. The 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. → Exosphere 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. __________________________________________________________________________________________