29 January 2015



Composition : Chapter Summary

The atmosphere is a layer of gases which protects us from the harmful rays of the sun and also provides us the air we breathe. It also regulates the Earth’s temperature and makes life possible on the planet. The atmosphere is a mixture of many discrete gases present in varying quantities and having their own physical properties.

The main constituents of the atmosphere are oxygen and nitrogen. Apart from these two gases, argon, carbon dioxide, ozone, helium, water vapour, some other components and dust particles are also present in the atmosphere.

Both nitrogen and oxygen are essential for all the living organisms. Nitrogen is used by the plants from the bacteria present in the soil or root nodules. The bacteria convert the atmospheric nitrogen into water soluble compounds to be absorbed by the plants. This process is called nitrogen fixation.

Plants use oxygen to prepare food by the process called photosynthesis. In this process they release oxygen and use carbon dioxide present in the air. Humans and animals inhale oxygen and breathe out carbon dioxide. The content of oxygen and carbon dioxide in the air remains constant owing to the exchange of gases through photosynthesis and respiration.

However, the balance of oxygen is affected majorly by the deforestation.

Structure : Chapter Summary

Our atmosphere is divided in to five different layers i.e. the Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere.

The troposphere is the lowermost layer in the atmosphere and is the densest of all layers. The average height of troposphere is 13 kilomteres from the earth’s surface. This layer is responsible for the entire weather phenomenon like rainfall, fog and hailstorm.

Stratosphere forms the next layer of the atmosphere and extends upwards from the troposphere up till a height of 50 kilometers. The words ‘Stratus’ means spreading out and layer provides suitable conditions for the supersonic jets, weather balloons and flying airplanes or commercial aircrafts. Stratosphere is less dense and clouds are almost absent.

The most important feature of this layer is the presence of Ozone layer; stratosphere has the maximum concentration of the Ozone Layer. Ozone layer plays a vital role in regulating the earth’s temperature and absorbing the harmful ultraviolet radiations from the sun.

Above the stratosphere lies the mesosphere layer. The height of mesosphere is 50-80 kilomteres above the earth’s surface and has the coldest temperatures in the atmosphere. Falling stars are nothing but meteorites that burn up in the mesosphere when entering the atmosphere.

The fourth layer in the atmosphere above mesosphere is the thermosphere layer. The temperature here rises rapidly with increase in the altitude. However, the boundary between mesosphere and the thermosphere, known as mesopause, is the coldest place on the earth’s atmosphere having a temperature of minus 100 degrees centigrade. The international space station which orbits the earth, lies in the thermosphere layer.

Above the thermosphere layer is the ionosphere which extends up to 80-400 kilometers from the earth’s surface. This layer reflects the radio waves enabling radio transmissions from one point to another on the earth.

The exosphere is the uppermost layer of the earth’s atmosphere and extends up to 960-10,000 kilomteres above the earth’s surface. The main components of this layer gases like hydrogen and helium. 

Weather and Climate : Chapter Summary

Get to know about Our Air (Ncert / Cbse Solutions & Revision Notes), Chapter Summary Composition, Structure, Cyclones, Weather and Climate,CBSE / NCERT Revision Notes, CBSE NCERT Class VII (7th) | Social Studies | Geography, CBSE NCERT Solved Question Answer, CBSE NCERT Solution.Weather is the day-to-day and hour-to-hour condition of the atmosphere. Climate on the contrary is
the average condition of the weather in any area from a long time period like 25 years or so. Temperature is the degree of coldness or hotness of the air. Summers are hotter than winters, this means that the temperature in summers is high as compared to the winters.

Insolation is the measure of solar energy received by the earth’s surface in a given time. The amount of solar energy trapped decreases when we move towards the poles away from the equator. This is the reason why temperature also decreases at poles and they remain frozen.

The temperature in cities is comparatively higher than in villages. This is because of the concrete and metal used in the buildings and the asphalt in the roads in the cities get hot during the day. The high-rise and crowded buildings in the cities also trap warm air, thereby raising the temperature.

Degree Celsius was invented by Anders Celsius and is the standard unit to measure the temperature. As per Celsius scale, the freezing point of water is Zero degree and the boiling point is 100 degree Celsius.

The air above exerts great pressure on us, but we do not feel the pressure owing to the pressure exerted by our body. The pressure exerted by the air on the earth’s surface is called air pressure and it is the highest at the lowest level of the earth’s surface i.e. sea level and decreases as we move up.

The air pressure is distributed on the basis of the temperature of air at a particular place. The area which has high temperature will have a low pressure as the air gets heated and rises and vice-versa.

The movement of air from a high pressure area to a low pressure area is called wind. Winds are classified into three types i.e. seasonal winds, permanent winds and local winds. Winds that blow throughout the year are called as permanent winds like the trade winds, easterlies and westerlies.

Seasonal winds are the winds which change their direction as per the seasons like the monsoon winds in India, which blow only during the monsoon season.

Local winds are like the sea breeze and land breeze. In day time, the wind blows from the sea which has a high air pressure owing to the low temperature, hence is called sea breeze. Whereas, during night, land is cooler and the air pressure is high, hence the winds blow from land to sea called the land breeze.

Loo is another form of local wind, which is dry hot air that blows during the summers in the northern plains and the western parts of India. 

Humidity and Rainfall : Chapter Summary

The weather occurs in the lower part of the atmosphere which contains moisture as well, mostly in the form of water vapour, an invisible gas. The surface water from water bodies like lakes, oceans and rivers evaporate to form moisture in the form of water vapour. This process is known as evaporation.

The amount of water vapour present in  the atmosphere is called as humidity or absolute humidity. Water vapour when cools down and condenses in form of liquid water or snow. This process is called condensation.

Condensation takes place in different ways like dew or frost which happens when the moisture in the air is exposed to a cold surface; mist or fog happens when the moisture in the air condenses near the surface of the ground and clouds are formed when the moisture in the air condenses at high altitudes.

Rain, snow, sleet and hail are some other forms of condensation.

Clouds can be classified in three main groups on the basis of their shape and height.

Cirrus clouds are the highest clouds feathery or like hair curls, formed above 6000 meters or 20,000 feet. Cirrus clouds indicate the presence of moisture at high altitudes.

Cumulus clouds are puffy and look like puffs of cotton. They are quite in the sky and if they grow tall, they become thunderheads and bring down rain.

Stratus clouds are flat sheets of clouds, quite low in sky and look like a low gray blanket. These clouds are responsible to bring rain or snow.

Precipitation is the process where water vapour condenses in the air and returns to the earth in form of rain, dew or snow. Rainfall is the most common form of precipitation and occurs in three different ways i.e. convectional rainfall, orographic rainfall and cyclonic rainfall.

Convectional rainfall occurs when the hot air rises and cools down eventually during condensation, to fall as rain. When hot air rises, it cools and starts condensing to form clouds. When these clouds hit mountains, they cause rainfall in that region. This is called as orographic or relief rainfall.

Cyclones : Chapter Summary

A cyclone is a storm accompanied by high speed winds and heavy rains. Cyclones are able to cause large-scale destruction of life and property in the areas they hit. A cyclone has low pressure at the centre and high pressure in its surroundings. It blows towards the centre at high speed and spins in the same direction as the rotation of earth i.e. anti-clockwise in the northern hemisphere and clockwise in the southern hemisphere.

Orissa is the most cyclone prone state in India. It is a marine state located at the east coast of India having a coastline of 480 kilometers.  Between 1804 and 2000, coastal Orissa was hit by 128 cyclones of which 6 were extremely destructive. On 17th and 18th October, 1999, a cyclone hit five districts of Orissa which was followed by a super cyclone on 29th October 1999 and caused massive destruction.

Running at a speed of 260 kilometers per hour, the cyclone continued the destruction for 36 hours. Trees were uprooted and houses were collapsed owing to the high velocity winds. The major factors causing the destruction are heavy rain, wind velocity and tidal surge.

Cyclonic winds caused the tidal waves to rise up to 7-10 meters covering a land area of more than 20 kilometers. This also caused huge damage to the agricultural land and large tracks of sal, bamboo and teak plantations were vanished.

The super cyclone blew away the entire coast of Orissa, killed innumerable people, left thousands homeless, damaging houses and thousands of hectares of agricultural land. The cyclone affected around one-third of the state’s population.