JAC Board Class 7 Social Science Notes Geography Chapter 4 Air
→ Earth is surrounded by a huge layer of air known as atmosphere.
- On the earth all the living creatures depend on the atmosphere for their survival.
- With the absence of this layer of protection, we would have been baked by the scorching heat of the sun at daytime and frozen at night time. Hence, the temperature of the earth is liveable due to this layer of air.
→ Composition of the Atmosphere:
- The two main gases which make the most of the atmosphere are nitrogen and oxygen. Carbon dioxide, helium, ozone, argon and hydrogen are found in less quantities.
- Tiny and minute dust particles are also present in the atmosphere apart from these gases.
- We take some amount of nitrogen when we inhale into our lungs and exhale it.
- Plants take nitrogen from soil and roots of the plant which is brought by the bacterias and change its form so that plants can use it as they cannot take nitrogen directly.
- The second most abundant gas found is the oxygen. Green plants produces oxygen during photosynthesis and humans and animals use oxygen to breathe.
- The balance gets disturbed if we cut down the forests in large numbers.
- The other important gas is carbon dioxide. Plants use carbon dioxide to make their food and release oxygen. Humans and animals release carbon dioxide.
- The balance gets disturbed by burning of fuels like oils, coal. Hence, the earth’s climate and weather gets affected by the increased volume of carbon dioxide.
→ Structure of the Atmosphere:
- Starting from the earth’s surface, the atmosphere has been divided into five layers – Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere.
- Troposphere – Themost important layer of the atmosphere is Troposphere and its average height is 13 km. The air we breathe exists here. Most of the weather incidences such as rainfall, fog and hailstorm occur in this layer.
- Stratosphere – Stratosphere lies above the troposphere and it extends up to a height of 50 km. This layer is almost free from clouds and related weather incidences are making ideal conditions for flying aeroplanes. One of the important features is that it contains a layer of ozone gas.
- Mesosphere – The third layer of the atmosphere is mesosphere. Above the stratosphere it is situated and extends up to the height of 80 km. On entering from the space, meteorites bum up in this layer.
- Thermosphere – Temperature rises very rapidly and fast with increasing height in thermosphere.
- Ionosphere is a part of this layer and it extends between 80^100 km. Radio transmission happens in this layer. In fact, radio waves transmitted from the earth are reflected back to the earth by this layer.
- Exosphere- It is the upper most layer of the atmosphere. It has very thin air. Light gases are present such as helium and hydrogen float into the space from here.
→ Weather and Climate:
Weather is the hour-to-hour, day to day condition of the atmosphere. But, the average weather condition or phenomena of a place for a longer period of time is called as the climate of a place.
- The degree or level of hotness and coldness of the air is called as the temperature.
- The atmosphere’s temperature changes not only between day and night but also from season to season as well.
- The incoming solar energy intercepted by the earth is called the insolation. It is one of the important factors that influence the distribution of temperature.
- Towards the poles, the amount of insolation decreases from the equator hence the temperature also decreases.
- Temperature in cities is much higher than that of villages because of the concrete and metals in buildings and the asphalt of roads get heated up during the day and this is released during the night.
→ Air Pressure
- From all directions, the air presses us and our body exerts a counter pressure.
- The pressure exerted by the weight of air on the earth’s surface is called the air pressure. It is highest at sea level and decreases with height.
- Low pressure is related to cloudy skies and wet weather. A low-pressure is created in areas where temperature is high and the air gets heated and rises.
- High pressure is related to clear and sunny skies. Heavy air sinks and creates a high pressure area and the air always moves from high pressure areas to low pressure areas.
- Wind is the movement of air from high pressure area to low pressure areas.
- It is divided into three parts:
- Permanent winds
- Seasonal winds
- Local winds
- Permanent winds are the trade winds, westerlies and easterlies. They blow constantly throughout the year in a particular direction.
- Seasonal winds are the winds which change their direction in different seasons such as monsoons in India.
- Local winds are the winds which blow only during a particular period of the day or year in a small area such as land and sea breeze. The hot and dry local wind of northern planes of India is called loo.
- We know that when water evaporates from land and different water bodies, it becomes water vapour. Humidity is the moisture in the air at any time. When the air is full of water vapour then it is called as a humid day.
- Clouds are just masses of water droplets. The water vapour condenses which cause the formation of these droplets of water.
- Precipitation happens when these droplets of water become too heavy to float in air and then they come down.
- Rain is the precipitation that comes down to the earth in liquid form.
- The three types of rainfall are the convectional rainfall, the orographic rainfall and the cyclonic rainfall.
- For the survival of plants and animals, rainfall is very important as it brings fresh water to the earth’s surface. If rainfall is less then water scarcity and drought occur and if it is more, then floods take place.