AIR

 

CLASS-VII                           GEOGRAPHY

CHAPTER 4.       AIR

Understanding Atmosphere:

•        Our earth is surrounded by a huge blanket of air called atmosphere.

•        All living beings on this earth depend on the atmosphere for their survival.

•        It provides us the air we breathe and protects us from the harmful effects of the sun’s rays.

•        Without this blanket of protection, we would be baked alive by the heat of the sun during day and get frozen during night.

•        So it is this mass of air that has made the temperature on the earth liveable.

Green House Effect:

•        Carbon dioxide released in the atmosphere creates a green house effect by trapping the heat radiated from the earth. It is therefore called a greenhouse gas and without it the earth would have been too cold to live in.

Global Warming:

•        When Carbon dioxide level in the atmosphere increases due to factory smoke or car fumes, the heat retained increases the temperature of the earth. This is called global warming.

•        This rise in temperature causes the snow in coldest parts of the world to melt. As a result the sea level rises, causing floods in the coastal areas. There may be drastic changes in the climate of a place leading to extinction of some plants and animals in the long run.

Composition of the atmosphere

•        The air is actually a mixture of many gases.

•        Nitrogen and oxygen are two gases which make up the bulk of the atmosphere. Carbon dioxide, helium, ozone, argon and hydrogen are found in lesser quantities.

•        Apart from these gases, tiny dust particles are also present in the air.

•        Nitrogen is the most plentiful gas in the air.

NITROGEN	78%
CARBON DIOXIDE	0.03%
ARGON	0.93%
ALL OTHER GAS	0.04%

•         Oxygen is the second most plentiful gas in the air.

 

 

 

 

 

 

 

Importance of air

•        Humans and animals take oxygen from the air as they breathe.

•        Green plants produce oxygen during photosynthesis. In this way oxygen content in the air remains constant. If we cut trees then this balance gets disturbed.

•        Carbon dioxide is another important gas. Green plants use carbon dioxide to make their food and release oxygen. Humans or animals release carbon dioxide.

•        When air is heated, it expands, becomes lighter and goes up. Cold air is denser and heavy. That is why it tends to sink down. When hot air rises, cold air from surrounding area rushes there to fill in the gap.

•        Oxygen is the second most plentiful gas in the air.

•        Humans and animals take oxygen from the air as they breathe.

•        Green plants produce oxygen during photosynthesis. In this way oxygen content in the air remains constant.

•        Green plants use carbon dioxide to make their food and release oxygen. Humans or animals release carbon dioxide.

•        The amount of carbon dioxide released by humans or animals seems to be equal to the amount used by the plants which make a perfect balance.

 

 

STRUCTURE OF THE ATMOSPHERE

         Our atmosphere is divided into five layers starting from the earth’s surface. These are Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere.

Troposphere:

•        This layer is the most important layer of the atmosphere.

•        Its average height is 13 km.

•        Almost all the weather phenomena like rainfall, fog and hailstorm occur in this layer.

Stratosphere:

•        Above the troposphere lies the stratosphere.

•        It extends up to a height of 50 km.

•        This layer is almost free from clouds and associated weather phenomenon, making conditions most ideal for flying aeroplanes.

•        It contains a layer of ozone gas.

Mesosphere:

•        This is the third layer of the atmosphere.

•        It lies above the stratosphere.

•        It extends up to the height of 80 km.

•        Meteorites burn up in this layer on entering from the space.

Thermosphere:

•        In thermosphere temperature rises very rapidly with increasing height.

•        Ionosphere is a part of this layer.

•        It extends between 80-400 km.

•        This layer helps in radio transmission.

Exosphere:

•        The upper most layer of the atmosphere is known as exosphere.

•        This layer has very thin air.

•        Light gases like helium and hydrogen float into the space from here.

 

Weather

•        Weather is hour-to-hour, day to day condition of the atmosphere.

•        Weather can change dramatically from day to day.

Climate

•        The average weather condition of a place for a longer period of time represents the climate of a place.

Temperature

•        The degree of hotness and coldness of the air is known as temperature.

•        The temperature of the atmosphere changes not only between day and night but also from season to season.

•        An important factor that influences the distribution of temperature is insolation.

•        Insolation is the incoming solar energy intercepted by the earth. The amount of insolation decreases from the equator towards the poles. Therefore, the temperature decreases in the same manner.

•        If the earth’s temperature rises too high, it would become too warm for some crops to grow.

•        Temperature in cities is much higher than that of villages. The concrete and metals in buildings and the asphalt of roads get heated up during the day.

•        This heat is released during the night. Also, the crowded high rise buildings of the cities trap the warm air and thus raise the temperature of the cities.

•        The standard unit of measuring temperature is degree Celsius. It was invented by Anders Celsius.

•        On the Celsius scale the water freezes at 0°C and boils at 100°C.

Weather Instruments

•        Wind Vane: Shows the direction of the wind

•        Rain Gauge: Measures the amount of rainfall

•        Thermometer: Measures the temperature

•        Barometer: Measures atmospheric pressure

Air Pressure

•        The air above us presses us with a great force on our bodies. However, we don’t even feel it. This is because the air presses us from all directions and our body exerts a counter pressure.

•        Air pressure is defined as the pressure exerted by the weight of air on the earth’s surface.

•        As we go up the layers of atmosphere, the pressure falls rapidly.

•        The air pressure is highest at sea level and decreases with height.

•        Horizontally the distribution of air pressure is influenced by temperature of air at a given place.

•        In areas where temperature is high the air gets heated and rises. This creates a low-pressure area. Low pressure is associated with cloudy skies and wet weather.

•        In areas having lower temperature, the air is cold. It is therefore heavy. Heavy air sinks and creates a high pressure area. High pressure is associated with clear and sunny skies.

•        The air always moves from high pressure areas to low pressure areas.

Wind

•        The movement of air from high pressure area to low pressure areas is called wind.

•        Winds can be broadly divided into three types.

1. Permanent winds – The trade winds, westerlies and easterlies are the permanent winds. These blow constantly throughout the year in a particular direction.

2. Seasonal winds – These winds change their direction in different seasons. For example monsoons in India.

3. Local winds – These blow only during a particular period of the day or year in a small area. For example, land and sea breeze. Do you recall the hot and dry local wind of northern planes of India? It is called loo

Moisture

•        When water evaporates from land and different water bodies, it becomes water vapour.

•        Moisture in the air at any time, is known as humidity.

•        When the air is full of water vapour we call it a humid day.

•        As the air gets warmer, its capacity to hold the water vapour increases and so it becomes more and more humid. On a humid day, clothes take longer to dry and sweat from our body does not evaporate easily, making us feel very uncomfortable.

•        When the water vapour rises, it starts cooling. The water vapour condenses causing formation of droplets of water. Clouds are just masses of such water droplets. When these droplets of water become too heavy to float in air, then they come down as precipitation.

•        Precipitation that comes down to the earth in liquid form is called rain.

Rainfall

•        On the basis of mechanism, there are three types of rainfall: the convectional rainfall, the orographic rainfall and the cyclonic rainfall.

•        Rainfall is very important for the survival of plants and animals.

•        It brings fresh water to the earth’s surface.

•        If rainfall is less – water scarcity and drought occur. On the other hand if it is more, floods take place.

Convectional Rainfall

•        You may be outside on a picnic on a hot summers day. You are enjoying yourself and you are soaking in the rays of sunshine. Suddenly, the sky gets darker and a dark, grey cumulus cloud is coming your way. Without warning, the heavens open and it begins to rain, with an almost thundery feel. Convectional rain occurs frequently on hot days usually giving cumulus cloud and thundery showers.

•        This causes the air to warm and become very hot.

•        The air rises upwards. The air is then cooled and condenses to form cumulus cloud.

•        When the cumulus cloud is saturated, it begins to precipitate giving heavy and thundery showers.

•        That is why you get thundery showers on a hot day, because the sun warms the air and it rises, cools and begins to rain.

Relief Rainfall

•        Relief rainfall occurs very frequently near mountains beside the sea.

•        Moisture-laden wind blows in from the sea. Because the wind meets a high mountain, it is forced to rise upwards. As it rises upwards, it is cooled and cloud is formed.

•        The cloud becomes saturated with water vapour and it begins to precipitate on the side of the mountain facing the sea. This side of the mountain is known as the windward side.

•        The cloud precipitates the most on the windward side of the mountain. By the time the cloud meets the other side, which is called the leeward side, the cloud has already lost most of its moisture so it rains very little there.

•        This makes leeward sides of a mountain very sheltered from rain and they hardly ever get much rain.

•        There is a more moist climate on windward sides of slopes whilst there is a more dry, sheltered climate on the leeward side.

•        This rainfall is very common in Hawaii, Sierr.

Frontal/Cyclonic Rainfall

•        This occurs when a warm, tropical air mass comes in contact with a cold, polar air mass and is very common in Britain and Ireland.

•        A warm, tropical air mass comes in contact with a cold, polar air mass. Because the air in the warm front is well, warm then it rises over the cold front.

•        The air is cooled and condenses to form stratus cloud.

•        When the stratus cloud becomes saturated, it begins to precipitate.

 

………The end………