AIR
THE ATMOSPHERE AND ITS IMPORTANCE
The Earth is surrounded by a huge blanket of air called the atmosphere. All living things need this atmosphere to live. It gives us the air we breathe and protects us from the sun’s harmful rays.
Without the atmosphere, we would get too hot during the day and too cold at night. This air blanket keeps the Earth’s temperature just right for living things.
COMPOSITION OF THE ATMOSPHERE
l The air we breathe is not just one gas. It is a mixture of many gases.
l Nitrogen and oxygen are the main gases.
l Small amounts of carbon dioxide, argon, helium, ozone, and hydrogen are also present.
l There are also tiny dust particles in the air.
Nitrogen is the most common gas. We breathe it in and out, but plants need it too. However, they cannot take it directly. Special bacteria in the soil help change nitrogen so plants can use it.
Oxygen is the second most common gas. Humans and animals need oxygen to live. Plants give off oxygen during a process called photosynthesis, which keeps the balance of oxygen in the air. Cutting trees can disturb this balance.
Carbon dioxide is also important. Plants use it to make food and release oxygen. Animals and humans give off carbon dioxide when they breathe. Normally, plants use up the same amount that animals and humans release. But when we burn fuels like coal and oil, it adds too much carbon dioxide, which affects the Earth’s climate.
STRUCTURE OF THE ATMOSPHERE
The atmosphere has five layers, from bottom to top:
1. Troposphere – Closest to Earth (up to 13 km). Weather happens here – like rain, fog, and storms.
2. Stratosphere – Up to 50 km high. Planes fly here. It has the ozone layer which protects us from the sun.
3. Mesosphere – Up to 80 km. Meteorites burn up in this layer.
4. Thermosphere – Up to 400 km. Radio signals bounce off this layer.
5. Exosphere – The top layer. Very thin air. Light gases like helium and hydrogen escape into space.
WEATHER AND CLIMATE
Weather is the day-to-day condition of the air (like rain or sunshine)
Climate is the average weather of a place over a long time.
Weather affects how we feel and what we do. That’s why we have weather forecasts every day.
TEMPERATURE
l Temperature means how hot or cold the air is.
l It changes from day to night and from season to season.
l The main factor that affects temperature is insolation (sunlight energy).
l Areas near the equator get more heat; areas near the poles get less heat.
l Cities are usually hotter than villages because of buildings and roads that trap heat.
AIR PRESSURE
l Air presses down on us, but we don’t feel it because it pushes from all sides.
l Air pressure is the weight of air on the Earth.
l It is highest at sea level and decreases with height.
l Hot air rises and creates low pressure (often brings clouds and rain).
l Cold air sinks and creates high pressure (brings clear skies).
l Air always moves from high pressure to low pressure – this movement is called wind.
WIND
Wind is moving air.
There are three main types of wind:
l Permanent winds – Blow all year in the same direction (like trade winds).
l Seasonal winds – Change with seasons (like monsoons in India).
l Local winds – Blow in small areas for a short time (like land and sea breeze, or loo, a hot dry wind in North India).
MOISTURE
l When water evaporates, it becomes water vapor.
l Water vapor in the air is called moisture or humidity.
l Warm air holds more moisture. On a humid day, we sweat more and clothes take longer to dry.
l When water vapor cools, it becomes tiny water droplets and forms clouds.
l When these droplets get heavy, they fall as rain (precipitation).There are three main types of rain:
1. Convectional rainfall
2. Orographic rainfall
3. Cyclonic rainfall
ü Rain is very important. It gives us fresh water.
ü Too little rain causes drought; too much rain causes floods.
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