Air — Study Notes

1. Introduction

We cannot see air, but it is all around us, and without it, life on earth would be impossible. Air is a mixture of gases that forms a protective blanket around the earth, known as the atmosphere. The atmosphere extends about 1,600 kilometres above the earth's surface and is held to the earth by the force of gravity.

The atmosphere is essential for life in many ways. It supplies the oxygen we breathe and the carbon dioxide that plants need. It protects us from the harmful rays of the sun, keeps the temperature of the earth suitable for life, and gives us weather and wind. This chapter explores the composition and structure of the atmosphere, the weather and climate it creates, and the important issues of air pressure, winds, and air pollution.

3. Composition of the Atmosphere

The atmosphere is made up of a mixture of gases. The main gases are:

• Nitrogen: About 78 percent of the atmosphere
• Oxygen: About 21 percent of the atmosphere
• Carbon dioxide: A small but important amount
• Argon: About 0.93 percent
• Other gases: Small amounts of other gases, including water vapour, dust particles, and ozone

Nitrogen is essential for the growth of plants, oxygen is needed for breathing and burning, and carbon dioxide absorbs heat and helps keep the earth warm. Water vapour in the atmosphere is the source of all rainfall. Dust particles in the air help the water vapour to condense into clouds and rain drops.

4. The Structure of the Atmosphere

The atmosphere is divided into five layers based on temperature:

1. Troposphere: The lowest layer, where all weather phenomena occur. It extends to about 13 kilometres above the surface. The temperature decreases as we go higher in the troposphere.
2. Stratosphere: The layer above the troposphere, extending to about 50 kilometres. It contains the ozone layer, which protects us from the sun's harmful ultraviolet rays. Aeroplanes fly in this layer.
3. Mesosphere: The layer above the stratosphere, where most meteors burn up as they enter the earth's atmosphere.
4. Thermosphere (Ionosphere): The layer where radio waves are reflected, making radio communication possible.
5. Exosphere: The outermost layer of the atmosphere, where the atmosphere gradually merges with space.

5. Weather and Climate

Weather

Weather is the day-to-day state of the atmosphere of a place at a particular time. It is described in terms of temperature, humidity, precipitation, wind, and sunshine. Weather changes quickly; it can be sunny in the morning and rainy in the afternoon.

Climate

Climate is the average weather of a place over a long period of time, usually 25 to 30 years. Climate is more permanent and does not change quickly. The weather of a place can change daily, but its climate is a long-term average.

6. Air Pressure and Winds

Air Pressure

Air has weight, and the weight of air pressing down on the earth's surface is called air pressure. Air pressure decreases as we go higher, because there is less air above us. Air pressure also varies with temperature - warm air rises, creating areas of low pressure, while cool air sinks, creating areas of high pressure.

Winds

Air always moves from areas of high pressure to areas of low pressure, and this moving air is called wind. The difference in air pressure is caused by differences in temperature. The winds that blow from the sea to the land during the day are called sea breezes, and those that blow from the land to the sea at night are called land breezes.

There are also large-scale winds like the monsoon winds, which bring rain to the Indian subcontinent during summer, and the trade winds. The direction and speed of winds are important for weather forecasting, navigation, and the distribution of rainfall.

7. The Greenhouse Effect and Global Warming

The Greenhouse Effect

Carbon dioxide and some other gases in the atmosphere absorb heat from the sun and keep the earth warm. This is called the greenhouse effect. Without the greenhouse effect, the earth would be too cold for life. However, when the amount of these gases in the atmosphere increases too much, the earth becomes warmer than necessary, causing global warming.

Global Warming

Global warming is the gradual increase in the temperature of the earth caused by the increase in greenhouse gases, especially carbon dioxide. The burning of fossil fuels like coal and petroleum, the cutting down of forests, and the increasing number of industries and vehicles all add to the carbon dioxide in the atmosphere. Global warming is a serious threat to the planet, as it can melt the polar ice caps, raise sea levels, and change the climate all over the world.

8. Air Pollution

Air pollution is the contamination of the air by harmful substances. The main causes of air pollution are the burning of fossil fuels, smoke from factories and vehicles, and the burning of crop waste. Air pollution is harmful to human health, causing respiratory diseases, and it also damages plants, animals, and buildings.

The ozone layer in the stratosphere protects us from harmful ultraviolet rays, but certain chemicals called chlorofluorocarbons (CFCs), used in refrigerators and air conditioners, are damaging the ozone layer, creating a "hole" in it. Reducing air pollution and protecting the ozone layer are among the most important environmental challenges of our time.

Quick Revision Tables

Table 1: Composition of the Atmosphere

Table 2: Layers of the Atmosphere

Mind Map

Important Diagrams (SVG)

Diagram 1: Layers of the Atmosphere

Diagram 2: The Greenhouse Effect

Common Mistakes

1. Confusing weather with climate; weather changes daily, while climate is the average over 25-30 years.
2. Thinking that oxygen is the most abundant gas in the atmosphere; nitrogen makes up about 78 percent.
3. Believing that aeroplanes fly in the troposphere; they fly in the stratosphere, above the weather.
4. Confusing the layers where meteors burn (mesosphere) and where radio waves are reflected (thermosphere/ionosphere).
5. Believing that air pressure increases with height; it decreases because there is less air above.
6. Thinking that the greenhouse effect is entirely bad; it is natural and necessary, but excess carbon dioxide causes global warming.
7. Forgetting that ozone is in the stratosphere and is damaged by CFCs.

Exam Tips

1. Memorise the composition: nitrogen 78%, oxygen 21% - the two big numbers.
2. List the five layers of the atmosphere in order with one feature each.
3. Distinguish weather (daily) from climate (long-term average of 25-30 years).
4. Explain wind as air moving from high to low pressure, with sea and land breezes as examples.
5. For the greenhouse effect, present both the natural benefit and the danger of global warming.
6. Mention CFCs as the cause of ozone layer depletion.
7. For air pollution questions, list causes and harmful effects, and suggest remedies.

Conclusion

The air around us is a precious mixture of gases that makes life possible. The atmosphere, with its five layers, protects the earth, provides oxygen and carbon dioxide, creates weather, and makes winds blow. Air pressure differences drive the winds that distribute heat and rainfall across the globe. But human activity is threatening this delicate system - the burning of fossil fuels is increasing the greenhouse effect and causing global warming, while CFCs are damaging the ozone layer. Understanding the atmosphere and protecting the air we breathe is essential for the survival of life on earth.