COMPOSITION OF AIR

COMPOSITION OF AIR

We have already talked about the composition of air in the first chapter. It is a mixture of many gases like nitrogen, oxygen, carbon dioxide and water vapour. It is interesting to note that even the composition of air is the result of life on Earth. In planets such as Venus and Mars, where no life is known to exist, the major component of the atmosphere is found to be carbon dioxide. In fact, carbon dioxide constitutes up to 95-97% of the atmosphere on Venus and Mars.

Eukaryotic cells and many prokaryotic cells, discussed in Chapter 5, need oxygen to break down glucose molecules and get energy for their activities. This results in the production of carbon dioxide. Another process which results in the consumption of oxygen and the concomitant production of carbon dioxide is combustion. This includes not just human activities, which burn fuels to get energy, but also forest fires.

Despite this, the percentage of carbon dioxide in our atmosphere is a mere fraction of a percent because carbon dioxide is ‘fixed’ in two ways:

(i)  Green plants convert carbon dioxide into glucose in the presence of Sunlight and

(ii) many marine animals use carbonates dissolved in sea-water to make their shells.

Questions

How is our atmosphere different from the atmospheres on Venus and Mars?

 

Source: This topic is taken from NCERT TEXTBOOK

THE ROLE OF THE ATMOSPHERE IN CLIMATE CONTROL

THE ROLE OF THE ATMOSPHERE IN CLIMATE CONTROL

We have talked of the atmosphere covering the Earth, like a blanket. We know that air is a bad conductor of heat. The atmosphere keeps the average temperature of the Earth fairly steady during the day and even during the course of the whole year. The atmosphere prevents the sudden increase in temperature during the daylight hours. And during the night, it slows down the escape of heat into outer space. Think of the moon, which is about the same distance from the Sun that the Earth is. Despite that, on the surface of the moon, with no atmosphere, the temperature ranges from –190° C to 110° C.

Activity 14.1:

Measure the temperature of the following :

*   Take (i)  a beaker full of water,

        (ii)  a beaker full of soil/sand and

        (iii)  a closed bottle containing a thermometer.

*   Keep them in bright Sunlight for three hours.

*   Now measure the temperature of all 3 vessels.

*   Also, take the temperature reading in shade at the same time.

As we have seen above, sand and water do not heat up at the same rate. What do you think will be their rates of cooling? Can we think of an experiment to test the prediction?

Questions

1.   Is the temperature reading more in activity (i) or (ii)?

2.   Based on the above finding, which would become hot faster – the land or the sea?

3.  Is the thermometer reading of the temperature of air (in shade) the same as the temperature of sand or water? What do you think is the reason for this? And why does the temperature have to be measured in the shade?

4.   Is the temperature of air in the closed glass vessel/bottle the same as the temperature taken in the open air?

           (i) What do you think is the reason for this?

         (ii) Do we ever come across this phenomenon in daily life?

5.   How does the atmosphere act as a blanket?

Source: This topic is taken from NCERT TEXTBOOK

THE MOVEMENT OF AIR: WINDS

THE MOVEMENT OF AIR: WINDS

We have all felt the relief brought by cool evening breezes after a hot day. And sometimes, we are lucky enough to get rains after some days of really hot weather. What causes the movement of air, and what decides whether this movement will be in the form of a gentle breeze, a strong wind or a terrible storm? What brings us the welcome rains?

All these phenomena are the result of changes that take place in our atmosphere due to the heating of air and the formation of water vapour. Water vapour is formed due to the heating of water bodies and the activities of living organisms. The atmosphere can be heated from below by the radiation that is reflected back or re-radiated by the land or water bodies. On being heated, convection currents are set up in the air. In order to gain some understanding of the nature of convection currents, let us perform the following activity:

Activity 14.2:

*   Place a candle in a beaker or wide-mouthed bottle and light it.

*   Light an incense stick and take it to the mouth of the above bottle (Figure 14.1).

*   Which way does the smoke flow when the incense stick is kept near the edge of the mouth?

*   Which way does the smoke flow when the incense stick is kept a little above the candle?

*   Which way does the smoke flow when the incense stick is kept in other regions?

Figure 14.1: Air currents being caused by the uneven heating of air.

The patterns revealed by the smoke show us the directions in which hot and cold air move. In a similar manner, when air is heated by radiation from the heated land or water, it rises. But since land gets heated faster than water, the air over land would also be heated faster than the air over water bodies.

So, if we look at the situation in coastal regions during the day, the air above the land gets heated faster and starts rising. As this air rises, a region of low pressure is created and air over the sea moves into this area of low pressure. The movement of air from one region to the other creates winds. During the day, the direction of the wind would be from the sea to the land.

At night, both land and sea start to cool. Since water cools down slower than the land, the air above water would be warmer than the air above land.

On the basis of the above discussion, what can you say about:

1.   the appearance of areas of low and high pressure in coastal areas at night?

2.   the direction in which air would flow at night in coastal areas?

Similarly, all the movements of air resulting in diverse atmospheric phenomena are caused by the uneven heating of the atmosphere in different regions of the Earth. But various other factors also influence these winds – the rotation of the Earth and the presence of mountain ranges in the paths of the wind are a couple of these factors. We will not go into these factors in detail in this chapter, but think about this: how do the presence of the Himalayas change the flow of a wind blowing from Allahabad towards the north?

Questions

1.   What causes winds?

Source: This topic is taken from NCERT TEXTBOOK

RAIN

RAIN

Let us go back now to the question of how clouds are formed and bring us rain. We could start by doing a simple experiment which demonstrates some of the factors influencing these climatic changes.

Activity 14.3:

*   Take an empty bottle of the sort in which bottled water is sold.

*   Pour about 5-10 mL of water into it and close the bottle tightly.

*   Shake it well or leave it out in the Sun for ten minutes.

*   This causes the air in the bottle to be saturated with water vapour.

*   Now, take a lighted incense stick.

*   Open the cap of the bottle and allow some of the smoke from the incense stick to enter the bottle.

*   Quickly close the bottle once more. Make sure that the cap is fitting tightly.

*   Press the bottle hard between your hands and crush it as much as possible.

*   Wait for a few seconds and release the bottle.

*   Again press the bottle as hard as you can. 

The above experiment replicates, on a very small scale, what happens when air with a very high content of water vapour goes from a region of high pressure to a region of the low pressure or vice versa.

When water bodies are heated during the day, a large amount of water evaporates and goes into the air. Some amount of water vapour also get into the atmosphere because of various biological activities. This air also gets heated. The hot air rises up carrying the water vapour with it. As the air rises, it expands and cools. This cooling causes the water vapour in the air to condense in the form of tiny droplets. This condensation of water is facilitated if some particles could act as the ‘nucleus’ for these drops to form around. Normally dust and other suspended particles in the air perform this function.

Once the water droplets are formed, they grow bigger by the ‘condensation’ of these water droplets. When the drops have grown big and heavy, they fall down in the form of rain. Sometimes, when the temperature of the air is low enough, precipitation may occur in the form of snow, sleet or hail.

Rainfall patterns are decided by the prevailing wind patterns. In large parts of India, rains are mostly brought by the south- west or north-east monsoons. We have also heard weather reports that say ‘depressions’ in the Bay of Bengal have caused rains in some areas (Figure 14.2).

Figure 14.2: Satellite picture showing clouds over India.

Activity 14.4:

*   Collect information from newspapers or weather reports on television about rainfall patterns across the country.

*   Also find out how to construct a rain- gauge and make one.

*   What precautions are necessary in order to get reliable data from this rain-gauge?

*   Now answer the following questions :

          1.  In which month did your city/town/ village get the maximum rainfall?

          2.  In which month did your state/union territory get the maximum rainfall?

          3.  Is rain always accompanied by thunder and lightning? If not, in which season do you get more of thunder and lightning with the rain?

Activity 14.5:

*   Find out more about monsoons and cyclones from the library.

*   Try and find out the rainfall pattern of any other country.

*   Is the monsoon responsible for rains the world over?

Questions

1.   When did you observe that the air inside seemed to become ‘foggy’?

2.   When does this fog disappear?

3.   When is the pressure inside the bottle higher?

4.   Is the ‘fog’ observed when the pressure in the bottle is high or when it is low?

5.   What is the need for smoke particles inside the bottle for this experiment?

6.   What might happen if you do the experiment without the smoke from the incense stick? Now try it and check if the prediction was correct. What might be happening in the above experiment in the absence of smoke particles?

7.   How are clouds formed?

Source: This topic is taken from NCERT TEXTBOOK

AIR POLLUTION

AIR POLLUTION

We keep hearing of the increasing levels of oxides of nitrogen and sulphur in the news. People often bemoan the fact that the quality of air has gone down since their childhood. How is the quality of air affected and how does this change in quality affect us and other life forms?

The fossil fuels like coal and petroleum contain small amounts of nitrogen and sulphur. When these fuels are burnt, nitrogen and sulphur too are burnt and this produces different oxides of nitrogen and sulphur. Not only is the inhalation of these gases dangerous, but they also dissolve in rain to give rise to acid rain. The combustion of fossil fuels also increases the amount of suspended particles in the air. These suspended particles could be unburnt carbon particles or substances called hydrocarbons. Presence of high levels of all these pollutants cause visibility to be lowered, especially in cold weather when water also condenses out of the air. This is known as smog and is a visible indication of air pollution. Studies have shown that regularly breathing air that contains any of these substances increases the incidence of allergies, cancer, and heart diseases. An increase in the content of these harmful substances in the air is called air pollution.

Figure 14.3: Lichen

Activity 14.6:

*   Organisms called lichens are found to be very sensitive to the levels of contaminants like sulphur dioxide in the air.

*   Lichens can be commonly found growing on the barks of trees as a thin greenish-white crust.

*   See if you can find lichen growing on the trees in your locality.

*   Compare the lichen on trees near busy roads and trees some distance away.

*   On the trees near roads, compare the incidence of lichen on the side facing the road and on the side away from the road.

What can you say about the levels of polluting substances near roads and away from roads on the basis of your findings above?

Questions

1.   List any three human activities that you think would lead to air pollution.

Source: This topic is taken from NCERT TEXTBOOK