My Science School

          Man has always been curious to know about the internal structure of the Earth. He tried digging and other direct methods and, finally, found out an indirect method to know the interior of the Earth - through the study of earthquake vibrations of Seismic waves. Studies reveal that our Earth has three main layers - the outer surface on which we live is called the ‘Earth’s crust’, below it is the ‘mantle’ and then comes the ‘core’.

          The outer layer or the Earth’s crust, has two sub-layers - the first layer is the lighter one and is called Sima (for silica – magnesium) while the second sub – layer is heavier than the first and is called ‘Sial’ (for silica - aluminium). Thus, the Earth’s crust is mainly composed of silica. Its depth varies from 16 km to 50 km on land and about 5 km under the oceans. The volume of this crust is only 1% of the Earth’s volume, while its weight is around 4% of the Earth’s total weight. As we go down the Earth’s crust, the temperature increases. At every 35 metres in depth, there is an increase of about 1° C. At a depth of 3 km, the temperature is around 100° C (boiling point of water) and at 50 km, the temperature is 1,200° C - hot enough to melt rocks.

          The next layer below the crust is called ‘mantle’. It is 2,880 km thick. It is mainly composed of silicon magnesia and iron. The rocks in the mantle are denser than sial and sima. Its total volume is 84% of the Earth’s volume. Its weight is around 67% of the Earth’s weight.

          The central portion of the Earth is called ‘core’ which is made up of high-density solid materials, in Free State or in combined form in rocks. Its thickness is about 3482 km. Its temperature is around 4800° C. Its volume is 15% of that of the Earth, while its weight is 32% of the Earth’s weight. This solid core is surrounded from all sides by molten iron and nickel. Its temperature is around 3900°C. The thickness of this molten mass is around 2,240 km. The centre of the Earth is some 6,336 km from the Earth’s surface.

 

          When we want to know the mass of a particular object we weigh it in a balance. The size of the balance varies with the size of the object to be weighed. But Earth is a giant body and to think of a balance for weighing it is almost impossible. Then, what is the way out? Scientists have simplified matters in this regard. Newton’s law of gravitation is used to determine Earth’s mass. According to this law, there exists a force of attraction between any two bodies of the universe and this is dependent on the masses of the two bodies as well as on the distance between them. The force of attraction is directly proportional to the product of the masses of the two bodies, and inversely proportional to the square of the distance between them.

          An experiment is performed to determine Earth’s mass with the help of the above mentioned law. In this experiment, a small metallic ball is suspended with the help of a thin thread. The position of this ball is accurately determined. Now a big lead-ball weighing a ton is brought near this ball. The small ball is attracted towards the big ball due to the gravitational pull and as such it is slightly displaced from its initial position in respect to the big ball. The change in the position of the small ball is even less than one-tenth of an inch. This displacement is very accurately measured with the help of the precision instruments. Using the value of this displacement in a formula of physics, the mass of the Earth is calculated. The mass of the Earth has been found to be (598, 000, 000, 000, 000, 000, 000, 0) tons. 

               Anything that is dropped falls to the ground; it does not go towards the sky. Similarly fruits from trees fall on the ground. Why does it happen like this? Questions like this used to bother the ancient man also, but he could not find their answers. But today, it is an accepted fact that the Earth attracts everything towards its centre. That is why the fruits from trees or the ball thrown up, are all attracted towards the Earth. This invisible force of attraction between the Earth and any other body is called the force of gravity. The centre of gravity of the Earth lies in its centre. Imagine what would happen if a hole is drilled in the Earth from one side to the other, passing through its centre and a ball is dropped in this hole. The ball, in fact, will stop at the centre of Earth; it will not go to the other side. The weight of a body will be more, if it is nearer the centre of gravity of the Earth. Similarly the weight will be less if the body is away from it. This is why a body weighs more at the poles than at the equator, since the poles are nearer the centre than the places on the equator. Not only the Earth, but all other planets also have this force of gravity. As a matter of fact, every body in this universe attracts every other body with this force of gravitation and it is this force which keeps all the planets and stars suspended in the sky. It is this gravitational pull that keeps the moon revolving round the Earth and the Earth revolving round the sun. Naturally the moon also attracts the Earth and tides in the sea are mainly due to the gravitational pull of the moon. 

               Up to the end of the fifteenth century, it was assumed that if two bodies were simultaneously dropped from the same height in vacuum, the heavier body will hit the ground first. But this assumption was baseless. The famous scientist Galileo proved it in 1590 for the first time that irrespective of their masses, all the objects dropped simultaneously from the same point in a vacuum will reach the ground at the same time. From the Leaning Tower of Pisa, he dropped one ball of 100 pounds and another of just half pound at the same time and he demonstrated it in the presence of thousands of people that both the balls hit to the ground simultaneously.

               Subsequently Newton propounded the law of gravitation. According to this law, the force of attraction between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. It follows from this that the force of attraction will be doubled if the mass of one of the two bodies is doubled. On the other hand if the distance between them is doubled, the force will be reduced to one-fourth of the initial value.

               Scientists have not been able to fully explain the existence of the gravitational force of the Earth or of other heavenly bodies. The velocity of a freely falling body towards the Earth increases by 9.8 metres or 32 feet every second. This is called acceleration due to gravity.

 

          Till a few hundred years ago, it was believed that the Earth is the centre of the universe and that the Sun, the Moon and the stars revolve around it. This fact was based on the observation that the Earth is stationary while the position of stars is changing. In 1545, a Polish astronomer Copernicus suggested that the Earth revolves round the Sun. It was proved that it revolves round the Sun and completes one revolution in 365 days. This period is called a year. Secondly, it also rotates on its own axis and takes 24 hours to complete one rotation.

          Naturally the question arises: if the Earth moves, why don’t we feel its motion? The answer is: because of gravity, all the things situated on the Earth including the atmosphere move with the Earth and hence we can’t feel it is moving. You can understand this in a different way. If you rotate a football with an ant on it, the ant will not feel that the ball is rotating. Exactly like the ant on the football, we are situated on the surface of the Earth and we don’t feel the movement of the Earth. 

          The biggest proof of the Earth’s motion is the change in seasons. Seasons occur due to the Earth’s motion round the Sun as well as due to its rotating on its own axis. Day and night are caused by the Earth’s motion on its axis. The portion of the Earth which faces the sun experiences day while the remaining portion has night. If the Earth did not rotate on its axis, the part of the Earth facing the sun would always have had day while the rest would have had night for ever. The Earth’s axis makes an angle of 23° with the vertical. As such each pole faces the sun continuously for six months and for the next six months, it does not. This explains the six monthly duration of day and night on the poles. All these observations confirm the motion of the Earth round the Sun as well as on its own axis.

 

          Climate is the average weather experienced by an area over a period of years. It depends upon the temperature, rains, atmospheric pressure, wind directions, mountains, height from the sea level and latitude of that place. Different kinds of climate are found in different parts of the world. Different instruments like thermometer, barometer, rain gauge, etc. are used to study the climate, atmospheric pressure, wind directions, rains, clouds, humidity etc.

          The world has been divided into 12 major climatic regions. For convenience, these climatic regions have been grouped into three on the basis of their latitudinal positions and extent. These are - the low latitude climatic zone, the mid-latitude climatic zone and the high latitude climatic zone. 

 

Low latitude climatic zone: It includes the humid tropical region, trade wind coastal region, tropical desert and steppe, tropical monsoon and savanna regions. The temperature in this region is very high and dry. In all seasons, there are heavy rains.

 

 

 

 

Mid-latitude climatic zone: It includes China type. West European type, Mediterranean, mid-latitude desert and steppe and Manhuria type climatic region. The summers are less hot, but the winters are very cold.

 

 

 

 

High latitude climatic zone: It includes Taiga type, Tundra type, ice-cap and high mountain type regions. In these places, temperature is very low during the winter and cold even during the summer.

 

 

            The 12 major subdivisions of climate within the zones are: 1. tropical wet, 2. tropical wet and dry, 3. highlands, 4. desert, 5. steppe, 6. subtropical, 7. subtropical moist, 8. oceanic moist, 9. continental moist, 10. Subarctic, 11. Polar and 12. Ice cap. Climate influences the types of houses we live in, the clothes we wear, the food we eat and the type of transportation we use. Climate differs due to the differences in latitude, land and water temperatures and surfaces of land.

 

 

 

          We know that the Earth revolves round the Sun and also rotates on its own axis. Days and nights are caused by the rotation of the Earth on its axis. The axis of the Earth makes an angle of 23° with the vertical. It is this inclination which causes changes in seasons. With its inclined axis, when the Earth revolves round the Sun, the Sun rays make different angles at the same place at different times. Due to the variations in angles, the distribution of the solar heat is not the same at the same place. This uneven distribution of solar heat on the Earth leads to the summer or winter season.

          If we look at the picture, we see that in June, when the northern hemisphere is tilted towards the Sun, it is summer in Europe, Asia and North America (northern hemisphere) and winter in the southern hemisphere. Six months later, in December, the southern hemisphere is tilted towards the Sun, so it is summer in southern hemisphere but winter in northern hemisphere.

          On 21st March and 23rd September every year, the Sun is exactly over the equator. On these two days, the duration of the day and the night is the same (12 hours) at every place on the Earth. From 21st March to 21st June, the Sun advances from the equator to the Tropic of Cancer. This results in hot season in the northern hemisphere, where days become longer and nights shorter. During this period, it is winter in the southern hemisphere. From 21st June to 22nd December, the Sun advances towards the tropic of capricon. This causes the summer season in the southern hemisphere and winter in the northern hemisphere. In the northern hemisphere, the days are shorter and the nights are longer during this period. After 22nd December, the Sun again starts moving towards the north and reaches the equator again on 21st March. During this period, the days in northern hemisphere start getting longer and the nights shorter.

          In March and September the Sun is overhead at the equator. Both hemispheres are enjoying either autumn or spring.

          Thus, the revolution of the Earth round the Sun and its rotation on its own inclined axis changes the seasons as well as the duration of the days and the nights.

            Atmosphere is the mass of air that envelops the Earth from all sides. It contains many gases and particles of various materials. Amongst the gases, nitrogen constitutes 78.1%, oxygen 21%, carbon dioxide and argon 0.03% and 0.9% respectively of the atmosphere. Along with these, the atmosphere also contains minute particles of water vapour, methane, nitrous oxide, carbon monoxide, hydrogen, ozone, helium, neon, and krypton and xenon gases. In addition, sand-particles, smoke, salt-particles, volcanic ash-particles, meteoric dust and pollen are also present in the atmosphere.

            The atmosphere is quite dense near the Earth’s surface, but becomes rarefied as one goes above it. It is estimated that the atmosphere extends up to a height of 1,000 km.

            It is made up of many layers. The pressure, density and temperature of the atmosphere vary with its distance from the Earth. At a height of 6 km, the air pressure is reduced to half of what it is at the Earth’s surface. Similarly, the temperature falls by 1°F for every 91 metres.

             On the basis of its physical properties, the atmosphere has been divided into the following five layers:

1. Troposphere: Troposphere extends from the Earth’s surface to a height of 17 km. It accounts for 75% of the total weight of the atmosphere. Almost all the living beings live in this part. As one goes up, the temperature decreases and becomes the minimum at a height of 10 km. Rains, clouds, storms and snow form in this very part. This is the most important atmospheric layer for living beings.

2. Stratosphere: Stratosphere extends up to a height of 48 km. Its upper portion contains ozone which absorbs ultra-violet rays coming from the sun. These rays are very dangerous for life. There are neither strong winds nor varying temperatures in this part.

3. Mesosphere: Mesosphere starts after a height of 50 km. Here the temperature is considerably low and it is the minimum at a height of 85 km.

4. Ionosphere: Atmospheric layer above the mesosphere and up to a height of about 500 km is called Ionosphere. It contains only charged particles. These charged particles reflect radio waves towards the Earth and make radio communication possible.

5. Exosphere: It is the outermost layer of the atmosphere. In this, the density of the atmosphere is very low. This part contains helium and hydrogen. So, the temperature is very high here.

             The atmosphere is extremely useful for life. Without it we cannot survive. It protects us from the dangerous radiations of the Sun. The meteors also get destroyed after getting burned due to the atmospheric friction.

 

            Air envelopes the entire surface of Earth. It is invisible, tasteless and has no smell. Air extends great distances above the Earth. One half of the air, by weight, is within 5.63 km of the earth’s surface. The other half is spread over hundreds of kilometres beyond that.

            Air is essential for life. No living being - plant or animal - can survive without it. It gives us energy and plants get their food from the carbon dioxide present in it.

            What is air? It is a mixture of various gases - oxygen, nitrogen, carbon dioxide, inert gases (helium, neon, argon etc.) and water vapours etc. It contains 78% nitrogen, 21% oxygen, and 1% other gases.

            First of all, oxygen helps in burning. If there is no oxygen in the air, nothing will burn. Nitrogen, too, is a very useful gas. Molecular nitrogen is inert because of the strong triple bond between the two atoms, but it will react with some elements, especially the alkaline-earth metals, to give nitrides; with oxygen, and hydrogen. On the other hand, activated nitrogen, formed in an electric discharge, consists of nitrogen atoms and is much more reactive. Nitrogen compounds in the form of fertilizers are very useful for trees and plants. Carbon dioxide is used by plants for breathing. The water vapours in the air help to produce rain. The quantity of water vapour in the air varies from place to place. It determines the amount of rainfall at a place. 

            As we go up the Earth’s surface, the pressure of the air decreases. The atmospheric pressure at the mountains is less because the molecules of air are separated from each other by larger distances. Therefore air is light there. This is why mountaineers need extra oxygen for breathing and carry oxygen cylinders with them.

               The Earth we live on is enveloped on all sides by air. Air is a gaseous state of matter. It is a mixture of nitrogen, oxygen, carbon dioxide, dust particles, water vapours, etc. which are all made up of molecules. That means air is a mixture of molecules of many kinds. We know that molecules have their own weight and therefore air which is composed of all these molecules, also has weight.

            This fact can be proved by a simple experiment. Take an empty football bladder and a small cord to tie around its neck. Weigh the bladder and the cord in a balance. Now fill the bladder with air and tie its neck with the cord and weigh it again, it weighs more than it weighed when it was empty. Thus, increase in the weight of the bladder is due to the air filled in it. This proves that air has weight. At sea level 0.03 cu m (1 cu ft) of air weighs 0.037 kg.

 

            Because of the weight of air, the atmosphere exerts pressure on every object. This pressure is about 1 kg per sq. cm. Our palm measuring nearly 80 sq. cm experiences a force of 82 kg. If we calculate the total force exerted by it on our body, we find that it is more than the weight of three elephants. We do not feel it because our bodies are supported by equal pressure on the inside of our bodies.          

           On mountain tops, this pressure is very low. This is because air pressure decreases with the increase in height. But the pressure (inside the body) remains the same at those heights. If the atmospheric pressure is very low as compared to the blood pressure, blood can burst out of nose or ears.

 

            Wind is moving air. Slow winds are gentle breezes. Fast winds are gales. You can see the speed of the wind by its effect on trees and buildings, but can not measure it because it is invisible.

            The instrument used for measuring the velocity of wind is called the ‘anemometer’ or wind-gauge. It was invented by the English scientist Robert Hooke in 1667. There are many kinds of anemometers. Most of them consist of three or four small aluminium cups attached to an axle. These cups can freely rotate on being struck by winds. The higher the velocity of the wind, the faster is the speed of rotation of these cups. The rate of rotation of these cups in a fixed time is used to calculate the velocity of wind. This instrument has a meter whose pointer gives the speed of rotation of the cups. This meter is so calibrated that it directly gives the velocity of the wind.

             You might be wondering why it is necessary to measure the velocity of wind. When man started flying in aeroplanes, it became essential for him to know the wind’s velocity. Initially he used to measure it by flying balloons in the sky, but with the invention of anemometers, it became easier. Scientists have succeeded in developing small anemometers of the size of 10 to 12 cm. Knowledge of the velocity of wind has proved very useful in meteorological studies especially in weather forecasting. Sailors also need to know the speed of wind.

We all know that clouds give us rain. Whenever the sky is cloudy, we know that it is going to rain. The formation of raindrops in clouds is a long and complicated process of nature. We can understand this natural process in the following way.

During summer, the water of rivers, lakes and seas is converted into vapour by the heat of the sun. These vapour mixed with the air give hot and humid air. Since water vapours are lighter than air, they go on rising high in the sky and even a slight decrease in the surrounding temperature changes them into clouds. We know that there are dust particles in air. When clouds move from hotter regions to colder ones, water vapours start condensing on these dust particles, forming droplets of rain in the clouds. Only when the size of these droplets increases due to further condensation, they start falling down on the Earth due to the force of gravity, and we say that it is raining.

Thus for rains, it is essential that hot and humid air should be lifted up so that water vapour get condensed. Lifting of air takes place in several ways such as orographic lifting, frontal lifting and convectional lifting. Orographic lifting occurs when air is forced upwards by a natural barrier, like a mountain. In frontal lifting cool air of one air mass pushes beneath the warm air of the other air mass. Convectional lifting occurs due to the heat of the sun. Do you know that the highest rainfall of the world is at Cherapunji (Assam) in India? The average annual rainfall of this place is 1,200 cm. In 1861 the total rainfall recorded here was 2,175 cm.

In colder regions, condensation of water vapour takes place not only on the dust particles, but also on the small particles of ice formed in the clouds. Condensation also takes place on ions produced by lightning in the air. Water condensed on ice particles also falls down on the Earth in the form of rains. 

          The word ‘monsoon’ is derived from the Arabic word ‘mausim’ meaning a season. It is a seasonal wind of South Asia blowing in summer from the ocean towards the land and in winter from the land to the ocean.

          In South Asia, the wind from the Indian Ocean blowing towards the shore is the monsoon wind. This wind indicates the chances of rain. Monsoon is of two kinds: (i) southwestern or summer monsoon, and (ii) northeastern or winter monsoon. India gets 90% of its total rainfall from the summer monsoon. These winds advance from the Indian Ocean towards the shore in mid June and, after being obstructed by the Himalayas, cause rains in the plains. Contrary to this, in Central Asia and north India, very cold, dry and strong winds blow off-shore in winter. They are called winter monsoon or the retreating monsoon. They cause some rain in the coastal areas.

            It is interesting to see how monsoon winds change the coastal weather. It is a scientific truth that big sub-continents warm up or cool off much faster than the seas adjoining them. The areas of central and south Asia start warming up in the spring and by summer they become very hot in comparison to the Indian Ocean in the south and the Pacific Ocean in the east. Due to the high temperature, there is a reduction in the air pressure on the land and, consequently, winds start blowing from the sea to the shore. This is the summer monsoon. With the onset of the autumn, the entire Asia starts cooling fast and by the start of winter, the temperature is much less than that of the adjoining oceans. This increases the atmospheric pressure and, as such, in the winter, monsoon winds start blowing from the dry shore to the seas. The south and East Asia have a monsoon climate because of their large areas of land.

 

          The formation of clouds has always been very important to man. You must be curious to know how the clouds are formed and how many kinds of clouds are there.

          We know that the water of rivers, ponds, lakes and seas is converted into vapour by the solar heat and this vapour moves in air. Hot air, mixed with water vapour, being lighter, goes up high in the sky. When the air mixed with water vapour accumulates at one place, it gives the appearance of smoke. We call it a cloud. 

          On the basis of their different shapes and sizes, clouds have been divided into mainly four kinds:

1. Cirrus clouds: Cirrus clouds are formed at great heights. They are white in colour and look like birds’ feathers. Their height ranges from 8,000 to 11,000 metres. They are made up of small ice particles.

 

 

 

2. Stratus clouds: Stratus clouds are formed at a height of about 2,438 metres. They look like layers of fog. They foretell bad weather and drizzle.

 

Continue reading "How are clouds formed? "

             When wind moves with great speed and creates disturbance in the atmosphere, it is called a storm. Storms of summer are mainly caused by the rise in atmospheric temperature. Due to high temperature, air expands and there is a fall in the atmospheric pressure in that area. To balance it, air from colder regions where there is more pressure, rushes to the hotter regions. Carrying with it a large amount of dust and we call it a dust-storm.

              Tornadoes, cyclones and hurricanes are some of the dangerous storms in which wind makes fast whirling motion. When the temperature of a small area rises to a high degree and, consequently, the atmospheric pressure falls, air from surrounding areas rushes in to fill this vacuum. Because of its fast velocity, wind’s motion in that area becomes whirling. The hot air goes up fast, forming a funnel-shaped cloud. This is called a tornado. The pressure inside a tornado is so low that any object coming in its way gets sucked in. Wind velocity of a tornado goes up to 200 kilometres per hour.

 

               Hurricanes generally belong to tropical areas. They engulf areas within a radius of 80 to 320 kilometers and the wind-speed goes up to 120 to 200 km per hour. The central portion of the hurricane extending from 5 to 15 kilometers is completely calm. This is called the eye of the hurricane. When this portion reaches a particular area the air there comes to a standstill: This gives the impression that the hurricane is over. But, as soon as it leaves the area, it is followed by strong gusts of wind. Since hurricane also is a kind of whirling storm, it advances with a very fast whirling motion. It is called typhoon in east India and in the areas adjoining the Chinese Sea.

 

                The cyclone is also a very dangerous storm. It devastates very large areas. It is caused by the rush of air towards low-pressure areas resulting in large-scale destruction. The violent cyclone which hit East Pakistan (now Bangladesh) on November 13-14, 1970 claimed lives of one million people.

 

             Occasionally, you must have seen, some round balls of ice accompanying heavy rains. These round balls are called hails. They are seen more often in summer than in winter. Hailstones are of varying sizes. You may be curious to know how hails are formed in the atmosphere!

              When raindrops start falling from the clouds towards the Earth, sometimes they have to pass through very cold regions and due to this low temperature, these raindrops get frozen. These frozen raindrops are called snow flakes. Sometimes these snow flakes are lifted up by strong winds to regions already having raindrops. As a consequence they get stuck to these snow flakes. When they fall through colder regions of the atmosphere, they are again frozen. This way the size of the frozen raindrops goes on increasing. When the weight of these frozen drops is more than the up thrust of the air, they fall down on the Earth in the form of hailstones.

            If you cut a hailstone into slices, you will notice many layers of transparent and semi-transparent snow. These layers are formed by the repeated freezing of the water-drops. The diameter of hailstones varies from 1 cm to 8 cm. A hailstone may weigh even more than half a kilogram. On 6 July, 1928, a very big hailstone fell at a place named Potter, Nebraska. It weighed 717 gm and had a diameter of 15 cm.

            Hailstones can cause extensive damage. It has been observed that some animals and human beings have even succumbed to the injuries caused by hailstones. Hailstones are very damaging to the crops - they destroy the standing crops. On 30th April 1886, 246 people died of injuries caused by hailstones in Moradabad in UP (India).