My Science School

          Most people think that the water in the oceans is still. But it is not so. It is always moving in regular patterns. This movement of water is called ocean currents. There are many kinds of currents. One kind is a ‘stream’. A stream is a current with distinct boundaries. Another kind is a ‘drift’. A drift does not have distinct boundaries. Do you know how ocean currents are produced?

          There are three main reasons for the occurrence of the ocean currents. (1) The density of sea water varies from place to place, because the salt content is not same at different places. Water flows from the regions of higher density to regions of lower density thus producing currents. (2) Sun’s rays fall on the surface of the sea at different angles and as such produce unequal heating. This generates ‘convection currents’ in the sea. (3) Winds blowing on the sea surface push water into current.

          Currents are also caused by the rotation of the earth. Generally, the earth’s rotation produces clockwise currents in the northern hemisphere and counter clockwise currents in the southern hemisphere.

          Amongst these, the Gulf Stream is the most important. The water of this stream is blue and warm. This stream flows from the Gulf of Mexico north to Canada like a river in the middle of the Atlantic Ocean. London and Paris are less cold, because of this stream. It keeps ports in Norway free of ice all year. In addition to these, Brazil current, Japan Current North Equatorial current, North Pacific current etc., are the main ocean currents of the world.

          A cool current like the Labrador Current, which flows from the Arctic to the Atlantic, may carry icebergs into the trans-Atlantic shipping lanes. Ocean currents are important to the plants and animals in the oceans. The movement of water brings them food and nutrients. It also helps them during migration. Currents also affect the climate of the land. Ocean currents move water from warm to cold areas and vice versa.

          The collective name for a complex system of ocean currents flowing in the environs of the equator in Atlantic, Pacific and Indian Ocean is known as ‘Equatorial Current’. The pacific north equatorial current is the part of clock-wise ocean current system with Japan Current, North Pacific current and California current. Similarly in Atlantic, the North Equatorial Current is the part of the clockwise system with Gulf Stream, and Canaries current.

 

          The giant water waves which rise and fall periodically in the sea are called tides. Sometimes these tides are so high that they appear like high walls moving on the surface of the sea. Do you know what causes these tides?

          The main cause behind the formation of these tides in the sea is the force of attraction between the earth and the moon. We know that all the bodies in the universe attract one another with a force known as gravitational force. This force also acts between the earth and the moon. The moon is pulled towards the earth and the earth towards the moon. This force of gravitation acting between the earth and the moon is so strong that it can easily break a steel rod with a diameter of 400 kms. This force affects the solid part of the earth slightly but it causes rise and fall in the sea water noticeably.

           At any point in the sea, the water level keeps on rising for six hours and subsequently for next six hours it keeps on falling. This rise and fall of the water level is known as tide. The water level rises after every 12 hours 25 minutes. This means that during one day and one night, i.e., 24 hours and 50 minutes, every place in the sea experiences tides two times. Tides are formed both in the seas facing the moon as well as those on the other side. Since the moon comes to the opposite side of the earth after 12 hours and 25 minutes, tides are again formed on the same two spots.

          The force of attraction between the sun and the earth also affects the formation of tides but to a very small extent. When the sun, earth and moon come in a straight line, the earth experiences the maximum force of attraction. And the tide caused on such occasions is very large. It is known as the ‘spring tide’. The sea experiences such tides on the full moon and the new moon days. When the sun and moon are in a perpendicular direction, the earth experiences the minimum force of gravitation. The tide formed on such occasions is very small and is called the ‘neap tide’. Such tides occur between the full moon and the new moon days. Tides can be as high as 15 metres.

          Tides are useful in many ways. They bring many precious items of sea, on to the shore. The tidal power of the sea is also being used to generate electricity.  

          Snow is formed of tiny crystals of frozen water that fall from the sky. Snow flakes have an infinite variety of shapes. They are usually hexagonal and may be flat, needle shaped or star shaped. No two snowflakes are exactly alike. Freshly fallen snow reflects about 95% of the sun’s heat back into the space. Do you know how snow fall takes place?

          Due to sun’s heat there is a continuous evaporation of water from the seas, rivers, lakes, ponds etc. Since water vapour is lighter than air, it goes up in the atmosphere and turns into cloud. We know that the temperature decreases as we go higher in the atmosphere. Since the capacity of air to hold water vapour decreases with the fall in its temperature at a certain height a large quantity of water vapour gets accumulated in the air. The air having excess of water vapours is said to be supersaturated. Under this condition the water vapours condense on the dust and smoke particles present in the air. On further cooling, they get converted into snow particles. These particles combine with each other to form crystals of snow. When the air cannot bear their weight, they fall down as snow flakes and go on accumulating on the mountains.

          Now the question arises: why does it snow on the mountains only? Why does it not fall in the plains too? In fact the possibility of snowfall at any place depends upon two factors - the altitude of the place above the sea-level and its distance from the equator. The higher the altitude of the place, the greater is the probability of snowfall there. Similarly the larger its distance from the equator, more are the chances of snowfall there. Even though the amount of snow formed in the atmosphere is very large, only a very small fraction of it falls down on the mountains in the form of snow. The remaining portion comes down as rains because while passing through hotter regions, it melts and turns into water. However, the temperature being low at the mountains, the snow does not melt into water there. It goes on piling up as snow and under the increasing weight of its accumulation layer after layer, it becomes harder and harder.

          Snowfall is very useful for us. When snow melts in summer, the melted water flows to rivers and is used for irrigation purposes. Fresh snow is very light and a good insulator, protecting the underlying plants from severe cold. Snow is a bad conductor of heat because of the air trapped between its particles. As such it acts as a blanket for the earth. In colder regions, people make houses of snow as it provides excellent protection against cold weather. In hilly areas, the roofs of the houses are made in a slanting shape, so that snow does not accumulate there. It saves the inhabitants from the adverse-effects of snow and also prevents the roofs from collapsing.

 

          Before the early part of the 20th century, the only known method of determining the depth of the sea at any point was by means of a cable. The ship stopped and a hemp line or wire with a weight at the end was lowered to the bottom. This was a slow, laborious task and not very accurate.

          Today man has developed innumerable instruments that help him to go deep into the seas and acquire more and more knowledge about their bottoms. Do you know how the depth of the sea is measured with the help of modern instruments?

          The equipment which is used to measure the depth of the sea is called a ‘fathometer’. It is installed on a ship. It produces sound waves having frequencies of more than 20,000 hertz. These are called ultrasonic waves. They cannot be heard by human ears. These waves are transmitted inside the sea. They are reflected back by the bottom of the sea. The reflected waves are received by a receiver and the total time taken by them to travel from the sea surface to the bottom and back to the surface is measured. Half of this time multiplied by the velocity of sound in sea water gives the depth of the sea at the point of the experiment. Thus the depth of the sea can be measured at any point. This technique is known as echo sounding or echo ranging. 

          This technique has been used to measure the depths of various seas and the deepest spot in each of the seas has been located using this technique. The Pacific Ocean is the deepest ocean. The average depth of this ocean is 4,282 metres (14,048 feet). The deepest spot in this ocean is located near Guam and has a depth of 10,668 metres (35,000 feet). Guam is an island in the Pacific Ocean. It is about 5,000 kms (300 miles) to the west of the Hawaii islands. A more recent survey located a spot with a depth of 10,924 metres (35,839 ft.) — as per a data obtained by Japan Maritime Safety Agency, using a narrow multi-beam echo sounder. Using depth as a criterion, the Indian Ocean occupies the second place. Its average depth is 3963 metres (13,002 feet). The Atlantic Ocean, coming third, has an average depth of 3926 metres (12,880 feet). Amongst the shallow seas, the Baltic Sea occupies the first place. Its average depth is 55 metres (1 80 feet).

            Studies of the oceanic crust have revealed that they are also as uneven as the surface of the earth. They are full of the skeletons of the aquatic animals as well as big and small sea plants. Volcanic ash is also found there.

          The most modern method of studying the oceanic crust is to use the laser beams. The laser beams are transmitted into the water of the ocean and the time taken by them from the surface to the bottom and back is measured. On multiplying the velocity of speed of light with this time, one gets twice the distance of the bottom of the sea. This method is very accurate. 

          In the Southern Hemisphere, the Antarctic ice sheet overflows its land support to form shelves of ice on the sea; huge pieces, as much as 200 km. across, break off to form icebergs. In Northern Hemisphere icebergs are generally not over 150 metres across. However most icebergs are from some 20 glaciers on the west coast of Greenland. When glaciers, drifting from mountains and valleys, reach the sea, they got broken into big pieces by sea waves and tides. These big pieces of ice are called icebergs. However there are some glaciers which do not break even after floating for long distances in the sea and appear like mountains of ice.

          The sizes of the different icebergs vary. The smallest iceberg measures 5 to 6 metres in length and breadth but the bigger ones might be hundreds of metres long. Some icebergs with lengths and breadths of half a kilometre have been seen floating in the oceans. They float in sea water because ice is lighter than water. Their one-tenth part remains above water and the remaining under water. For example, if a 50 metre high iceberg is seen above water, then 450 metres of it will be under water. These icebergs contain huge quantities of snow. Some of them may contain 200,000,000 tons of ice. As an iceberg floats, some of the ice melts and pieces break off. Eventually, it completely disappears.

          Because of their huge weight and volume, the icebergs do not move in the sea on their own. They are pushed ahead by the sea waves. They are very dangerous for ships. A collision with them can cause a ship wreck. Though they can be detected with the help of modern instruments, yet mishaps do take place occasionally. On 14th April 1912, a ship named Titanic was broken to pieces when it collided with an iceberg. This ship was sailing to New York; 1513 passengers, who were aboard this ship lost their lives.

          A similar accident took place on January 30, 1959 when the ship Hans Hedtoft collided with an iceberg in southern Greenland and was wrecked. Now the United States and other countries have formed an International Ice Patrol. The patrol uses ships, planes and radars to locate icebergs.

 

          We know that air is the mixture of oxygen, nitrogen, carbon dioxide, water vapour and dust particles. Air in motion is called the wind. Do you know how the wind blows?

          When any place on the earth gets heated by the sun’s rays, the air of that place also gets heated. Due to this heating the air expands and hence its density decreases or it becomes lighter. The hot air, due to this lightness, goes up in the atmosphere. And this causes a decrease in atmospheric pressure in that area. Under such a condition air from high pressure cold regions rushes to that place to create an equilibrium. This movement of air is called the blowing of wind.

          In the areas adjoining the sea, the earth becomes hot during the day. Due to this air becomes lighter and goes up in the atmosphere. To restore balance, cold air from the sea blows towards the land. During the night, a reverse movement takes place, that is, the earth becomes colder than the sea water and as such the air moves from the land towards the sea.

          It is very hot in the regions surrounding the equator. It creates an equatorial belt of low pressure. So there is a constant upward movement of hot air from these regions. This hot air flows out to the north and south.

          The rotation of the earth in its axis considerably affects the directions of the winds. The westerly winds are the direct result of the earth’s rotation from the west to the east. The spinning of the earth makes all in the northern hemisphere deflect towards the right and in the southern hemisphere, towards the left. Superimposed on the general wind systems are local winds. These are caused by temperature differentials associated with topographical features such as mountains and coastal belts. The presence of mountains also influences the wind direction. The mountains obstruct the winds and change their directions. The speed and direction of wind is measured by an instrument called an anemometer.