ICSE Class 6 Geography Chapter 11 Hydrosphere

Chapter Eleven: Hydrosphere

Meaning Of Hydrosphere

The earth appears blue when viewed from outer space. This is because approximately 71 percent of the earth is covered in water. Hydrosphere refers to the combined mass of water found on the earth such as in oceans, rivers, streams, lakes, in the atmosphere and underground. The movement of water around and through the earth is called water cycle, a key process of the hydrosphere.

Teacher's Note

The water cycle is constantly happening around us - when you boil water for tea, you're watching evaporation in action, and the steam condensing on your kitchen window is precipitation.

Main Sources Of Water On The Earth

The main sources of water on the earth are the following:

(a) Ocean Water: About 97 per cent of the earth's water supply is in the oceans which is unfit for human consumption and other uses due to high salt content. Of the remaining 3 per cent, 2.3 per cent is locked in the polar ice caps. The balance 0.7 per cent is available as fresh water but 0.66 per cent is ground water and the rest 0.03 per cent is available to us as fresh water in rivers, lakes and streams. Thus, we see that we have a very limited stock of usable water. 0.03 per cent surface water (rivers, streams and ponds) and 0.66 per cent ground water.

(b) Water On Land: Water on land is the result of precipitation or seepage from underground which forms streams and rivers flowing on land and finally joining the sea.

(c) Underground Water: The water that is from rain or snow flows on the surface. Some of the water evaporates and the rest sinks into the soil. The water which gets collected under the surface of the land is known as underground water which remains in soil, sub-soil or bedrock. Most of the water ultimately reaches the sea. An underground water stream that is saturated with water and transmits water readily is known as an aquifer.

(d) Water In Atmosphere: Water is also present in the form of vapours in the air which gives rise to humidity. Water evaporates from oceans and other water bodies due to heating of surface water. Water vapours condense, form clouds or fog and finally precipitate, causing rain or snowfall.

Teacher's Note

You can observe the water cycle in a simple way by noticing how puddles disappear after rain (evaporation), how clouds form on humid days, and how rain returns water to the ground.

Tides - Formation And Pattern

Tides - formation and pattern

Waves

Waves are oscillatory movements in water, manifested by an alternate rise and fall of the sea surface. These moving ridge-like curves on the surface of the sea are mainly produced by winds on the surface of the oceans.

Teacher's Note

When you watch waves at a beach or even in a bathtub, you're seeing the direct effect of wind and water surface tension working together to create these beautiful patterns.

Tides

The rise and fall of sea water due to gravitational forces of the sun and the moon are called tides. Tidal currents affect the whole water mass from the sea surface to the bottom. The sea waves produced by tides are called tidal waves. The rise of sea water and its movement towards the coast is called tide and the resultant high water level is known as high tide water. The fall of sea water and its movement towards the sea is called ebb and the resultant low water level is called low tide water. The difference between high tide water and low tide water is called tide range.

Factors That Cause Tides

The tides have their origin in the gravitational forces of the sun and the moon. The earth rotates from west to east and revolves round the sun following an elliptical orbit. Similarly, the moon, rotates from west to east and revolves round the earth along an elliptical orbit so that the distance between the moon and the earth changes during different times every month. The period of the farthest distance between the moon and the earth is called apogee while the period of the nearest distance is called perigee.

The surface of the earth facing the moon experiences maximum gravitational force of the moon, while it will be minimum at the opposite side of the earth. Consequently, the water on the earth's surface facing the moon is attracted and pulled up thus, the high tide occurs. High tide is also formed at the opposite side of the earth simultaneously, because of the reactionary (centrifugal) force of the gravitational (centripetal) force of the moon causing outward bulge of the water.

Nature Of The Tides

Two tides and ebbs are experienced twice at every place on the earth's water surface in 24 hours.

High Tides: When the sun, the earth and the moon are in the same line (at the time of full moon and new moon) their gravitational forces work together and high tides are formed.

Low Tides: When the sun and the moon are at the position of right angle with reference to the earth, the gravitational forces of the sun and the moon work against each other and hence low tides are formed (Fig. 11.5). This situation occurs during the 8th day of each fortnight of a month.

(ii) Neap Tides: The sun, the earth and the moon come in the position of quadrature (i.e., form right angle) on the seventh or eighth day of every fortnight of a month. The tide producing forces of the sun and the moon work in opposite direction, with the result, a low tide occurs. Such a tide, which is lower in height than that of the normal tide, is called neap tide. The height of neap tides is generally 20 per cent lower than that of the normal tides.

Tidal range is not constant but changes depending on the locations of the Moon and Sun. The most extreme tidal range occurs during spring tides, when the gravitational forces are almost in the same line. Such high tides are called Spring Tides. The gravitational forces of the sun and the moon work together with combined force and thus a high tide is caused. The height of such spring tides is 20 per cent more than that of the normal tides. Such tides take place twice every month (during the full moon and the new moon) and their timing is fixed.

Teacher's Note

Tides affect fishermen and coastal communities significantly - they determine the best times for fishing, navigation, and even access to certain coastal areas.

Periodicity Of Tides

On an average, every place experiences tides twice a day. Since the earth completes its rotation in roughly 24 hours, every place should experience tide after 12 hours but this never happens. Each day tide is delayed by 26 minutes because the moon also rotates on its axis (west to east) while revolving round the earth. Since the earth rotates from west to east, the tide centre shifts westward. When the tide centre completes one round, the moon's position is ahead of the tide centre by that time. The moon also revolves round the earth, with the result, the tide centre takes another 52 minutes to come under the moon. Thus, a particular tide centre takes 24 hours 52 minutes to come under the moon but by that time there is another tide at the opposite side of the referred tide centre and this happens after 12 hours 26 minutes.

Types Of Tides

The oceanic tides are caused due to tide producing forces of the sun and the moon. There is a lot of temporal and spatial variation in the tide producing forces because of different positions of the sun and the moon with the earth. Because of variations in the intensity of tide producing forces several types of tides are caused. Two important types of tides are given below.

(i) Spring Tides: Very high tide is caused when the sun, the moon and the earth are almost in the same line. Such high tides are called Spring Tides. The gravitational forces of the sun and the moon work together with combined force and thus a high tide is caused. The height of such spring tides is 20 per cent more than that of the normal tides. Such tides take place twice every month (during the full moon and the new moon) and their timing is fixed.

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