Geography Notes - for Competitive Exams

Part - 2

Tides refer to the phenomenon of regular rise and fall of the level of water in the seas and oceans. Water in the oceans and seas rises and falls twice a day, the successive high tides being abut 121/2 hours apart.

In between the two high tides occur two low tides when the levels of the water in these water bodies become lower.

Tides are primarily a result of the attraction of the moon. This force is more potent upon water since the mass involved is liquid and can easily rise under the pull exerted by the moon.

High tides are of a greater magnitude than normal when the tide-producing forces of the moon and the sun are acting in such a way that they complement each other. Such tides of higher magnitude are called spring tides.

Spring tides occur on the days of full moon and new moon when the sun, moon and the earth in a straight line.

Tides of lower magnitude are called neap tides. With regards to neap tides, the tide-producing forces of the sun and the moon act in such a manner that they neutralize each other, hence the tidal range is much lower than normal.

Neap tides are observed at the time of the first and the last quarters of the moon (half moon) when the sun and the moon form three corners of a triangle and the tide producing forces of the sun and the moon act at right angles to each other.

The tides can be between 15 to 20 per cent greater or lesser than the average depending upon whether the moon is in perigee or apogee. These are called as perigean tides and apogean tides.

Perigean tides are stronger than the apogean tides, as the distance of the moon and earth is short during the perigee when compared to apogee.

The difference between the water levels at the time of the high tide and the low tide is called as tidal range. The tidal range varies not only from time to rime but also from place to place.

Generally, a higher tidal range is observed in narrow bays and gulfs where a large amount of water may be pushed into a narrow area by tidal waves originating in open seas.

Bay of Fundy (bay on the Atlantic coast of North America) has the highest tidal range in the world.

The head of the Bay of Bengal and the Gulf of Katchch have a higher tidal range along the Indian coast.

Locations experiencing a high tidal range are ideal for generating tidal energy.

Tides also affect the water level of rivers, estuaries etc.

Sometimes the rising water of the seas and oceans may advance upstream in rivers as a nearly vertical wall several feet high. This is known as tidal bore.

The mouth of the river Hooghly in West Bengal is well known for the phenomenon of tidal bore.

The term atmosphere refers to the mass of air surrounding the earth up to a height of several hundreds of kilometres. It is this envelope of air that has made life possible on earth.

The atmosphere is held close to earth because of the earth's gravitational force.

The structure of the atmosphere refers to its complex, layered arrangement.

The atmosphere can be thought of as having five major layers, the troposphere, the stratosphere, the mesosphere, the ionosphere and the exosphere.

The atmosphere is also divided into two major zones, the homosphere and the heterosphere.

The homosphere has an almost uniform chemical composition and is divided into three layers which are the troposphere, the stratosphere and the mesosphere.