The oceans are a vast reservoir of heat, its main source being the solar radiation absorbed by the ocean water. That is why water gets heated and cooled in longer time than land, so that there is less variability in the temperature of the ocean surface from day to night and from season to season than on land in the same latitudinal zone.

Thus, both in tropical and polar regions the annual range of temperature on the surface of the oceans is never more than 6° at any locality. Because of this characteristic of water the coastal areas have a moderate climate where the difference in temperatures of the warmest and the coldest months is nominal.

On the contrary, the interior locations have extremely hot summers and cold winters. Its effect is more marked in the temperate regions. The coastal locations have larger amount of clouds and the resultant more precipitation.

The effect of ocean currents upon the temperature of adjacent lands is more variable depending on the direction of the prevailing winds. Where the prevailing winds are onshore, they carry the moderating influence of the oceans far inland.

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The warm ocean currents help to raise the temperature of the coastal regions. For example, the prevailing westerlies keep the winter time temperatures in Great Britain and much of the Western Europe warmer for the latitude, because of the presence of the North Atlantic Drift, a warm ocean current in the nearby ocean.

Another example is offered by the cold ocean current of California because of which summer temperatures in the subtropical coastal Southern California are lower by 6° Celsius.

The out-flowing water from the Arctic and Antarctic regions is of great importance in modifying the climate of those regions, because the cold water that sinks to the bottom creeps to the other zones, and the surface currents carry both cold and ice to warmer regions.

In the absence of this distribution the climatic conditions in the polar zone would be far different. The influence of the cold ocean currents is felt in the adjacent east-facing lands. Thus, the cold climate of Labrador, Newfoundland and New England is partly due to the effect of cold Arctic current transmitted to the land by the onshore ocean winds.

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Warm currents distribute the tropical heat and reduce the temperature of the tropical water. Partly enclosed seas like the Red Sea, the Mediterranean and the Gulf of Mexico have higher tem­peratures than the open ocean in the same latitude.

Without ocean currents the temperatures of the tropical waters would be much higher than what they are. In fact, the ocean currents equalize the latitudinal imbalance of energy and preserve the heat balance in the various latitude zones of the earth. Let us remember that about 25 percent of heat transfer is carried out by the ocean currents.

Besides, the circulation of the ocean waters is of great significance to ocean life influencing its distribution, in transportation of food supply, and in distributing the much needed oxygen.

Without such circulation the marine life would be very different. For example, the distribution of reef building corals is largely controlled by the ocean currents. Corals abound on such sea coasts as are washed by sufficiently warm ocean currents.

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Navigation too is affected by the ocean currents. Ocean currents influence the navigation by aiding or retarding movement of ships. Strong currents affect the movement of small ships.

Tides are also very important to man. The time and height of tides are important for coastal navigation. It is a common experience that sometimes vessels are drifted out of their course by strong tidal currents.

These are among the most frequent causes of shipwreck. They are also important in coastal cities as means of removing the waste materials and as a cause for maintaining a supply of pure salt water.

As stated earlier, the oceans are a great reservoir of heat. Enormous quantities of water vapour are supplied to the atmosphere from the surface of the oceans. When the water vapour condenses into the atmosphere, the vast amount of the latent heat of condensation is released into the atmosphere.

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Thus, the oceans are the source of supply of temperature and moisture to the atmosphere which actually control the climates found in different regions of the world with different characteristics.

As a matter of fact, oceans are the starting points of the hydrological cycles in the absence of which there would be no precipitation and the earth would have remained barren and uninhabitable.