Like land masses, ocean water varies in temperature from place to place both at the surface and at great depths. Since water warms up and cools down much more slowly than the land, the annual range of temperature in any part of the ocean is very much smaller.

It is less than 5.5°C (10°F) for most of the open surface ocean water. Ocean water temperature decreases from about 21°C (70°F) in equatorial areas to 13°C (55°F) at latitudes 45°N and S, and drops almost to freezing-point at the poles. The reduction of temperature with latitude is however, never constant, because of the interference by warm and cold currents, winds and air masses. Unlike the solid earth, ocean water is mobile and variations in the temperature between different parts of the oceans can be expected. Water flowing out from the Arctic and Antarctic as cold currents, such as the Labrador Current off north-east Canada, tends to reduce the surface-water temperature.

Ports of eastern Canada even at 45°N are thus icebound for almost half the year. In the same way, coasts warmed by warm currents, such as the North Atlantic Drift, have their surface temperature raised. The Norwegian coast, even at latitudes of 60° to 70°N, is ice-free throughout the year.

The highest water temperatures are found in enclosed seas in the tropics, e.g. the Red Sea which records a temperature of 29° to 38°C (85° to 100°F). The Arctic and Antarctic waters are so cold that their surface is permanently frozen as pack-ice down to a depth of several metres. In the warmer summer, parts of the ice breaks off as icebergs that both dilute the water and lower the surface temperature of surrounding ice-free seas.

The temperature of the oceans also varies vertically with increasing depth. It decreases rapidly for the first 200 fathoms, at the rate of 1°C for every 18 fathoms (1°F for every 10 fathoms), and then more slowly until a depth of 500 fathoms is reached. Beyond this, the drop is scarcely noticeable, less than 1°C for every 180 fathoms (or 1°F for every 100 fathoms). In the ocean deeps below 2000 fathoms (3700 m/12000 feet), the water is uniformly cold, just a little above freezing-point. It is interesting to note that even in the deepest ocean trenches, more than 10 km (6 miles) below the surface, the water never freezes. It is estimated that over 80 per cent of all ocean waters have a temperature between 2°C (35°F) and 4°C (40°F).

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