Generally, one talks about the cold current and the warm current. These two terms are relative. Generally, a pole-ward-moving current is warm as compared with general atmospheric temperatures and those of the adjacent waters of the ocean.

An equator-ward-moving current is cool or cold because its temperatures are lower than those of the atmosphere or of adjacent waters. The criteria of warm or cold current are not based on their absolute temperatures.

There is another classification of ocean currents on the basis of their mode of origin, velocity, volume and boundaries: drift current and stream.

(i) Drift:

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Drift is a slow motion of the upper layers with no very definite boundaries. The velocity of a drift varies from 16 to 24 km a day. The drift is generally a broad movement induced mainly by the wind movement on the surface of the ocean.

Thus, drifts are caused by the friction between the winds and the surface water. The drifts move in the direction of the wind. They vary in their position and strength with the seasonal winds.

West wind Drift, for example, is a slow, eastward movement of water in the west wind belt. It covers a broad belt in between latitude 35° and 45° in the northern hemisphere and latitude 30° and 60° in the southern hemisphere.

(ii) Current:

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Current moves more rapidly and more definitely than the drift. A current may have a velocity of 6 or even 8 km an hour. It is commonly a part of the oceanic circulation modified in such a way as to concentrate the movement along a more or less definite belt with consequent increase in velocity. The Florida Current may be cited as an example.

(iii) Stream:

The Stream is still more definite. In other words, its boundaries are clearly discernible and are more definite. The Gulf Stream in some parts is quite definitely bounded so as to suggest a stream of warm water in the ocean.

In most parts, however, there is gradation of such slow degree that hours of shipping are required to pass from normal ocean water to the definite Gulf Stream water. However, its most clear-cut boundary lies on the western side, where a cold, south-moving current parallels it on the landward side.