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When we analyze the salt salinity（盐溶度 ） of ocean waters， we find that it varies only slightly from place to place. Nevertheless， some of these small changes are important. There are three basic processes that cause a change in oceanic salinity. One of these is the subtraction of water from the ocean by means of evaporation. In this manner， the salinity is increased， since the salt stays behind. If this is carried to the extreme， of course， white salt would be left behind; this， by the way， is how much of the table salt we use is actually obtained.
The opposite of evaporation is precipitation， such as rain， by which water is added to the ocean. Here the ocean is being diluted so that the salinity is decreased. This may occur in areas of
high rainfall or in coastal regions where rivers flow into the ocean. Thus salinity may be increased by the subtraction of water by-evaporation， or decreased by the addition of fresh water by precipitation.
Normally， in hot regions where the sun is very strong， the ocean salinity is somewhat higher than it is in other parts of the world where there is not as much evaporation. Similarly， in coastal regions where rivers dilute the sea， salinity is somewhat lower than in other oceanic areas.