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The oceans contain 97 per cent of the world's water. All of the Earth's natural elements are present in this water, the most common being sodium and chloride, which together form salt. The salinity of ocean water varies between about 3.3 and 3.7 per cent. Areas where the evaporation rate is high and there is little rain generally have a high salinity. Other areas, such as the Baltic Sea, which receive large amounts of fresh water from rivers, are much less saline. Variations in the salinity and temperature of the water in the oceans create density differences in the water, which in turn cause ocean currents.

These currents flow through all of the oceans, redistributing the water, transferring heat, and modifying the climate. However, most of the familiar surface currents, such as the warm Gulf Stream which brings mild weather to northwestern Europe, are caused by prevailing winds. The effect of currents on climate were well demonstrated in 1997-1998, when a phenomenon known as El Niño, caused by currents in the Pacific Ocean, caused freak weather conditions in many parts of North and South America, eastern Asia, and Australia.

Approximately 80% of all evaporation is from the oceans, with the remaining 20% coming from inland water and vegetation. Winds transport the evaporated water around the globe, influencing the humidity of the air throughout the world. For example, a typical hot and humid summer day in the Midwestern United States is caused by winds blowing tropical oceanic air northward from the Gulf of Mexico. Evaporation is the process by which molecules in a liquid state (e.g. water) spontaneously become gaseous (e.g. water vapor). It is the opposite of condensation. Generally, evaporation can be seen by the gradual disappearance of a liquid when exposed to a significant volume of gas.

On average, the molecules do not have enough heat energy to escape from the liquid, or else the liquid would turn into vapor quickly. When the molecules collide, they transfer energy to each other in varying degrees, based on how they collide. Sometimes the transfer is so one-sided for a molecule near the surface that it ends up with enough energy to escape. Liquids that do not evaporate visibly at a given temperature in a given gas (e.g. cooking oil at room temperature) have molecules that do not tend to transfer energy to each other in a pattern sufficient to frequently give a molecule the heat energy necessary to turn into vapor. However, these liquids are evaporating, it's just that the process is much slower and thus significantly less visible.

Evaporation is an essential and mysterious part of the water cycle. Solar energy drives evaporation of water from oceans, lakes, moisture in the soil, and other sources of water. In hydrology, evaporation and transpiration (which involves evaporation within plant stomata) are collectively termed evapotranspiration. Evaporation is ultimately caused when water is exposed to air and the liquid molecules turn into water vapor which rises up and forms clouds. Although this value will vary considerably from location to location, hydrologist and climatologists estimate that an average of about 5 feet or 60 inches of water evaporates from the oceans each year.

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