The average salinity of the ocean is about 35 parts per thousand (ppt), which means that there are about 35 grams of salt in every 1,000 grams of seawater. This value can vary depending on location and depth, but most of the ocean has a salinity between 34 ppt and 36 ppt. The salinity of the ocean is based on a balance between the evaporation of fresh water caused by the wind at the surface and the input of fresh water through rainfall or runoff. The salts and minerals, too heavy to evaporate, remain in the ocean. The two seas that produce the saltiest water in the world are the Mediterranean and the Red Sea, partly because of their semi-enclosed nature. In contrast, the Black Sea has low salt content because it experiences more rainfall and runoff than evaporation of water. The polar regions are also areas of low salinity due to the seasonal melting of ice in the Arctic Ocean.