6. The Properties of Sea Water

The Baltic Sea lies in the temperate region. Due to lower temperatures, the rate of evaporation is less. Additionally, the melting of snow from the surrounding regions provides an ample and continuous supply of freshwater to the sea. Therefore, despite being land-locked, the salinity of the Baltic Sea is very low (around 7‰).

The northern part of the Red Sea is highly landlocked and surrounded by hot deserts, leading to an extremely high rate of evaporation with virtually no supply of freshwater from rivers. This concentrates the salt. In contrast, the southern part connects to the Gulf of Aden and the open waters of the Arabian Sea, allowing some exchange and mixing of water, which keeps the salinity comparatively lower.

Salinity does not depend on latitude alone. It is affected by the unequal distribution of land and water, varying rates of evaporation, the volume of freshwater supplied by large rivers, and the mixing caused by hot and cold ocean currents. Hence, oceans at the same latitude can have different salinity levels depending on these local geographical factors.

While most sunrays radiate back from the surface, some penetrate the water. However, the intensity of sunrays decreases rapidly with increasing depth. Therefore, the temperature of sea water decreases up to a depth of about 2000m. After 2000m, sunrays cannot penetrate at all, and the temperature remains uniform (around 4°C) everywhere, from equatorial to polar regions.

The Arabian Sea on the western coast has a higher salinity (35‰) compared to the Bay of Bengal on the eastern coast (34‰). The eastern coast receives a massive supply of freshwater from large rivers like the Ganga, Godavari, and Krishna, which lowers its salinity. The western coast receives less freshwater and experiences higher evaporation. Higher salinity makes the extraction of salt more efficient, leading to more salt-pans on the western coast.

In the mid-latitudinal zones (25° to 35° North and South), rainfall is less and the supply of freshwater from rivers is also low. This zone contains most of the hot deserts of the world, leading to high temperatures and high rates of evaporation. The combination of high evaporation and low freshwater supply causes the salinity to increase in these zones.

The two main factors affecting the salinity of sea water are:

• Rate of evaporation: Driven by temperature and the sun's heat. Higher evaporation leaves salts behind, increasing salinity.
• Supply of freshwater: Provided by rainfall, large rivers meeting the sea, or melting snow. An abundant supply of freshwater dilutes the sea water, lowering its salinity.

Around the Tropic of Cancer and Tropic of Capricorn (mid-latitudes 25° to 35° N and S), the salinity of the sea water is exceptionally high. This is because these regions experience very low rainfall and a minimal supply of freshwater from rivers. Furthermore, these zones host the world's hot deserts, meaning temperatures are high, leading to a very high rate of evaporation. Consequently, the salinity levels peak in these areas.

The surface temperature of sea water is primarily dependent on latitude (it decreases from the equator towards the poles). Besides latitude, it is affected by cyclones, rainfall, sea waves, salinity, pollution, seasons, and ocean currents (warm currents increase surface temperature, while cold currents decrease it).

Surface water receives direct heat from the sun, but the intensity of sunrays decreases with depth. Therefore, the temperature of sea water decreases up to a depth of 2000 meters. The rate of this decrease is rapid in equatorial areas and lesser in polar areas. Beyond 2000m, sunrays cannot reach, and the temperature becomes uniform everywhere (around 4°C), regardless of whether it is an equatorial or polar region.

The primary factors affecting salinity are temperature (which dictates the rate of evaporation) and the supply of freshwater (which comes from rainfall, river discharge, and melting snow).

Temperature is inversely proportional to the density of sea water and is the most dominant factor controlling it. As the temperature of the water reduces, it contracts, causing its density to increase. Therefore, cold water is denser than warm water. Even if a body of cold water is highly saline, it is primarily its low temperature that keeps its density exceptionally high.

Temperature directly controls the rate of evaporation. Higher temperatures lead to faster evaporation of sea water. When water evaporates, the dissolved salts are left behind in the ocean. Therefore, in regions with high temperatures and a low supply of freshwater, the high rate of evaporation significantly increases the salinity of the sea water.