Properties of Sea Water Class 9 Geography Notes Maharashtra State Board
Temperature is a major property of the seawater. The surface temperature of the seawater is not uniform everywhere. This is dependent on different factors. Latitudinally speaking, the surface temperature of the seawater decreases from the equatorial areas towards the poles. The average temperature in equatorial areas is around 25° C, it is 16° C in mid-latitudes while it is about 2°C near the poles. Besides this, cyclones, rainfall, sea waves, ocean currents, salinity, pollution, convectional currents, seasons, etc. also affect the surface temperature Ocean currents also have similar effects on the temperature of the seawater. In regions where cold ocean currents flow, the surface temperature of ocean water is less while in the regions where the warm currents move, the temperatures increase. The changes in the temperature of seawater concerning depths in different latitudes are shown in the figure.
While most of the sunrays radiate back from the surface of the sea, some of them penetrate to certain depths in the water. As a result, the intensity of sunrays decreases with increasing depth. Temperature decreases up to 2000m depth. After 2000m, the temperature of the seawater is uniform everywhere. It is around 4 0C everywhere from the equatorial regions to the polar areas. Temperature reduces only up to 4 0C according to depth. And therefore, the water at greater depths does not freeze. The temperature of the seawater changes rapidly with depth at the equatorial areas. The temperature difference is lesser in polar areas. There is also a difference between open seas and landlocked seas. As the salinity of the landlocked seas is higher, the temperature of these landlocked seas is higher than the open seas. This is true for low latitudinal areas.
Salt is used in the food we eat. Salt is also used for making various chemicals and medicines. Salt is also used to preserve things for longer periods. Salt is also used in ice factories. (Why did you use the salt in the activity from lesson five?) We obtain salt from the salt pans. The buoyancy of the seawater increases because of salinity. This is useful for water transport. But if the salinity is more than bearable limits, the life in the water gets destroyed.
Take 1.5 liters of water in a big container. Put 100 gm salt in it and stir. Now take 3 containers of the same size. To identify them put three dots of different colours on them. Put the water containing salt equally in the three containers. Make sure the containers are half-empty after filling the water. Taste the water out of the three containers, and keep one in the sun outside. Cover it with a meshed lid. Keep the remaining two in the classroom (Cover them with net lids too).
Put half a glass of fresh water daily in one of the containers in the classroom. After 3-4 days, bring all the containers together in one place. Observe the level of water in all the three containers. Taste the water in all three containers. Experience the difference in the taste. Write about all three tastes in one line each.
You must have realized that because of the sun’s heat, evaporation happens at a faster rate. Evaporated water turns into water vapor and reduces in quantity. But the amount of salt remains the same in the remaining water and therefore the salinity of the water increases.
- In seas where the rate of evaporation is high, the supply of freshwater salinity is high.
- In seas where the supply of freshwater exceeds the rate of evaporation, salinity is low.
- Salinity is not affected much in seas where both the supply of freshwater and evaporation of water are low.
How is the salinity of the seawater measured?
The weight of all dissolved salts in water in a ratio of parts per thousand of water is called the salinity of seawater. For example, if the weight of dissolved salts in 1000g (1 kg) of seawater is 40g, then the salinity is 40‰ i.e. 40 per thousand parts. Hydrometers, refractometers, and salinometers are also used to measure salinity.
The uneven distribution of temperature on earth, uneven supply of fresh water, etc. affects the salinity of seawater. In the tropical zone, the temperature is higher and the rate of evaporation is also higher, therefore, salinity is higher. Around 5 0N and S of the equator, in the equatorial calm belt, the sky is cloudy for a long period and convectional rainfall occurs every day. Large rivers like the Congo and Amazon in the equatorial regions meet the sea. Therefore the supply of freshwater is abundant. However because of higher temperatures, the rate of evaporation is higher, and therefore, the seas in these areas are more saline.
In mid-latitudinal zones (25° to 35° N and S), rainfall is less and the supply of fresh water from rivers is also low. This zone has the hot deserts of the world. Thus, the salinity of the seas is found to be higher here. In temperate regions, the sun’s rays are slanting, and therefore, the temperatures are lower. Because of the melting of the snow, the supply of water is also more, and therefore, in this zone, salinity decreases with increasing latitudes. In the polar areas, temperatures are very low. Evaporation is also very low in polar areas. So salinity is low.
Landlocked seas have higher salinity than open seas as the rate of evaporation is more and there is a lack of supply of fresh water from large rivers. Thus, there is a difference in the salinities of open and enclosed seas. For example, the average salinity of the Mediterranean Sea is 39‰ while the salinity of the most saline ocean, the Atlantic Ocean, is 35‰.
The name of the sea itself gives an idea of the conditions there. The sea lying on the border of Israel and Jordan has a salinity of 332‰. The average salinity of the ocean is 35‰. Jordan is the only large river meeting this sea. Low rainfall, low supply of freshwater, and high evaporation are the reason for high salinity. There is no life here except a few unicellular organisms. The fish coming from the river Jordan die as soon as they come here. Because of high salinity, saline pillars have developed. Some of them come out on the surface. The density of water is also high because of higher salinity. Therefore, one can never drown in the sea. We can easily float when we go in the seawater. Another characteristic of the Dead Sea is that the land here is below the mean sea level. It is one of the places with the lowest elevations on the earth. In some areas, the altitude is -400m.
Temperature and salinity are the two properties of seawater that control the density of the seawater. If the temperature reduces, the density of water increases. Cold water is denser and so is saline water. As compared to salinity, temperature affects density more. Hence, sometimes, more saline water has a lower temperature at the surface. But still, the density of such water is more than that of other water. On the contrary, seawater having higher temperature and low salinity can have lower density.
The graph shows salinity, temperature, and density concerning the depth of the seawater. You know that the density of the water is dependent on temperature and salinity. If you see the graph carefully, you will realize that after a certain depth, there is no change in these factors according to the depth. The change can be seen up to a depth of about 500m. The slopes of the curves of the graph vary for all three factors but below the depth of 1000 m, there is no change in all three factors.
Generally, we can call the seawater up to the depth of 500m as surface water. This water gets affected by ocean currents and sun rays. We can see the movement of these waters in the form of ocean currents. At greater depths, winds, sunrays, and currents do not affect these properties. Therefore, after around 1000m depth, there is no change in these three factors. The ocean currents are generated because of differences in the properties of seawater. They act as temperature controllers at the global level. The distribution of temperature gets controlled due to the ocean currents. Thus, the climate of the earth is affected by ocean currents.
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