Temperature of Sea Water
In summer the upper layer of sea water feels warm but a little deeper it is cold. Near the poles it is very cold. So sea temperature changes with place and with depth.
Temperature is a major property of sea water. Surface temperature changes with latitude, ocean currents and other factors and falls with depth.
Key idea 1 : Temperature by latitude — Surface temperature decreases from the equator towards the poles: about 25 °C at the equator, about 16 °C in mid-latitudes and about 2 °C near the poles. Cyclones, rain, waves, salinity, pollution, seasons and ocean currents also have an effect.
Key idea 2 : Effect of currents — Where cold currents flow surface temperature is lower, and where warm currents flow it is higher. Landlocked seas are warmer than open seas in low latitudes because their salinity is higher.
Key idea 3 : Temperature by depth — Sun rays reach only up to a certain depth in water, so temperature falls with depth. It falls down to about 2000 m and below that it is about 4 °C everywhere from the equator to the poles. So deep water does not freeze. At the equator temperature changes sharply with depth; in polar areas the difference is small.
- Easy : Average surface temperature near the equator? — About 25 °C.
- Medium : Temperature of water deeper than 2000 m? — About 4 °C.
- Exam-type : Temperature where a warm current flows? — Higher.
In the figure: Temperature falls from equator to poles.
Higher latitude = lower temperature; more depth = lower temperature (down to 4 °C).
Deep sea water is not frozen; it is about 4 °C everywhere there.
Memory clue: 25 → 16 → 2; deep = 4. Memory clue: "Twenty-five, sixteen, two; deep four".
Spot the difference
| Place | Average temperature | Reason |
|---|---|---|
| Equatorial | About 25 °C | Direct rays |
| Mid-latitude | About 16 °C | Moderate rays |
| Polar | About 2 °C | Slanting rays |
Practice questions
- 1How does sea surface temperature change with latitude?
- 2State factors affecting surface temperature.
- 3What effect do ocean currents have on temperature?
- 4How does sea temperature change with depth?
- 5Why does deep sea water not freeze?
- 6State the temperature difference by depth in equatorial and polar areas.
- 7Why are landlocked seas warmer?
- 8Write what you think would happen if the sea bed became cold enough to freeze.
Show answers (try them yourself first)
- 1It decreases from the equator to the poles.
- 2Cyclones, rain, waves, currents, salinity, pollution, seasons.
- 3Cold current lowers it; warm current raises it.
- 4Falls to about 2000 m; then about 4 °C.
- 5Because temperature does not fall below 4 °C there.
- 6Sharply at the equator; less near the poles.
- 7Because their salinity is higher.
- 8Life and currents of the sea would be badly hit.
Salinity and Its Factors
If a curry has too much salt it tastes salty. If water boils away the salt stays the same so the curry becomes saltier. In the sea too, when evaporation reduces water, salinity rises.
Salinity is the amount of dissolved salts in sea water. It is measured as the weight of salts in 1000 parts of water (‰, per thousand). If 1 kg of water has 40 g salts the salinity is 40 ‰.
Key idea 1 : Uses of salts — Salt is used in food, to make chemicals and medicines, and to preserve things. Salinity increases the buoyancy of water which helps water transport. But if salinity goes beyond bearable limits life in water is destroyed.
Key idea 2 : How salinity changes — Heat of the sun causes evaporation; water becomes vapour and reduces but salts stay the same, so salinity rises. Where evaporation exceeds the supply of fresh water salinity is high; where fresh water supply exceeds evaporation salinity is low. If both are low salinity is not much affected.
Key idea 3 : Instruments to measure salinity — Salinity is stated in parts per thousand (‰). Hydrometer, refractometer and salinometer are used to measure it. The average ocean salinity is about 35 ‰.
- Easy : 1 kg water has 35 g salts; salinity? — 35 ‰.
- Medium : 5 kg water has 200 g salts; salinity? — 40 ‰.
- Exam-type : If evaporation increases what happens to salinity? — Rises.
In the figure: Evaporation raises salinity.
Salinity = grams of salts ÷ kg of water (per thousand).
Salt does not evaporate; only water becomes vapour and salt stays behind.
Memory clue: Evaporation ↑ → salinity ↑; fresh water ↑ → salinity ↓. Memory clue: "More heat, more salty".
Spot the difference
| Condition | Result | Example |
|---|---|---|
| Evaporation > fresh water | High salinity | Hot desert coasts |
| Fresh water > evaporation | Low salinity | Seas with big rivers |
| Both low | Little change | Polar areas |
Practice questions
- 1What is salinity?
- 2How is salinity measured?
- 3If 3 kg of water has 120 g salts, what is the salinity?
- 4Why does evaporation raise salinity?
- 5In which seas is salinity low?
- 6State uses of salinity.
- 7What are the dangers if salinity is too high?
- 8Name three instruments for measuring salinity.
Show answers (try them yourself first)
- 1Amount of dissolved salts in sea water.
- 2By the weight of salts per thousand parts (‰).
- 340 ‰.
- 4Water reduces but salts remain the same.
- 5Where fresh water supply exceeds evaporation.
- 6Food, chemicals, medicine, preserving, buoyancy.
- 7Life in the water can be destroyed.
- 8Hydrometer, refractometer, salinometer.
Distribution of Salinity
On the world map some seas are more salty and some less. Salty water kept in the sun in a vessel dries fast; so does the sea in hot regions.
The average salinity of the sea is about 35 ‰ but it differs from place to place with latitude, temperature, evaporation, rain and supply of rivers.
Key idea 1 : Salinity by latitude — In the tropics temperature and evaporation are high so salinity is high (about 37 ‰). In the equatorial calm belt the sky is cloudy, convectional rain falls daily and big rivers like the Congo and Amazon meet the sea; so salinity is somewhat lower (about 34 ‰). Between 25° and 35° rain and river supply are low and there are hot deserts so salinity is high.
Key idea 2 : Temperate and polar regions — In temperate regions the rays are slanting, temperature is low and melting snow adds water; so salinity falls with latitude (about 33 ‰). In polar areas temperature is very low and evaporation very small so salinity is low (about 31 ‰).
Key idea 3 : Landlocked and open seas — Landlocked seas have high evaporation and little supply from big rivers so salinity is high. The Mediterranean averages about 39 ‰ while the Atlantic Ocean (the most saline ocean) has about 35 ‰. West of India the Arabian Sea (about 35 ‰) is more saline than the Bay of Bengal to the east (about 34 ‰).
- Easy : Salinity in the tropics? — About 37 ‰.
- Medium : Why is polar salinity low? — Very little evaporation.
- Exam-type : Arabian Sea or Bay of Bengal — higher salinity? — Arabian Sea.
In the figure: Salinity by latitude.
The Dead Sea on the Israel-Jordan border is said to have a salinity of about 332 ‰ (as per the textbook). Only the Jordan, a big river, enters it; rain is low and evaporation high. Because the water is dense a person floats easily. (Check with the textbook)
Tropics = highest; poles = lowest; landlocked = high.
Salinity is not highest at the equator; it is highest in the tropics (25°–35°).
Memory clue: 37 tropics, 34 equator, 33 temperate, 31 polar. Memory clue: "37, 34, 33, 31 - falling steps".
Spot the difference
| Region | Salinity | Main reason |
|---|---|---|
| Tropics | About 37 ‰ | High evaporation |
| Equatorial | About 34 ‰ | Rain and rivers |
| Polar | About 31 ‰ | Low evaporation, ice |
| Landlocked sea | High | Little river supply |
Practice questions
- 1State the order of salinity by latitude.
- 2Why is salinity high in the tropics?
- 3Why is salinity a little lower near the equator?
- 4Why is salinity low in polar areas?
- 5Why is salinity high in landlocked seas?
- 6State the reason for higher salinity in the Arabian Sea.
- 7Compare salinity of the Mediterranean Sea and the Atlantic Ocean.
- 8Why can two oceans at the same latitude have different salinity?
Show answers (try them yourself first)
- 1Tropics > equatorial > temperate > polar.
- 2Because temperature and evaporation are high there.
- 3Because it rains daily and big rivers enter.
- 4Because temperature is very low and evaporation little.
- 5Because evaporation is high and fresh water supply low.
- 6More evaporation and less fresh water.
- 7Mediterranean about 39 ‰; Atlantic about 35 ‰.
- 8Because river supply, rain and evaporation differ.
Density and Depth
An egg floats in salted water but sinks in plain water. So salty water is denser. Cold water is also denser.
Density of sea water depends on two properties: temperature and salinity. Density rises when temperature falls and when salinity rises.
Key idea 1 : Temperature, salinity and density — Cold water is dense and salty water is also dense. Temperature affects density more than salinity. So sometimes more saline surface water is cold and its density is higher than other water. On the other hand warm water with low salinity can be lighter.
Key idea 2 : Change with depth — Water down to about 500 m can be called surface water. It is affected by ocean currents and sun rays. Change with depth in temperature, salinity and density is seen up to about 500 m and below about 1000 m there is no change in all three because wind, sun rays and currents do not reach there.
Key idea 3 : Currents and climate — Ocean currents arise due to differences in properties of water. They act as global temperature controllers and keep the temperature distribution in balance. So currents affect the earth’s climate.
- Easy : Cold salty water or warm less-salty water — higher density? — Cold salty.
- Medium : Is there a change in properties below 1000 m? — Almost none.
- Exam-type : Which affects density more? — Temperature.
In the figure: Cold and salty water is heavy.
Cold ↑ density; salty ↑ density; temperature matters more.
Salty water is not always the densest; if very warm its density can be lower since temperature matters more.
Memory clue: Cold + salty = heavy. Memory clue: "Cold and salty is heavy".
Spot the difference
| Depth | Change | Reason |
|---|---|---|
| 0 – 500 m | Big change | Currents and sun rays |
| 500 – 1000 m | Reduces | Effect fades |
| Deeper than 1000 m | No change | No effect reaches |
Practice questions
- 1State the two properties affecting density of sea water.
- 2What happens to density if temperature falls?
- 3Temperature or salinity — which affects density more?
- 4Surface water means water up to what depth?
- 5Why do properties not change below 1000 m?
- 6How are ocean currents produced?
- 7How do ocean currents affect climate?
- 8Write how you will float an egg by adding salt to water.
Show answers (try them yourself first)
- 1Temperature and salinity.
- 2Density rises.
- 3Temperature.
- 4Up to about 500 m.
- 5Because wind, sun rays and currents have no effect there.
- 6Due to differences in water properties.
- 7They control temperature and shape the climate.
- 8Take water in a glass, the egg sinks; add salt and stir, the egg floats.