GeographyCBSE Class 11 · Water (Oceans)

Temperature and Salinity of Ocean Water (Class 11 Geography, Fundamentals of Physical Geography, Chapter 12)

Updated 17 October 2026 · Fact-checked

Ocean surface temperature averages about 27 degrees C and falls poleward, controlled by latitude, land-water distribution, prevailing winds and currents; vertically there is a warm layer, a thermocline and cold deep water. Salinity is grams of salt per 1,000 g of sea water (parts per thousand): 33-37 in the open ocean, controlled by evaporation, precipitation, river inflow, freezing and thawing, winds and currents.

Temperature stays near 27°C at the surface, drops rapidly through the thermocline, then stays close to 0°C in the deep water.

Illustration drawn for QuestionBuddy.

Understand Temperature and Salinity of Ocean Water: Factors, Distribution, Thermocline and Halocline

Why this topic matters. Factors, the three-layer structure of temperature and salinity numbers with their controls are standard short and long answers.

1. Temperature factors. Water heats and cools more slowly than land. (i) Latitude: surface temperature falls from the equator to the poles because insolation decreases. (ii) Land and water distribution: northern hemisphere oceans receive more heat because they touch a larger extent of land. (iii) Prevailing winds: offshore winds drive warm surface water away, causing upwelling of cold water; onshore winds pile up warm water near the coast. (iv) Ocean currents: warm currents raise temperature in cold areas (the Gulf Stream warms the east coast of North America and the west coast of Europe); cold currents lower it in warm areas (the Labrador current cools the north-east coast of North America). Enclosed seas are warmer than open seas in low latitudes and colder in high latitudes.

2. Distribution. Average surface temperature is about 27 degrees C, decreasing poleward; the northern hemisphere averages about 19 degrees C and the southern about 16 degrees C. Vertically, the maximum is at the surface and heat moves down by convection. Temperature falls rapidly to about 200 m and then more slowly. The thermocline is the boundary where temperature falls rapidly; about 90 per cent of ocean water lies below it at temperatures approaching 0 degrees C. In low and middle latitudes there are three layers: a warm surface layer of about 500 m (20 to 25 degrees C), the thermocline (500 to 1,000 m) and cold deep water. In polar waters there is a single cold layer.

3. Salinity. The amount of salt in grams per 1,000 g of sea water, in parts per thousand (per mille). Water with 24.7 per mille is the upper limit for brackish water. Open ocean salinity is 33 to 37 per mille; the Red Sea reaches 41; the Indian Ocean averages 35 and the Atlantic 36.

4. Controls. Evaporation and precipitation (high evaporation raises salinity, heavy rain lowers it), river inflow, freezing and thawing of ice, wind and ocean currents which redistribute salt.

5. Vertical salinity. Salinity generally increases with depth, and the halocline is the zone in which salinity increases sharply.

Temperature and Salinity of Ocean Water: Factors, Distribution, Thermocline and Halocline: key facts

Ocean temperature: Average surface about 27 degrees C
Northern hemisphere about 19
Southern about 16
Falls poleward; Latitude is the main control.
Three layers: Warm surface about 500 m, 20 to 25 degrees C
Thermocline 500 to 1,000 m
Deep cold to the floor; Polar waters: one cold layer.
Salinity: Grams of salt per 1,000 g of water
Open ocean 33 to 37 per mille
Red Sea 41
Brackish limit 24.7; Parts per thousand.
Controls of salinity: Evaporation and precipitation
River inflow
Freezing and thawing
Wind
Currents; Halocline: sharp rise with depth.

How to answer Temperature and Salinity of Ocean Water: Factors, Distribution, Thermocline and Halocline questions

Salinity numericals use salt mass divided by total mass of sea water, times 1,000.

1Write the salt mass from the salinity and original mass.
2Subtract the water lost or add the water gained.
3New salinity = salt / new mass x 1,000 per mille.
4State the unit (per mille) and the change.

Temperature and Salinity of Ocean Water: Factors, Distribution, Thermocline and Halocline in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Evaporating pan

Suppose 2 kg of sea water at 34 per mille loses 200 g of water by evaporation.

What is the new salinity?

Try it first, then show the solutionSolution

Salt = 34 per mille of 2,000 g = 68 g.
New mass = 1,800 g; salinity = 68 / 1,800 x 1,000 = 37.8 per mille.

Answer: About 37.8 per mille.

Memory hook: Less water, more salty.

Warm coast, cold coast

Suppose two ships at the same latitude record 24 degrees C off one coast and 12 degrees C off another coast.

What may explain the difference?

Try it first, then show the solutionSolution

Warm currents raise temperature, cold currents lower it.
Offshore winds can also cause upwelling of cold water.

Answer: Ocean currents and winds.

Memory hook: Currents and winds.

Tricks to remember Temperature and Salinity of Ocean Water: Factors, Distribution, Thermocline and Halocline

Short memory aids. Say each one aloud once, then use it on a question the same day.

LLPC
Latitude, Land-water, Prevailing winds, Currents
Use it when: Temperature factors.
500-1,000 m
Thermocline depth
Use it when: Layer.
35 per mille
Average ocean salinity near 35
Use it when: Salinity.

Temperature and Salinity of Ocean Water: Factors, Distribution, Thermocline and Halocline: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Average surface temperature · 1 mark · MCQBoard level

The average temperature of the surface water of the oceans is about:
(A) 10 degrees C
(B) 19 degrees C
(C) 27 degrees C
(D) 35 degrees C

Try it first, then show the solutionSolution

About 27 degrees C (1 mark).

Answer: (C)

Example 2: Thermocline · 2 marks · Very short answerBoard level

What is the thermocline and at what depth is it found in low and middle latitudes? (2 marks)

Try it first, then show the solutionSolution

The boundary layer where temperature falls rapidly (1 mark).
500 to 1,000 m (1 mark).

Answer: The thermocline is the layer between surface and deep water in which temperature decreases rapidly with depth; in low and middle latitudes it lies between about 500 and 1,000 m.

Example 3: Salinity sum · 3 marks · Short answerHarder board level

A 400 g sample of sea water contains 12 g of salt. Find its salinity in per mille and say whether it is brackish. (3 marks)

Try it first, then show the solutionSolution

Salinity = 12 / 400 x 1,000 (1 mark).
= 30 per mille (1 mark).
30 is above 24.7, so it is not brackish (1 mark).

Answer: Salinity = 12 / 400 x 1,000 = 30 per mille. Since brackish water has up to 24.7 per mille, this sample is above that limit and is not brackish.

Example 4: Assertion and reason: onshore winds and coastal temperature · 1 mark · Assertion-reasonHarder board level

Assertion (A): Surface water warmed by onshore winds raises coastal sea temperature.
Reason (R): Onshore winds pile up warm water near the coast.
(A) Both true, R explains A
(B) Both true, R does not explain A
(C) A true, R false
(D) A false, R true

Try it first, then show the solutionSolution

Piling up of warm water near the coast explains the warming (1 mark).

Answer: (A)

Example 5: HOTS: compute salinity change · 5 marks · Long answerHOTS / competency

A 4 kg sample of sea water at 36 per mille is diluted by heavy rain adding 400 g of fresh water and then loses 200 g by evaporation. Find the salinity after each step and explain the controls illustrated. (5 marks)

Try it first, then show the solutionSolution

Salt = 36 per mille of 4,000 g = 144 g (1 mark).
After rain: mass 4,400 g; salinity = 144 / 4,400 x 1,000 = 32.7 per mille (1 mark).
After evaporation: mass 4,200 g; salinity = 144 / 4,200 x 1,000 = 34.3 per mille (1 mark).
Precipitation lowers salinity (1 mark).
Evaporation raises salinity; salt mass stays the same throughout (1 mark).

Answer: The salt mass is 36 per mille of 4,000 g = 144 g. After adding 400 g of fresh water the mass is 4,400 g and the salinity is 144 / 4,400 x 1,000 = about 32.7 per mille. After losing 200 g the mass is 4,200 g and the salinity is 144 / 4,200 x 1,000 = about 34.3 per mille. Precipitation lowers salinity and evaporation raises it, two of the controls named in the chapter; the salt mass does not change.

Where students lose marks in Temperature and Salinity of Ocean Water: Factors, Distribution, Thermocline and Halocline

The slips examiners see most often, and what to write instead.

Mistake: Using salinity as per cent
Units slip.
Wrong working
35 per mille = 35 per cent.
Right working
Per mille means parts per thousand: 3.5 per cent.
Do this: Per mille is parts per 1,000 and per cent is parts per 100, so divide per mille by 10 to get per cent (35 per mille = 3.5 per cent).
Usually costs: 1 mark.
Mistake: Writing temperature rises with depth
Heat from below assumed.
Wrong working
Ocean temperature rises with depth.
Right working
Temperature falls with depth, rapidly through the thermocline.
Do this: Surface is warmest.
Usually costs: 1 mark.
Mistake: Saying heavy rain raises salinity
Water amount misread.
Wrong working
More rain means higher salinity.
Right working
Heavy rain dilutes the water and lowers salinity.
Do this: Link rain with dilution.
Usually costs: 1 mark.

Exam tips

Show the salt mass as a separate step in salinity sums.
State that salinity is in parts per thousand.
Draw the three-layer temperature profile with depths.

Practice questions on Temperature and Salinity of Ocean Water: Factors, Distribution, Thermocline and Halocline

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The zone where salinity increases sharply with depth is the:
(A) thermocline
(B) halocline
(C) shelf break
(D) trench
Hint
Salinity.
Answer
(B)
Q2 · 2 marks · Very short answerBoard level
Name two factors affecting the salinity of ocean water. (2 marks)
Hint
Evaporation, rivers.
Answer
Evaporation and precipitation, and river inflow (also freezing and thawing of ice, winds and currents).
Q3 · 3 marks · Short answerHarder board level
Why do northern hemisphere oceans record higher surface temperatures than southern ones? (3 marks)
Hint
Land.
Answer
The northern hemisphere oceans are in contact with a larger extent of land, so they receive more heat. The average annual surface temperature is about 19 degrees C in the north and 16 degrees C in the south.
Q4 · 3 marks · Short answerHarder board level
Describe the vertical structure of temperature in middle and low latitudes. (3 marks)
Hint
Three layers.
Answer
There are three layers: a warm surface layer about 500 m thick at 20 to 25 degrees C; the thermocline between 500 and 1,000 m where temperature falls rapidly; and a deep cold layer to the sea floor. About 90 per cent of ocean water lies below the thermocline at temperatures near 0 degrees C.
Q5 · 4 marks · Short answerHOTS / competency
A sample of 3 kg of sea water contains 99 g of salt. Find the salinity, and compare it with the Red Sea's 41 per mille. Suggest two reasons why salinity might be high in some seas. (4 marks)
Hint
Salt over mass.
Answer
Salinity = 99 / 3,000 x 1,000 = 33 per mille, which is lower than the Red Sea's 41 per mille. High salinity may arise from strong evaporation and low precipitation, or from limited river inflow and little mixing with the open ocean.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is the Red Sea salinity?Show answer
41 per mille.
2.Average ocean salinity?Show answer
About 35 per mille.
3.Which layer has rapid temperature fall?Show answer
The thermocline.
4.Which current warms Europe?Show answer
The Gulf Stream.

Temperature and Salinity of Ocean Water: Factors, Distribution, Thermocline and Halocline: frequently asked questions

What is brackish water?

Water with salinity up to 24.7 per mille.

Why are enclosed seas different in temperature?

Warmer than open seas in low latitudes and colder in high latitudes.

What is upwelling?

Cold deep water rising when offshore winds push surface water away.