Global distribution of temperature: isotherms and ranges

Detail

1. Isotherms: what they are and how to read them

• An isotherm is a line on a map that joins places with the same temperature.
• Reduced to sea level: on world maps, every station's temperature is first corrected to what it would be at sea level. This removes the effect of height, so the map shows only the effect of latitude, land, sea and currents.

• Without this step, a hill station like Shimla would look "cold" only because it is high up.

• Isotherms broadly follow latitudes (they run roughly east–west).

• The reason is that insolation (incoming solar radiation) mainly depends on latitude.

• They bend wherever land and sea, or ocean currents, interrupt the pattern.

• Land heats and cools faster than water.

• Warm currents carry heat towards the poles. Cold currents carry cold water towards the equator.

• Isotherms bend more in January than in July.

• In January, the large northern continents are very cold while the oceans next to them stay mild. This big land–sea contrast bends the lines.

• Isotherms bend more in the Northern Hemisphere.

• The Northern Hemisphere has much more land. The Southern Hemisphere is mostly ocean, so its temperatures are more even.

2. January pattern (Fig. 8.4 a): northern winter

• Isotherms bend poleward (towards the North Pole) over the warm North Atlantic.
• The Gulf Stream and the North Atlantic Drift (warm currents) carry heat to north-west Europe.

• This keeps the ocean, and the coasts next to it, warmer than other places at the same latitude.

• Isotherms bend equatorward (towards the equator) over the cold continents.

• Land loses heat quickly in the long winter nights. The coldest air sits over the inner continent.

• The bend is sharpest over Siberia.

• Along 60°E: the mean January temperature is −20°C at both 80°N and 50°N.

• Normally a place 30° nearer the equator should be much warmer. Here the deep cold of the inner continent at 50°N is as bad as the Arctic Ocean edge at 80°N.

• January temperature belts:

Region	Mean January temperature
Equatorial oceans	Over 27°C
Tropics	Over 24°C
Middle latitudes	2°C to 0°C
Interior of Eurasia	−18°C to −48°C

• Southern Hemisphere (summer in January): isotherms run almost parallel to the latitudes, because the hemisphere is mostly ocean.

Isotherm	Approximate latitude
20°C	35°S
10°C	45°S
0°C	60°S

3. July pattern (Fig. 8.4 b): northern summer

• Isotherms are mostly parallel to the latitudes.
• Equatorial oceans: over 27°C.
• Subtropical Asia: over 30°C along about 30°N.

• The large land mass heats strongly under the high summer Sun.

• Note on the printed text: NCERT's text says the 10°C isotherm runs along 40°N in July. This does not fit the map. 40°N in July is mid-summer and much warmer than 10°C. A 10°C value in July fits about 40°S, where it is winter. Always read the value from the map.

4. Thermal equator

• Thermal equator: the belt of the highest mean temperature on Earth. It joins the hottest places on each line of longitude.
• It lies north of the geographical equator, at about 5°N on the annual average.

• The Northern Hemisphere has more land, and land heats up more than water.

• It moves with the overhead Sun:

• It goes north in July and south in January.
• It moves much further over the continents than over the oceans, because land heats and cools quickly.

• In July, it reaches well into South Asia and the Sahel (the dry belt just south of the Sahara in Africa).

• It tracks the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt near the equator where the trade winds of the two hemispheres meet).

• Strongest heating → air rises → low pressure forms.

• Link to the Indian monsoon:

• In summer, the north Indian plains become very hot.
• The thermal low and the ITCZ move over north India and form the monsoon trough (a long low-pressure belt).
• Moist south-west winds are drawn in from the Indian Ocean, which gives monsoon rain.

5. Annual range of temperature

• Annual range of temperature: the mean temperature of the warmest month minus the mean temperature of the coldest month.
• Greatest range: over 60°C in north-eastern Eurasia (Verkhoyansk–Oymyakon region, East Siberia, Russia) (Fig. 8.5).
• Cause: continentality. This means being far from the sea, so the moderating effect of the ocean cannot reach the place.
• Long, very cold winter nights plus warm summers with long days give a huge gap between the two seasons.

• The region's winter extremes are shown by Oymyakon (63°28'N, 142°23'E, 800 m), which recorded −67.8°C on 6 February 1933. This is the WMO record low for Asia [6].

• Least range: about 3°C between 20°S and 15°N.

• The noon Sun is high all year and day length hardly changes, so every month is about equally warm.

• Why Siberia's range is so large (NCERT exercise):

• It lies at high latitude, so the Sun's angle and the length of day change a lot between seasons.
• It is deep inside the continent, far from the sea.
• Clear, dry winter air lets heat escape quickly into space.
• Cold air is not carried away, because winds are weak under the strong winter high pressure.

6. Diurnal range of temperature

• Diurnal range of temperature: the day's highest temperature minus the day's lowest temperature.
• Large in deserts and in dry seasons with clear skies.
• No clouds by day → strong sunshine → very hot afternoons.
• No clouds at night → heat escapes quickly into space → cool nights.

• Example: the Thar Desert (Rajasthan), where days can be near 50°C and nights are cool.

• Small over oceans, on coasts and in humid, cloudy places.

• Water vapour and clouds trap heat at night and block sunlight by day.
• Example: Kerala, with a diurnal range of about 7–8°C.

Feature	Large diurnal range	Small diurnal range
Sky	Clear	Cloudy
Air	Dry	Humid
Surface	Sand, rock, interior land	Sea, coast
Indian example	Thar (Rajasthan)	Kerala coast

7. Worked example: Safdarjung, New Delhi (1951–80 data, NCERT project work)

• Daily mean temperature = (daily maximum + daily minimum) ÷ 2.
• Monthly mean = the average of the daily means for that month.
• Safdarjung (1951–80):
• January (coldest month) mean: 14.2°C.
• May (warmest month) mean: 32.75°C.

• Annual range = 32.75 − 14.2 = 18.55°C.

• Delhi's range is much larger than Kerala's because Delhi is inland in the north Indian plains (continentality). It is still far smaller than Siberia's.

8. Temperature extremes

• Al Aziziyah (El Azizia), Libya: 58°C on 13 September 1922.
• This was cited for about 90 years as the world's highest temperature.
• The WMO rejected it in 2012.

• Reasons: the reading was taken over a paved courtyard that reflected heat, and a new, inexperienced observer may have misread the instrument by as much as 7°C [4].

• Official world highest: 56.7°C (134°F) at Furnace Creek, Death Valley, California, USA, on 10 July 1913.

• The WMO certified it as the world record on 12 September 2012 [3].

• World lowest: −89.2°C at Vostok Station, Antarctica, on 21 July 1983.

• Vostok lies at 77°32'S, 106°40'E, at a height of 3,420 m on the Antarctic ice sheet [5].

• It is so cold because of its high altitude, the long polar night (no sunlight in July) and clear, dry air.

• Northern Hemisphere lowest: −69.6°C at the Klinck automatic weather station, Greenland (3,105 m), on 22 December 1991.

• The WMO verified it in 2020 [7].

• It replaced the older record of −67.8°C, which was shared by Verkhoyansk (February 1892) and Oymyakon (1933) in Russia [7].

• India's highest: 51.0°C at Phalodi, Jodhpur district, Rajasthan, on 19 May 2016. The WMO notes this as India's highest temperature on record [2].

Prelims Hooks

• Isotherms bend poleward over warm currents (North Atlantic in January) and equatorward over cold continents (sharpest over Siberia).
• Isotherms bend more in January than in July, and more in the Northern Hemisphere than in the Southern Hemisphere.
• January, along 60°E: −20°C at both 80°N and 50°N.
• Southern Hemisphere in January: 20°C, 10°C and 0°C isotherms near 35°S, 45°S and 60°S.
• The thermal equator lies at about 5°N on average, not at 0°. It shifts most over land and tracks the ITCZ.
• Largest annual range: over 60°C in north-east Siberia. Smallest: about 3°C between 20°S and 15°N.
• Trap: El Azizia's 58°C is no longer the world record (WMO rejected it in 2012). The record is 56.7°C at Death Valley (1913) [3][4].
• World lowest: −89.2°C, Vostok, Antarctica (21 July 1983) [5]. Northern Hemisphere lowest: −69.6°C, Klinck, Greenland (1991), not Oymyakon [7].
• India's highest: 51.0°C, Phalodi, Rajasthan (19 May 2016) [2].
• Diurnal range is large in the Thar (clear, dry skies) and small in Kerala (humid coast, about 7–8°C).

Mains Points

• Land–sea contrast and ocean currents shape temperature more than latitude does in the Northern Hemisphere.
• The Gulf Stream and North Atlantic Drift keep north-west Europe mild in January.
• Siberia at the same latitude falls to −40°C or lower.

• This explains why the Northern Hemisphere has bent isotherms and the ocean-dominated Southern Hemisphere has nearly straight ones.

• Continentality controls the temperature range.

• The annual range is over 60°C in north-east Eurasia, 18.55°C at Delhi (Safdarjung, 1951–80) and very small on the Kerala coast.

• These differences shape crop seasons, energy demand for heating and cooling, and housing design.

• The northward shift of the thermal equator over South Asia drives the Indian monsoon.

• Intense summer heating → thermal low and ITCZ over north India (the monsoon trough) → south-west winds bring rain.

• Any change in this heating pattern affects the timing and strength of the monsoon.

• Extreme heat is a growing hazard in India (GS-III, disasters).

• Phalodi's 51.0°C (2016) [2] and the large diurnal ranges of the Thar show how exposed north-west India is.
• This calls for heat action plans, early warning and checking extreme readings to WMO standards. The rejected El Azizia reading shows why checking matters [4].