Isotherm

Geography glossary

Topic: Solar Radiation, Heat Balance and Temperature · NCERT: Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"; Class 12, Ch 3 "Graphical Representation of Data"

Meaning

An isotherm is a line on a map that joins places with the same temperature. On world maps, each place's temperature is first "reduced to sea level", so the effect of height is removed. Isotherm maps show how latitude, land, sea and ocean currents control heat on Earth. They are a common base for UPSC questions on currents, continentality and the monsoon.

Explanation

How isotherms are drawn and read

  • Step 1: collect the data. Temperature is recorded at many weather stations.
  • Step 2: reduce to sea level. Each reading is corrected to what it would be at sea level.
  • Without this step, a hill station like Shimla would look "cold" only because it is high up.

  • After the correction, the map shows only the effect of latitude, land, sea and currents.

  • Step 3: join equal values. Places with the same temperature are joined by a smooth line.

  • An isotherm is one kind of isopleth (a line joining places with an equal value of something). This drawing method is taught in NCERT Class 12, Ch 3.

Why isotherms run east–west, and why they bend

  • Isotherms broadly follow the latitudes (they run roughly east–west).
  • Insolation (incoming solar radiation) mainly depends on latitude.

  • So places on the same latitude get about the same heat.

  • They bend wherever land and sea, or ocean currents, break this pattern.

  • Land heats and cools faster than water.
  • Warm currents carry heat towards the poles.

  • Cold currents carry cold water towards the equator.

  • The bending rule:

  • Over a place warmer than its latitude, isotherms bend poleward.
  • Over a place colder than its latitude, isotherms bend equatorward.

January pattern (northern winter, Fig. 8.4 a)

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

  • So the ocean and the coasts next to it stay warmer than other places at the same latitude.

  • Isotherms bend equatorward 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.

  • A place 30° nearer the equator should normally be much warmer.

  • Here, the cold inner continent at 50°N is as cold 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 in January (its summer): isotherms run almost parallel to the latitudes, because this hemisphere is mostly ocean.
Isotherm Approximate latitude
20°C 35°S
10°C 45°S
0°C 60°S

July pattern (northern summer, Fig. 8.4 b) and the two "bending" rules

  • In July, isotherms are mostly parallel to the latitudes.
  • Equatorial oceans: over 27°C.

  • Subtropical Asia: over 30°C along about 30°N, because the large land mass heats strongly under the high summer Sun.

  • A note on the NCERT text: it 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.

  • Rule 1: Isotherms bend more in January than in July.
  • In January, the large northern continents are very cold, but the oceans next to them stay mild.

  • This big land–sea contrast bends the lines.

  • Rule 2: 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.

Where it occurs

  • North Atlantic and north-west Europe: isotherms bend poleward in January because of the Gulf Stream and the North Atlantic Drift.
  • Siberia (Russia): isotherms bend equatorward most sharply in January. The interior of Eurasia has January means of −18°C to −48°C.
  • Along 60°E: the −20°C January value appears at both 80°N and 50°N.
  • Southern Hemisphere oceans: nearly straight isotherms. In January the 20°C, 10°C and 0°C lines lie near 35°S, 45°S and 60°S.
  • Subtropical Asia (about 30°N): over 30°C in July.
  • South Asia and the Sahel (the dry belt just south of the Sahara in Africa): in July the thermal equator (the line joining the hottest places on each line of longitude) moves well into these regions.
  • India: in summer, the hot north Indian plains pull the thermal low and the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt near the equator where the trade winds of the two hemispheres meet) northward. Together they form the monsoon trough (a long low-pressure belt).

Don't confuse with

  • Isobar: joins places with equal air pressure. An isotherm joins places with equal temperature.
  • Thermal equator: this is not just any isotherm. It is the belt of the highest mean temperature. It lies at about 5°N on the annual average, not at 0°, and it shifts with the overhead Sun.
  • Annual range of temperature: this is the warmest month's mean minus the coldest month's mean, for one place. An isotherm map shows the temperature itself for one month across many places. The range is over 60°C in north-east Eurasia and about 3°C between 20°S and 15°N.
  • Actual (unreduced) temperature map: world isotherm maps use temperatures reduced to sea level. So a high place is not shown as cold just because of its height.

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: the mean is −20°C at both 80°N and 50°N.
  • Southern Hemisphere in January: the 20°C, 10°C and 0°C isotherms lie near 35°S, 45°S and 60°S.
  • July: over 30°C along about 30°N in subtropical Asia, and over 27°C over equatorial oceans.
  • Trap: "the 10°C isotherm runs along 40°N in July" does not fit the map. In July, 10°C fits about 40°S (southern winter).

Mains Points

  • In the Northern Hemisphere, land–sea contrast and ocean currents can matter more than latitude.
  • 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 is why northern isotherms are bent and southern ones, over a mostly ocean hemisphere, are nearly straight.

  • The July isotherm pattern explains the Indian monsoon (GS-I).

  • Strong summer heating over subtropical Asia creates a thermal low over north India.
  • The thermal equator and the ITCZ move over north India and form the monsoon trough.

  • Moist south-west winds from the Indian Ocean are drawn in and bring rain. Any change in this heating can change the timing and strength of the monsoon.

  • Isotherm maps help explain differences in climate and their effects on people.

  • Sharply bent isotherms point to strong continentality (being far from the sea, so the ocean cannot moderate temperature) and large temperature ranges.
  • These differences shape crop seasons, the energy needed for heating and cooling, and the way houses are designed.

Related concepts

Read more

Sources

  1. 1Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"; Class 12, Ch 3 "Graphical Representation of Data" (primary)