UPSC Prelims 2024 Q4: January Isotherms and Ocean Currents — Explained

Which of the following is/are correct inference/inferences from isothermal maps in the month of January?

1. The isotherms deviate to the north over the ocean and to the south over the continent.

2. The presence of cold ocean currents, Gulf Stream and North Atlantic Drift make the North Atlantic Ocean colder and the isotherms bend towards the north.

Select the answer using the code given below:

  1. 1 only
  2. 2 only
  3. Both 1 and 2
  4. Neither 1 nor 2

Answer: (a) 1 only

Statement-wise explanation

  • 1: correct — In the Northern Hemisphere January is winter. Land cools faster and further than the sea, so the oceans stay warmer than the land at the same latitude. An isotherm keeps its temperature value, so it shifts poleward (north) over the warmer ocean and equatorward (south) over the colder continent. The NCERT Class XI text (Fundamentals of Physical Geography, 'Solar Radiation, Heat Balance and Temperature') states this inference in almost the same words.
  • 2: incorrect — The Gulf Stream and the North Atlantic Drift are warm currents, not cold ones. They carry tropical water north-east across the Atlantic, so in January the North Atlantic is warmer than other seas at those latitudes. The isotherms bend north there because the ocean is warmer, not colder. Over Europe the land cools sharply, so the isotherms bend south.
  • 2: incorrect — The statement gets the conclusion right (the isotherms bend north) but gets both causes wrong: it calls the currents cold and the ocean colder. A colder ocean would pull the isotherms south, towards the equator, so the statement contradicts itself.

How to eliminate

Statement 2 can be eliminated with basic knowledge. The Gulf Stream is one of the best-known warm currents, so calling it a 'cold ocean current' is wrong. The statement is also wrong on its own logic: if the North Atlantic were colder, the isotherms would bend south, towards the equator, not north. That rules out the two options that include Statement 2. For Statement 1, use one rule: in winter, land is colder than sea at the same latitude, and an isotherm moves equatorward wherever it is colder. So in January, Northern Hemisphere isotherms bend south over the land and north over the sea. If you know that rule, you can solve the question.

Know more

Isotherms are lines that join places with the same temperature. On world maps, temperatures are first reduced to sea level so that altitude does not distort the pattern. If latitude were the only factor, isotherms would run straight along the parallels. They bend because of land–sea contrast, ocean currents and relief.

The rule for bending: an isotherm moves poleward over a surface that is warmer than its latitude would suggest, and equatorward over one that is colder.

Feature January (N. Hemisphere winter) July (N. Hemisphere summer)
Over continents Land colder, so isotherms bend south Land hotter, so isotherms bend north
Over oceans Sea warmer, so isotherms bend north Sea cooler, so isotherms bend south
Classic example North Atlantic: the Gulf Stream and North Atlantic Drift push isotherms far north. Over Europe they dip sharply south. Highest land temperatures (over 30°C) over subtropical Asia near 30°N
Southern Hemisphere Isotherms are more regular and nearly parallel to latitude, because there is mostly ocean Same

Points from the NCERT text:

  • In January, the coldest readings are in the interior of Eurasia (Siberia). Average temperatures there are far below 0°C.
  • The annual range of temperature is largest, about 60°C, over north-eastern Eurasia, because of continentality. It is smallest, about 3°C, over the oceans between 20°S and 15°N.
  • Equatorial oceans stay above about 27°C in both months.

Ocean currents to remember: warm currents (Gulf Stream, North Atlantic Drift, Kuroshio, Brazil) raise coastal winter temperatures and keep ports such as those in Norway free of ice. Cold currents (Labrador, Canaries, Benguela, Humboldt/Peru, California) cool the coast and push isotherms towards the equator.

NCERT Concepts to Revise

  • Isotherm: Both statements are inferences about how isotherms behave on a January map, so first be clear about what the line joins.
  • Gulf Stream: Statement 2 depends on the nature of the Gulf Stream current and how it affects North Atlantic temperatures.
  • Factors controlling temperature distribution: Land–sea contrast and ocean currents are the controls that explain where the January isotherms bend.
  • Annual range of temperature: Continentality, which shows up as the annual range of temperature, explains why winter land surfaces cool so much more than the sea.
  • Isopleth: Isotherms are one type of isopleth; the same reading rules apply to isobar and isohyet maps that UPSC also tests.

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