UPSC Prelims 2025 Q27: January Isotherms over Land and Ocean
Consider the following statements:
Statement I: In January, in the Northern Hemisphere, the isotherms bend equatorward while crossing the landmasses, and poleward while crossing the oceans.
Statement II: In January, the air over the oceans is warmer than that over the landmasses in the Northern Hemisphere.
Which one of the following is correct in respect of the above statements?
- Both Statement I and Statement II are correct and Statement II explains Statement I
- Both Statement I and Statement II are correct but Statement II does not explain Statement I
- Statement I is correct but Statement II is not correct
- Statement I is not correct but Statement II is correct
Answer: (a) Both Statement I and Statement II are correct and Statement II explains Statement I
Statement-wise explanation
- Statement I: correct — In the Northern Hemisphere in January, land loses heat fast and becomes colder than the sea at the same latitude. A given temperature therefore lies further south (towards the equator) over the continents and further north (towards the pole) over the oceans. NCERT's January isotherm map shows this most clearly over the North Atlantic, where the isotherms swing north between North America and Europe and dip south over Eurasia and North America.
- Statement II: correct — Water has a higher specific heat than land, mixes heat through a deep layer and cools slowly. So in the Northern Hemisphere winter the oceans stay warmer than the continents at the same latitude. The Gulf Stream and North Atlantic Drift add to this: Britannica notes that winter air over the ocean west of Norway is more than 22°C warmer than the average for its latitude. This warmer air over the oceans is the direct reason the isotherms bend poleward over the sea and equatorward over land, so Statement II explains Statement I.
How to eliminate
Treat this as a cause-and-effect question. An isotherm joins places with the same temperature. If it bends poleward over the oceans, the oceans must be warmer than land at that latitude. So Statement I and Statement II say the same thing in two ways: one describes the map and the other gives the temperature reason behind it. If you accept one, you have to accept the other, and the second explains the first. You can also get there from basic land-water contrast: land heats and cools quickly, water slowly, so in January the continents are colder. Remember the seasonal reversal: in July, Northern Hemisphere isotherms bend poleward over the hot continents and equatorward over the cooler oceans.
Know more
Why isotherms bend
If temperature depended only on latitude, isotherms would run parallel to the lines of latitude. They bend because of three main controls:
- Land vs water. Land heats and cools quickly. Water has a high specific heat, mixes heat downward, and is partly transparent, so it changes temperature slowly.
- Ocean currents. Warm currents such as the Gulf Stream and North Atlantic Drift push isotherms poleward. Cold currents such as the Labrador and California currents pull them equatorward.
- Altitude and relief. High plateaus and mountains are colder than lowlands at the same latitude.
| Feature | January (NH winter) | July (NH summer) |
|---|---|---|
| Over NH continents | Isotherms bend equatorward (land colder) | Isotherms bend poleward (land hotter) |
| Over NH oceans | Isotherms bend poleward (sea warmer) | Isotherms bend equatorward (sea cooler) |
| Clearest example | North Atlantic; very cold Siberian interior | Hot interiors of Asia and North Africa |
| Southern Hemisphere | Isotherms nearly parallel to latitudes | Same |
Why the Southern Hemisphere looks different. It is mostly ocean, so the land-water contrast is weaker. Its isotherms run nearly east-west and are more evenly spaced.
Annual range of temperature. Continentality also explains the annual range: the difference between the warmest and coldest months. The range is largest in the interior of north-east Eurasia, at over 60°C, and smallest over the equatorial oceans, at only a few degrees.
Exam link. UPSC Prelims 2024 also asked what January isotherm maps show. Learn the January and July maps as a matched pair.
NCERT Concepts to Revise
- Isotherm: Statement I is about the shape of isotherms, so you need to know exactly what an isotherm joins.
- Differential heating of land and water: How land and water heat and cool at different rates is the physical process both statements depend on.
- Continentality: Distance from the sea decides how cold the continents get in January compared with the oceans.
- Annual range of temperature: The land-ocean contrast behind the bending isotherms also produces very different annual temperature ranges over continents and oceans.