CDS (II) 2020 GK Q9: The Isotherm Line, which divides India North-South in

Q: What is the answer to CDS (II) 2020 GK Q9?

D. Correct — D, 20 degrees Celsius. In January the sun is over the southern hemisphere, so peninsular India stays warm while the northern plains cool sharply — Chennai near 25 degrees, Delhi near 14. Somewhere in between lies the line joining places with the same January mean, and the 20 degree isotherm is the one that runs roughly west to east across central India, close to the Tropic of Cancer, cutting the country into a warmer southern half and a cooler northern half of about equal size. The other three values sit in the wrong places: 25 degrees would run through the far south, while 15 and 10 degrees would lie across the northern plains and the Himalayan foothills, leaving very unequal halves. Note that the line is not a straight parallel. Along the coasts the sea keeps January temperatures up and the isotherms bend to run parallel to the shoreline rather than east to west.

CDS (II) 2020 General Knowledge · Question 9 of 120

Question text verified against the official CDS question paper.

The Isotherm Line, which divides India North-South in almost two equal parts in the month of January, is

(a)10 °C
(b)25 °C
(c)15 °C
(d)20 °C

Answer

Why

Correct — D, 20 degrees Celsius. In January the sun is over the southern hemisphere, so peninsular India stays warm while the northern plains cool sharply — Chennai near 25 degrees, Delhi near 14. Somewhere in between lies the line joining places with the same January mean, and the 20 degree isotherm is the one that runs roughly west to east across central India, close to the Tropic of Cancer, cutting the country into a warmer southern half and a cooler northern half of about equal size. The other three values sit in the wrong places: 25 degrees would run through the far south, while 15 and 10 degrees would lie across the northern plains and the Himalayan foothills, leaving very unequal halves. Note that the line is not a straight parallel. Along the coasts the sea keeps January temperatures up and the isotherms bend to run parallel to the shoreline rather than east to west.

Why the others are wrong

(a)10 °C — A January mean of 10 degrees belongs to the far north — Punjab, Haryana and the Himalayan foothills. A line there would leave a very thin northern sliver, not two equal halves.
(b)25 °C — That is roughly the January mean of Chennai and the deep south. The 25 degree line hugs the southern tip and the coasts, nowhere near the middle of the country.
(c)15 °C — Closer to Delhi's January mean, so this line runs across the northern plains. It divides India, but into a small north and a large south.

Concept

An isotherm is a line on a map joining places of equal temperature. Maps of January mean temperature for India show values falling from about 24 to 25 degrees in the extreme south to about 10 to 15 degrees in the north-western plains, and lower still in the mountains. Because the land cools faster than the sea, the isotherms over the peninsula bend away from a simple east-west course and run parallel to the coastline, so that temperature along the coasts does not fall steadily from south to north the way it does inland.

The stem gives away its own method. It does not ask which isotherm is warm or cold; it asks which one divides India into two nearly equal parts. India runs from about 8 degrees to about 37 degrees North, so the dividing line has to sit near the middle latitudes of the country, in the neighbourhood of the Tropic of Cancer at 23 and a half degrees North. Then it is enough to know roughly what January feels like there — around 20 degrees in Nagpur or Kolkata — to pick the value. Worth carrying alongside it: a 1997 UPSC prelims question asked for the January isotherm used to divide India into tropical and sub-tropical zones and its answer is 18 degrees. The two are different questions with different purposes, one about a climatic classification boundary and one about a rough geographical bisection, so the two numbers do not contradict each other, but the pair is a reminder to read which division the stem is after.

Key facts

January mean temperatures in India fall from about 24 to 25 degrees Celsius in the far south to about 10 to 15 degrees in the north-western plains.
The 20 degree January isotherm runs roughly west to east across central India, near the Tropic of Cancer.
NCERT notes that along the coasts the isotherms run parallel to the coast, so temperature there does not decrease from south to north but increases from the coast inland.
In the cold weather season the peninsula has no well-defined winter — Thiruvananthapuram's mean maximum for January is about 21 degrees against 29.5 degrees for June.
December and January are the coldest months over most of the country.

Where each January isotherm would actually lie

25 degrees Celsius→Through the extreme south and along the southern coasts. Leaves almost nothing to the south of it.

20 degrees Celsius→Across central India near the Tropic of Cancer — a warmer south and a cooler north of about equal size. This is the answer.

15 degrees Celsius→Across the northern plains, near Delhi's January mean. A large south and a small north.

10 degrees Celsius→Across the far north-west and the Himalayan foothills. A thin northern sliver.

The stem asks for a line that bisects the country, so the arithmetic of latitude does most of the work before any temperature is recalled.

Study next

Reading January and July isotherm maps of India and explaining the difference between themWhy isotherms bend along coastlines, and the moderating influence of the seaThe cold weather season in India — western disturbances, winter rain and the absence of a real winter in the peninsulaThe Tropic of Cancer and the States it passes throughFactors controlling temperature distribution — latitude, altitude, distance from the sea and prevailing winds

Common traps

Reaching for the tropical and sub-tropical dividing isotherm of 18 degrees when the stem asks for a rough geographical bisection.
Assuming isotherms must run east to west everywhere. Over the coastal strips they run parallel to the coast.
Thinking January is uniformly cold across India. In the far south the January mean is around 25 degrees.

As a which-isotherm item for January or July, or as a reason question on why coastal isotherms run parallel to the shore.

Related PYQs

UPSC_1997_GS1_Q101997Open & attempt →

The January isotherm taken as a basis for dividing India into tropical and sub-tropical zones is

(a) 21 °C
(b) 18 °C
(c) 12 °C
(d) 15 °C

Answer(b) 18 °C

The same map, read for a different purpose. That item wants the line that marks off tropical from sub-tropical India, a climatic classification boundary at 18 degrees; this one wants the line that cuts the country into two roughly equal halves. Read the stem before reaching for a remembered number.

Practice

practice — not a real PYQ
In the month of January, the isotherms along the Indian coasts run roughly parallel to the coastline. This is mainly because of which one of the following?
- (a)The high altitude of the coastal plains
- (b)The moderating influence of the sea
- (c)The north-east monsoon winds
- (d)The presence of coral reefs offshore
Answer(b) The moderating influence of the sea — the sea cools far more slowly than the land, so coastal temperatures do not fall from south to north the way inland temperatures do.
practice — not a real PYQ
The Tropic of Cancer, close to which the 20 degree January isotherm runs, does NOT pass through which one of the following States?
- (a)Gujarat
- (b)Madhya Pradesh
- (c)Odisha
- (d)West Bengal
Answer(c) Odisha — the Tropic of Cancer crosses Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram, but not Odisha.