In January, the isotherm that divides India into roughly two equal northern and southern parts is the:

In January, the 20 degree Celsius isotherm runs roughly east-west across the middle of India, close to the Tropic of Cancer, dividing the country into two nearly equal parts.

Temperature pattern in January:

• January is the coldest month in most of India.
• The temperature generally decreases from south to north.
• Areas south of the 20 degree Celsius isotherm, such as the peninsula, remain warm, with average temperatures above 20 degrees and often above 25 degrees in the far south.
• Areas north of it, such as the northern plains and the Himalayan region, are much colder; temperatures in Punjab and Haryana may fall to about 10 to 15 degrees Celsius, and below freezing in the mountains.

Why the south is warmer in winter:

• It is closer to the Equator and receives more direct sunlight.
• The moderating influence of the sea keeps temperatures mild along the coasts.

Why the north is colder:

• It is farther from the Equator.
• Cold winds and western disturbances from the north-west bring cold waves and fog.

The 20 degree Celsius January isotherm and climate:

• It is sometimes used to divide India into a tropical south and a subtropical north.
• North of it, winters are distinctly cold, and frost can occur.

Effect of the Himalayas:

The Himalayas block the bitterly cold winds of Central Asia from entering India in winter. Without them, northern India would be much colder.

What an isotherm is:

• An isotherm is a line joining places with the same temperature at a given time or the same average temperature over a period.
• Temperatures are usually reduced to sea level before plotting, to remove the effect of altitude.

Why not the other options?

• ✗The 10 degree Celsius isotherm passes through the far north in January.
• ✗Good to know
• ✗30 and 35 degree Celsius are summer temperatures, not January averages.