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

In the month of January, the 20°C isotherm line acts as a significant climatic boundary, effectively dividing India into two almost equal northern and southern parts. This line runs roughly parallel to the Tropic of Cancer, separating the subtropical and temperate northern regions from the tropical southern peninsula. While northern India experiences a distinct cold weather season with mean temperatures often dropping below 15°C in the Great Plains, the peninsular region remains relatively warm with temperatures staying above 20°C [1]. Specifically, the 20°C isotherm is frequently cited in Indian climatology as the line that demarcates the cooler northern continental climate from the warmer tropical climate of the south during the peak of winter. This thermal distribution is influenced by the latitudinal extent and the protective role of the Himalayas against cold Siberian air [1].

Sources

1. [1] Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 4: Climate of India > 1. The Cold Weather Season > p. 18