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

The standard January isotherm used to demarcate India’s tropical (southern) and subtropical (northern) zones is the 18°C line. Textbook description notes that January isotherms run nearly parallel to latitudes and specifically states that the 18°C isotherm for January traverses the middle of the country in an east–west direction, linking the Tapi estuary in the west with the Mahanadi delta in the east; this latitudinal position is therefore taken as the practical boundary between tropical and subtropical India [1].

Sources

1. [1] Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 4: Climate of India > 1. The Cold Weather Season > p. 17