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

1. 21°C
2. 18°C
3. 12°C
4. 15°C

Explanation

Option (b) is correct

• The standard January isotherm used to distinguish India’s tropical (southern) and subtropical (northern) regions is the 18°C line. In January, isotherms generally run almost parallel to the latitudes. The 18°C January isotherm passes across the central part of the country in an east–west direction, extending from the Tapi River estuary in the west to the Mahanadi River delta in the east.

Answer: (b) 18°C