The January isotherm taken as a basis for dividing India into tropical and sub-tropical zones is 18°C.

The January isotherm refers to the line on a map that connects areas with the same average temperature in January. In the context of India, the 18°C January isotherm is commonly used as a reference to divide the country into tropical and sub-tropical zones.

Areas with an average January temperature above 18°C are classified as tropical regions, characterized by warm to hot climates. These regions typically experience high temperatures throughout the year, with minimal variations between seasons.

On the other hand, areas with an average January temperature below 18°C fall into the sub-tropical zones. These regions generally have milder winters and cooler temperatures compared to the tropical zones.

It`s worth noting that isotherms are not fixed boundaries and can vary slightly from year to year. However, the 18°C January isotherm is commonly used as a general guideline for classifying India into tropical and sub-tropical regions.