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

1. India can be broadly divided into tropical and sub-tropical climate zones. 2. The Tropic of Cancer (23.5° N latitude) is often considered the geographical dividing line, but climatic divisions often use temperature criteria, especially isotherms (lines connecting points of equal temperature). 3. For climatic classification in India, the January isotherm is commonly used to delineate the boundary between the warmer tropical south and the cooler subtropical/temperate north. 4. Climatologists like Stamp and Kendrew used the 18°C January isotherm as a basis for separating the tropical zone (south of this line, where January mean temperature is above 18°C) from the sub-tropical zone (north of this line, where January mean temperature is below 18°C).