In the Köppen system, the letter A designates tropical climates.
Köppen’s first letter identifies one of five broad climate groups: A tropical, B dry, C temperate, D continental, and E polar. Group A climates have a mean temperature of at least 18°C in every month, although rainfall patterns can vary greatly.
The system was developed by climatologist Wladimir Köppen, whose botanical background influenced its emphasis on natural vegetation. Tropical climates are then divided by their precipitation regime: Af tropical rainforest, Am tropical monsoon, and Aw or As tropical savanna. The second letter therefore describes moisture seasonality, not the basic temperature group.
A common mix-up is confusing A with the letter used for arid climates. In Köppen’s notation, B means dry, while D means continental. Another useful distinction is that a place can be tropical in the Köppen sense without lying exactly between the Tropics of Cancer and Capricorn, because local elevation and other conditions affect temperature.