In the Trewartha climate classification, the letter A designates tropical climates.
Trewartha’s system, first published by American geographer Glenn T. Trewartha in 1966, modified the Köppen scheme to represent vegetation and middle-latitude climates more closely. It retained A for the tropical realm, where every month has a mean temperature of at least 18 °C (64.4 °F).
The A group is further divided by rainfall seasonality. Ar indicates tropical climates with no more than two dry months; Am identifies tropical monsoon conditions; Aw marks a dry winter; and As marks a dry summer. These codes can look similar to Köppen symbols because Trewartha adapted much of its structure.
A common mix-up is choosing C, D, or E because those letters also describe broad thermal zones in Trewartha’s system. C means subtropical, D temperate, E boreal, and F polar. B is reserved for dry climates, so A is the only canonical answer for tropical climates.