In Köppen climate codes, the second letter w marks a dry winter.
Köppen codes use letters to summarize temperature and precipitation patterns. The first letter identifies the broad climate group, while the second letter usually describes seasonal precipitation. In the C and D groups, w means dry winter, s means dry summer, and f means no dry season.
The same lowercase w also appears in tropical A-group codes, where Aw means a tropical savanna climate with a dry winter. Thus, the letter’s broad meaning is consistent, although the exact climate type depends on the first letter and the rest of the code.
For example, Cwa is a dry-winter humid subtropical climate, while Dwa is a dry-winter humid continental climate. The “w” does not mean winter is warm or cold; it describes rainfall distribution. Köppen’s definitions compare seasonal precipitation, so w should not be confused with the uppercase W used in BWh or BWk, where W means an arid desert climate.