In Köppen climate codes, the third letter d indicates a very cold winter.
This letter is used in the D, or continental, climate group. In the standard table, D climates can end in a, b, c, or d: a indicates a hot summer, b a warm summer, c a subarctic pattern with only one to three months above 10°C, and d an extremely cold winter. The d category is associated with a coldest-month average below −38°C.
The letter should not be interpreted as a general label for any chilly winter. It is part of a specific three-letter code and appears in combinations such as Dfd, Dwd, or Dsd. The first letter identifies the broad climate group, while the second letter describes seasonal precipitation, such as dry winter, dry summer, or no dry season.
A frequent confusion is that c means “cold winter.” In Köppen’s D group, c primarily describes the short, cool summer pattern, whereas d marks the extreme winter threshold. The exact boundary can vary slightly between versions using 0°C or −3°C for the continental-climate definition, but d’s very-cold-winter meaning is stable.