In Köppen codes, the third letter “b” indicates a warm summer. It is used mainly in temperate (C) and continental (D) climates, where the summer is warm but does not meet the stricter definition of a hot summer.
For C and D climates, “b” generally means the warmest month averages below 22°C while at least four months average above 10°C. By comparison, “a” marks a hot summer, usually with a warmest-month average of at least 22°C, while “c” describes a cool or short summer.
The letter’s meaning depends on the climate group. In dry climates, the third letter instead uses “h” or “k” to distinguish hot and cold conditions, so “b” is not a universal third-letter meaning across every Köppen code.
A code such as Cfb therefore describes a temperate climate with no dry season and a warm summer. The Köppen system combines temperature and precipitation letters, which is why reading one symbol in isolation can cause mix-ups.