Dry climates -- category B -- are divided into two subcategories, BS for steppe and BW for desert. Since average monthly temperatures sometimes fall below freezing, k, meaning kalt (German for "cold"), is appended to distinguish this climate from its warmer-weather counterpart.
Boreal forest climates ("boreal" means "northern") -- category D -- can exhibit extreme seasonal fluctuations, and winter precipitation often falls as snow. They are further distinguished by adding single lower-case letters, describing typical precipitation. There are two subcategories: f indicates a location that is wet year-round and w describes a region with a dry winter and a wet summer.
By Dave Bryant, copyright 2002