Dry climates -- category B -- are divided into two subcategories, BS for steppe and BW for desert. Since average monthly temperatures never fall below freezing, h, meaning heiss (German for "hot"), is appended to distinguish this climate from its colder-weather counterpart.
Mild, humid climates -- category C -- are further distinguished by adding single lower-case letters, describing typical rainfall. There are three subcategories: f indicates a location that is rainy year-round, s describes a region with a dry summer and a rainy winter, and w is just the reverse, with a dry winter and rainy summer.
By Dave Bryant, copyright 2002