The Köppen climate group designated by the letter “B” is Dry. It identifies climates where average precipitation is low relative to the region’s potential evaporation, rather than defining them by temperature alone.
Group B contains two major families: BW, for desert or arid climates, and BS, for semi-arid or steppe climates. A third letter may describe heat: “h” means hot and “k” means cold, producing classifications such as BWh for hot desert and BSk for cold semi-arid climates.
The letter B does not mean that a place receives no rain. Steppe regions can receive enough seasonal precipitation for grasses and shrubs, while deserts fall below a stricter dryness threshold. Köppen’s threshold also considers when precipitation occurs, because rain arriving during the hot season is lost more quickly through evaporation.
The system was developed by climatologist Wladimir Köppen and later refined with Rudolf Geiger. This is why “Dry” is more precise than choosing a landscape term such as “desert.”