Hot Desert versus Cold Desert Climates

The climatologist Wladimir Köppen first published his climate classification system in 1900 and substantially revised it in 1918, introducing the letter code that remains the global standard for distinguishing desert types: a leading "B" marks an arid climate, defined not by temperature alone but by a ratio between annual precipitation and potential evapotranspiration too low to sustain most vegetation. Within this arid category, a second letter separates true desert ("W," from the German Wüste) from semi-arid steppe ("S"), while a third letter separates hot arid climates ("h," mean annual temperature above 18°C) from cold arid climates ("k," mean annual temperature below 18°C, with a distinctly cold winter). Michael Peel, Brian Finlayson, and Thomas McMahon's widely cited 2007 update of the Köppen-Geiger map, published in Hydrology and Earth System Sciences, confirmed that arid "B" climates cover roughly 30% of the Earth's land surface, making drylands collectively the largest climate category on the planet, larger than either the tropical or the temperate zones.