How are arid BW climates different from semiarid BS climates?

In the Köppen climate classification system, dry climates are categorized as Type B and are defined by the relationship between precipitation and evaporative losses rather than temperature alone. The distinction between arid (BW) and semiarid (BS) subtypes is based on the total annual precipitation relative to the threshold for a B-type climate.1

The differences are defined as follows:

• Arid (BW) climates: These are true desert regions where the average annual precipitation ($r$) is less than one-half of the upper limit established for a B-type climate.1
• Semiarid (BS) climates: These are steppe regions where the average annual precipitation ($r$) is less than the upper limit for a B-type climate but is greater than one-half of that amount.1

Essentially, the BS subtype is slightly wetter than the BW subtype. Both can be further differentiated by temperature using the codes 'h' (warm, with an average annual temperature $\ge 18$ °C) or 'k' (cold, with an average annual temperature $<18$ °C).21