The Köppen climatic classification system in a modified format is the most widely applied system in use today. Mapping and analysis of hundreds of arid and semiarid climate stations has made the use of the additional fourth letter in BW/BS climates essential. The addition of "s," "w," or "f" to the standard three-letter designation adds significant information when mapping and analysis is being conducted. A new quantity is herein proposed, the potential excess precipitation value (PEP value). The PEP value is actual annual precipitation minus potential annual evapotranspiration. When mapped, PEP value equal zero is the boundary between BS and C or D climates. [This article is an addendum to: Larson, P. R., and C. F. Lohrengel II. 2011. A new tool for climatic analysis using the Köppen climate classification. "Journal of Geography" 110 (3): 120-130 (EJ925181).]