>>12336402jesus christ just use the usual scources on the wikipedia page rabbithole thing.
http://www.fao.org/nr/climpag/globgrids/KC_classification_en.aspwhich again uses references from:
eck, C., J. Grieser and B. Rudolf, 2005: A New Monthly Precipitation Climatology for the Global Land Areas for the Period 1951 to 2000. Klimastatusbericht 2004, 181-190, DWD.
Mitchell, T., and P. Jones, 2005: An improved method of constructing a database of monthly climate observations and associated high-resolution grids. Int. J. Climatol., 25, 693-712.
http://www.cru.uea.ac.uk/Rudolf, B., C. Beck, J. Grieser, U. Schneider, 2005: Global Precipitation Analysis Products of the GPCC. Internet publication at
http://gpcc.dwd.de/ please btfo that retarded teacher, if this isnt enough use google to go even deeper