Weather-sealed against ‘tropical’ conditions. The term was used to signify resistance against hot and humid conditions in a small number of watches in the 1960s and 70s, typically via better gaskets, added lubrication, and corrosion resistance. It may be an anglicization of the French term ‘tropicalisé’ which means ‘to make resistant against tropical conditions’. Not to be confused with ‘tropical dial’, a dial patinated by humid and sunny conditions.