出典:Wiktionary
Many speakers prefer to use the long a pronunciation of the homograph patent in its sense meaning "open" or "clear", whereas the short a pronunciation is the norm for its sense meaning "a government-issued temporary monopoly on an idea" (the senses are cognate but long since differentiated). In this view, prescriptively, the adverb patently is best pronounced with a long a, although descriptively it is true that speakers often pronounce it with a short a. See patent § 発音, patent § Etymology 1, and patent § Etymology 2.
patently (comparative more patently, superlative most patently)