Стало понятно, что ...
It seems that when the verb "стать" is used in the sense of "become ...", the instrumental case follows. According to Wiki, however, the instrumental case of the adjective "понятный" is listed as "поня́тным", and "понятно" seems to be its shortened form, whatever its case is.
So I wonder if this shortened form "понятно" applies to all cases of the adjective "понятный"?
And is it more common to use "понятно" than "поня́тным" in conversation?