If one happnes to see a rainbow at night (rare phenomenon) does that require a bracha,or only by day do we say a bracha?
There is a Teshuvah (2:124) from רב שלום שוואדרון which says that even though it is very unusual to see a rainbow at night, one should still make the blessing.
He brings proofs from various other subjects (with which I am not familiar) that even though the implication from the source text may be that the occurrence is during the day,
"אורחא דמילתא נקט ולאו דוקא"
"the source mentions the normal way of things and does not limit us to that way."