As Right as Rain

Recorded in Britain from the late 19th century, first appearing in the London newspaper The Weekly Dispatch in 1886 and fully established by 1894. It follows a whole family of older "right as ___" comparisons, including "right as a trivet," a reference to a stable three-legged stand, used by Charles Dickens in The Pickwick Papers (1837). Why rain specifically joined that pattern is genuinely unclear. Guesses range from simple alliteration to rain's association with fertile crops and prosperity, or its sheer reliability in Britain's notoriously damp climate, but none has ever been confirmed.