welcome rain in a drought; long-awaited relief
A yojijukugo (four-character idiom) used literally for beneficial rain during a drought, and figuratively for any long-awaited relief or realization of something eagerly anticipated. Literary and somewhat formal.
After the long drought, a welcome rain fell and the farmers rejoiced.
After the long recession, this new policy was truly a welcome relief.
A yojijukugo composed of 干天 (drought, literally 'dry heavens') and 慈雨 (merciful rain). The exact historical origin is uncertain, but it follows a common classical pattern of pairing natural phenomena with human-like virtues.