Meiji,
First is the original kanji for emperor (王), which is three horizontal lines connected by a vertical line. The three horizontal lines represent Heaven, the emperor, and earth. The idea is that the emperor was the connection for the common people between heaven and earth. (I have heard that these same three horizontal lines are important in flower-arranging.)
Which bring us to heaven (天). It is a picture of heaven, via the emperor, spreading its happiness upon the world.
Rain (雨 ) is the same idea, with the raindrops falling from the same upper horizontal (heaven) line.