The earliest quantitative rain gauge seems to be credited to a king in Korea called King Sejong who lived from 1397 to 1450. One of his greatest goals as king was to make his people literate, so not only did he invent a rain gauge, but more importantly, he invented a language and movable type for that language.
He decided that instead of digging into the soil to check for moisture, it would be better to have a standardized container about 30cm in depth and 14cm in diameter that stood on a pillar to measure the rainfall. His standard of measurement however is not known. These containers were to help villagers determine their potential harvest, to give him a better idea of how much the farmers should be taxed. So these standard containers were distributed to each village. The gauge was invented in the fourth month of 1441 according to records. The invention of the rain gauge in Korea came two hundred years before inventor Christopher Wren created a rain gauge (tipping bucket rain gauge circa 1662) in Europe.
Courtesy: CoCoRaHS and About.com