It is named after the first words of the Mass (Introit) of the day in which it was established, the Fourth Sunday of Advent. It truly is one of the most beautiful of all Introits in the Liturgical Year.
oráte, cæli, désuper, et nubes pluant justum: aperiátur terra, et gérminet Salvatórem. (Drop down dew, ye heavens, from above, and let the clouds rain the Just: let the earth be opened and bud forth a Savior.) Ps. 18, 2 Cæli enárrant glóriam Dei: et ópera mánuum ejus annúntiat firmaméntum. (The heavens show forth the glory of God: and the firmament declareth the work of His hands.) Glória Patri...
The imagery of celestial dew, a favorite of the Latin Church (it is mentioned, for instance, in the Traditional formula for the Consecration of Bishops), is often repeated in these words adapted from Isaias. The same words are repeated throughout Advent in responsories of the Divine Office, partly in the Introit of the Mass of the Blessed Virgin Mary (S. Maria in Sabbato) in Advent, as well as in several other traditional liturgical texts of the West.
The audio and image files of the Introit are available here and an actual audio recording of it is available here (mp3 file; the quality is poor, but one can clearly hear the metalic sounds of the chains of thuribles...).
