Contrary to what it says in the song, the rain in Spain does not stay mainly in the plain. It falls mostly in the mountainous regions of Cantabria and Asturias. Ask meteorologists why, and they will explain that the prevailing winds pick up moisture over the Atlantic, and that when this moist air hits Spain's northern mountain ranges it is forced up to higher altitudes, where the moisture condenses to form clouds, and then rain. So far so good - except that it's only half an answer.