It does not! That phrase is the name of a song from the musical My Fair Lady. The rainiest part of Spain is in the hilly northwest on the Bay of Biscay.
IN THE PLAINS. You know how they sang "the rain in Spain stays mainly in the plains!"
The rain in Spain stays mainly in the plains.
The rain in Spain stays mainly in the Plains. Last year was the hottest ever recorded.
No, it is an "assonance". Assonance repeats vow sounds, alliteration repeats consonant sounds as in "Peter picked a peck of pickled peppers".
The rain in Spain stays mainly on the plain
Mainly rain, if you're in Spain.
The rain on the plane falls mainly in Spain.
My Fair Lady
The rain on the plane falls mainly in Spain.
Cows nibble the flowers or geese wheel above the lake
Japan is mostly mountains. It has no rainforests or deserts.
King Cobra's mainly live in rain forests in Plains of India, Southern China, and Southeast Asia.