classic waka stream

though rain may fall
and even dew should not pass through—
on Mount Kasatori,
how has it come to be, I wonder,
that the leaves are dyed with autumn?

Meaning
Though rain may fall and even dew should not pass through, how has Mount Kasatori come to be colored with autumn leaves?
Commentary
Book Five Autumn Poems (Part Two)

Composed as an autumn poem.

It was believed that leaves turned color when moistened by dew or rain. Since the name “Kasatori” (taking a hat) suggests shelter from rain, the poet wonders how the mountain could have come to be colored with autumn leaves despite not being wetted.

“Mount Kasatori” is said to be located in present-day Uji, Kyoto.
Author
Ariwara no Motokata
Source
Kokin Wakashu
Other