薄雲西風 Thin Cloud And West Wind
但見那：但见那：
逐句解 Line by line
And this is what they saw:
The narrator stops the story to show a picture. 但見那 is one of the standard phrases that announces a set-piece poem, rather like a stage direction.
Thin cloud broken and scattered, the west wind keen; a crane calling from far peaks, the frosted woods like brocade.
Broken cloud, a hard west wind, a crane, frost. 薄 takes the literary bó in verse rather than the everyday báo. 岫 xiù is properly a cave in a mountain, and by extension the peak itself.
The prospect all bleak and grey; the mountains long, the waters longer.
長 is cháng, "long," at both places in this line — not the zhǎng of growing up. The parallel positions force it.
Travelling geese come down to the northern marches; the dark birds go home to the southern lanes.
Birds keeping the calendar. 塞 is sài, a frontier fort or the frontier itself, not the sāi of plugging a hole. 玄鳥, the "dark bird," is the swallow, whose departure south is one of the oldest markers of the season.
On a traveller's road one shrinks from being alone; a patched robe grows cold soon enough.
The poem finally comes down to a person. 衲衣 is the robe patched together from cast-off scraps that a mendicant wears — the standard shorthand for a monk. It ends on cold, which the very next line of prose will turn inside out.
The poem is a completely conventional autumn scene, and that is the point of it: broken cloud, a keen west wind, a crane on far peaks, frost-brocaded woods, geese heading north and swallows south. Any reader of the period would have known every item. What the verse does is certify the season on the authority of poetry itself, rather than on any character's say-so.
Only the last two lines get personal. 客路 — the traveller's road — and a patched robe that "grows cold soon enough" bring the whole wide panorama down onto four cold bodies a very long way from home. That narrowing is the only real move the poem makes.
Placed where it is, the poem works as evidence. The pilgrims start sweating a line later, and the reader has just been shown, in the most unimpeachable style available, that it ought to be freezing.