The poem's heavy alliteration helps reinforce its imagery and link important words and concepts together. In lines 2-3, for example, alliteration captures the insistent pounding of the rain:
Insomniac since four, hearing this narrow,
rigidly metred, early-rising rain
recounting, as its coolness numbs the marrow,
The /r/ sounds impose themselves on the reader's ear, much as the falling rain demands the speaker's attention.
In line 7, the speaker imagines that turning 40 brings a kind of reckoning with oneself. He calls this the "bleak modesty of middle age" (line 7). Growing older, in other words, makes you humble, stripping away the delusions of youth. The /m/ here connects "middle age" with that newly "modest[]" attitude.
In the following line, the speaker anticipates that he'll judge his work "as a false dawn, fireless and average[.]" Here, the fricative /f/ sounds highlight the negative qualities of the speaker's work; their harsh sound seems to convey some of his frustration.
The speaker turns his attention to his friend and fellow poet, John Figueroa. In lines 25-26, he imagines Figueroa facing middle/old age with quiet dignity, turning up daily for the difficult work of writing poetry:
[...] or you will rise and set your lines to work
with sadder joy but steadier elation,
Those sibilant /s/ sounds give the lines a whispery quality, hinting at the subdued life of middle or older age. They also give the lines themselves a "stead[y]" sonic consistency.
The speaker then compares this older and wiser type of poet to a "water clerk," or administrator measuring rainfall. Strong alliteration brings this simile to life:
[...] conventional as any water clerk
who weighs the force of lightly falling rain,
which, as the new moon moves it, does its work
even when it seems to weep. (lines 29-32)
Again, the repeating sounds (including the many soft /w/ sounds) reflect the image of steady rain. These sounds also lend extra "force" or emphasis to a passage about force—including the force of rainfall, the gravitational force of the "moon," and the emotional force of writing as a vocation.