Longfellow's poem is about the coming of rain in a draught or in a dry season. He heralds how the rain looks coming down in "fiery" streets and lanes. He tells of how it sounds as it "clatters" along dry rooftops and how it gushes out of dry, "choking" water spouts, tumbling down from the roofs. He speaks of how it clatters on the "window pane" while washing a "muddy tide" of accumulated dry dirt ahead of it. He ends by saying how welcome rain is to the plants on the plains: " How welcome is the rain!"
This poem begins with a description of rain following a dry, hot climate, maybe a summer day. It is a "welcome" rain, meaning the land had been too dry, the weather too hot. The rain is also described as being full of energy ("gushes and struggles" and "roars"). These literal descriptions will be used metaphorically and analogously in the rest of the poem.
The poem then describes how the rain affects other people and how its actions are similar to those people.
A sick man sees the rain's cooling and soothing effect on the outside world and his fever is likewise cooled and soothed. A group of rambunctious boys are coming from school and the gushing turbulence of the rain is compared to the energy they have. The water streaming down the streets mimics their own energetic play.
The rain is again a welcomed event "on every side" of the country. The oxen welcome the rain as well, with silent admiration and thanks:
Their large and lustrous eyes
Seem to thank the Lord,
More than man's spoken word.
Of course, the farmer welcomes the rain because it will help his crops to grow and helps him to make a living.
The speaker then invokes the Greek figure Aquarius who is known as a water carrier and a cup-bearer to the Greek gods. This adds a spiritual reference to the rain as sustenance for land, plants, animals similar to sustenance for the gods.
The speaker calls The Poet (referring to himself and poets in general) a "seer," one who can see or imagine things others can not. The poet sees the life cycle of water from rain to the ground and back up to the sky. The rain helps give life to plants, animals and the rain reaches the graves of the dead as well; almost like saying the rain knows the cycle of life intimately, like the Poet does.
The poem is about cycles, life cycles; the poet can see these cycles as part of a much larger, "immeasurable" really, cycle ("wheel"). The Poet "sees" (imagines and understands) this idea of cycles "From birth to death, from death to birth" - but still is in awe ("wondering eyes") of it all.
"How Beautiful is the Rain" is a poem all about what happens when rain comes to a dry place. In the poem, the falling rain has an impact on different people and in different places. The writer shows that rain is a beautiful thing in many different ways.
The rain quenches heat and settles dust. Rainwater flows all over houses and other buildings in cities. A sick man with a fever feels the coolness brought on by the rain. It gives him relief from the heat. He is also comforted by the falling rain. A group of schoolboys play in the rain. The rain falls on the thirsty grasses in the plains after a draught. Oxen who are used to plow fields feel the relief and rest from the rain. The farmer surveys his crops as the rain pours over them. He knows that his crops will benefit from the rain. The poet describes Aquarius and how he walks and the clouds moving.
The farmer thinks about the water and all the places where it will go. He ponders the journey each drop will take:
Down to the graves of the dead,
Down through chasms and gulfs profound,
To the dreary fountain-head
Of lakes and rivers under ground;