Theory:

How beautiful is the rain!
After the dust and heat,
In the broad and fiery street,
In the narrow lane,
How beautiful is the rain!
 
How it clatters along the roofs,
Like the tramp of hoofs
How it gushes and struggles out
From the throat of the overflowing spout!
 
Across the window-pane
It pours and pours;
And swift and wide,
With a muddy tide,
Like a river down the gutter roars
The rain, the welcome rain!
 
The sick man from his chamber looks
At the twisted brooks;
He can feel the cool
Breath of each little pool;
His fevered brain
Grows calm again,
And he breathes a blessing on the rain.
 
From the neighboring school
Come the boys,
With more than their wonted noise
And commotion;
And down the wet streets
Sail their mimic fleets,
Till the treacherous pool
Ingulfs them in its whirling
And turbulent ocean.
 
In the country, on every side,
Where far and wide,
Like a leopard's tawny and spotted hide,
Stretches the plain,
To the dry grass and the drier grain
How welcome is the rain!
 
       ~ Henry Wadsworth Longfellow
Summary:
  
The poet talks about the bliss that one gets when it rains after a long time of summer. It is unexpected and most welcome after a prolonged season of heat and dust. He describes how it falls over the roof and compares it to horses' hooves. He calls the sky an overflowing spout from which water gushes out. The rainwater pours over the windowpane where he sits, enjoying it. A sick man who looks at the rain feels calm again. The beauty of the rain heals his mind and body. A group of schoolboys play in the rain and it makes their day. The rain falls on dry grass and areas that are affected by drought. The rain is welcomed for its healing nature.