Theory:

When the humid shadows hover
Over all the starry spheres
And the melancholy darkness
Gently weeps in rainy tears,
Explanation:
 
The poet begins his narration by picturising a beautiful day. His words reflect the beauty of the events unfolding. He starts off  by describing the process of how rain is made. Rain is caused by the formation of clouds. When water from the earth's surface gets evaporated, it becomes water vapour which later on becomes clouds. The clouds have water droplets, which, when it gets too heavy to stay inside the cloud, falls into the earth as rain. He calls the movement of the clouds as humid shadows. When it is about to rain, the sky becomes dark due to the formation of clouds overhead, which is very similar to a dark shadow that is cast by an object.
 
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Dark clouds
 
The clouds move around the starry spheres, which is the sky. But when it is about to rain, the stars in the sky usually become dim because of the darkness, and therefore he says that it is a melancholy darkness. Melancholy, although it means a kind of sadness, here he signifies a warmth in one's heart. A lonely feelingand comfort in being with oneself is what is referred by the poet. The state of melancholy is further transferred into the next line when he says that rain weeps in tears. He beautifully compares rain to tears falling from the eyes of a lonely person. He makes the descriptions of the process of rain falling beautiful by making these comparisons.
 
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Starry Sphere
 
Words with Difficult Meaning:
 
S.No
Words
Meaning
1
HumidContaining a lot of water
2
ScatterTo move quickly in different direction
3
Starry SpheresA sky filled with stars
4
MelancholyA feeling of sadness
5
EvaporateTo change into steam and disappear
6
Water vapourWater turning into a gaseous state
Reference:
National Council of Educational Research and Training (2006). Beehive. Rain on the Roof -  Coates Kinney (pp 41-42). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.