I would consider the initial descriptions that Bradbury offers after the rain has settled in to display the intense change in mood.  When the rain starts, Bradbury's use of sensory details illuminates the change in the children:

A few cold drops fell on their noses and their cheeks and their mouths. The sun faded behind a stir of mist. A wind blew cool around them. They turned and started to walk back toward the underground house, their hands at their sides, their smiles vanishing away.  A boom of thunder startled them and like leaves before a new hurricane, they tumbled upon each other and ran.

The sensory language of how the rain started in droplets to land on their faces and bodies, along with the cool breeze accompanying the rain both become magnified with the children running and tumbling over one another.  All the while "their smiles vanishing away" helps to bring forth this basic idea of how life has changed back for seven years apart from what can be.  There is a definite mood shift in comparison to when the sun was out and the happiness that emanated from them and within them in seeing the sun.  I think that Bradbury's description when the rain resumes falling is so powerful because it shows how the physical reality that governs the children is also an emotional one.  This helps to bring forth the idea that the external conditions impact the subjective, emphasized in how the mood changes with the sensory descriptive experience of the children.