Such is the case of this world where magic and emotions are one and the same and fill the air with life and hearts. It is what brings forth spirits into being and what drives the people to eat, work and live. After enjoying a long period of peace, the denizens of this world see the rise of a new form of weather - the weather of emotions. Now when emotions of the same nature run too high and saturate an area, clouds of that emotion can form above and drift away with the wind. In time the emotions will rain back down on the world and shower all below it in that emotion and magic. If you're lucky, you'll walk away from only slightly moody and the magic in the air will stay benign and ripe for harvest but if you're not, your mind could stop being your own and the local spirits and wildlife can become something entirely different.
Often enough, these clouds do tend to be made of negative emotions like anger, hate and fear while other times they can be composed of joy, harmony and nostalgia. Even rarer than the latter and serving as the core of this story is a uniquely formed cloud - the emotion of stillness. Mysterious in nature and formed seemingly spontaneously without rhyme or reason, this cloud forcibly absorbs the emotions of those within its vicinity and deadens those unfortunate enough to have been rained down on by one. Recovery in the former case is fairly quick and the temporarily muted emotions can boost productivity but the latter case takes years to come undone without drastic treatments to cure the victims.
One such victim of the still rain showers is a young boy whose talents in magic and studies awakened because of what ails him. Having just come of age in his society he sets out on a journey to find a cure for people like him and begins hunting down these clouds and any and all clues he may find of their origins. His journey will see him steadily clear away the heavy cloud that hangs over his heart as unlocks the secrets of this mysterious phenomenon.
>just if there is an specific time and reason behind the origin of the clouds
A wizard did it and now that wizard's line of successors repeatedly manipulate the system to harness special mixes of liquid mana for their research.
Latest successor is something of a foil to the MC who'd rather trade off his talents so he and friends back home can feel emotions properly again as opposed to the successor's single minded pursuit after his workshop's grand legacy.
Intended ending would have MC willingly take the completed elixir to permaboost and permamute himself so he'd be able to create a cure for everyone else. No real bad guy to beat, just mainly about MC rediscovering the joys of life and how precious that is.
As self-sacrificial that ending sounds, I pictured it as a hopeful and meaningful one for him. Also, I'm pretty sure I got the title and base premise from Bleach and how Ichigo's inner world would rain when he was down. Even I find it strange how Kubo manages to inspire some of the more interesting and heartful plot ideas come up with. It's like he somehow holds power over his readers even years after the fact. Sasuga Kubo.