Writing a summer pop tune is easy. Just take an uptempo rhythm, add a catchy chorus, stir in some breezy lyrics and you have the soundtrack to a thousand summer romances and several lifetimes' worth of wistful reminiscences.
Making a summer album, on the other hand, is a more challenging endeavor. Just repeating the above formula 12 times will leave you with an awkward and unbalanced result. A good summer album should take a more subtle approach, evoking the season in all its differing textures and moods while retaining a coherent overall atmosphere.
Pop music in Japan traditionally worked on a seasonal cycle, with new singles every three months that were often themed according to the time of year, the result of which is that albums would be a kind of annual roundup of loose singles bulked out with some filler. However, the growth of more "serious" pop and rock music since the 1970s and especially the late '80s mostly killed off that style — outside of the idol scene at least. While the well-worn subject matters of love, friendship and the importance of following your dreams continued to dominate, some acts started thinking a bit more thematically about their albums. The seasons continued to be a theme, but this time were spread out over a wider musical canvas.