Since nobody can actually grasp a cloud, the expression denotes impossibility. From afar, a cloud has shape and form. Close-up, its gossamer essence dissipates at the touch. So when a not-so-talented sixteen-year-old declares that she’s going to become a movie star, you can respond by saying it’s a kumo o tsukamu yoona dream – a mild warning that the ambition or goal is highly unlikely to be realized.
(Style: spoken/casual/A=son, B=mother)
A: Sakkyokuka ni naroo ka na.
B: Sonna kumo o tsukamu yoona koto bakari kangoeteiru kara itsu made tatte mo hitoridachi dekinai n desu yo.
A: You know, maybe I’ll become a composer.
B: (Scolding) All you ever think about is impossible dreams like that! No wonder you’ve never been able to become financially independent.