Asuma dies in volume 37 of naruto. It's soo sad. I mean, everyone breaks down literalry because of that. And just as he passes away, it starts to rain.

But besisdes that, it also reveals that there was probably a love affair going on between him and Kureni. Ino caught him buying flowers for her, and just as it rains, you see Kureni back at Konoha watching the rain.