when i mega evolve to mega drampa, does cloud nine dissapear

Yes — if Mega Drampa works like other Mega Evolutions, Cloud Nine would stop working once it Mega Evolves , because Mega Evolution replaces the Pokémon’s regular form and ability with the Mega form’s ability. A competitive guide on Drampa specifically notes that you may not want to Mega Evolve immediately because losing Cloud Nine turns weather effects back on.

What that means in battle

• Before Mega Evolving, Drampa’s Cloud Nine suppresses weather effects while Drampa is active.

• After Mega Evolving, Drampa would no longer have Cloud Nine unless Mega Drampa’s Mega ability is also Cloud Nine, which is not shown in the sources I found.

• So if you Mega Evolve right away, weather-based advantages and penalties would resume.

Practical example

If rain or sun is up, Cloud Nine can shut off the battle effects of that weather while Drampa is in play. Once it Mega Evolves, those weather effects would come back into force, which is why some players delay Mega Evolving Drampa.

TL;DR: yes, Cloud Nine should disappear on Mega Evolution unless Mega Drampa’s own ability specifically keeps it.