“Nebulus” is the incantation for a spell that causes fog and then rain to appear (CG).
Albus Dumbledore used this spell to obscure himself and Newt Scamander from view when they met in London and Dumbledore told Newt that Credence was in Paris (CG).
from the Latin nebulosus, meaning cloudy or obscure
This was a very unsuspicious means of concealment when Dumbledore used in in London in CG, since London is known for its fog and damp weather. -BB