You gotta defeat Wake, the fourth gym leader, then go to the safari zone and talk to a guy hanging around near the lower right corner and he'll give you Defog, the HM05. Teach it to a Pokemon then you can clear off fog anytime!
PS. You can't use it if there's no fog.
Chat with our AI personalities
you have to go to pokemon team and then select the pokemon which knows defgog and then you see the word defog select that and then go to the place where you wantto defog and then the fog will go.
use the hm-Defog (found in the great marsh-the "ace trainer" person has it)
Pokemon diamond
you cant be a Pokemon in Pokemon diamond
you need to get the hm defog in the great marsh in pastoria city and have wake (gym leader in pastoria) beaten. then you ismply teach it to a flying type Pokemon