use the hm-Defog (found in the great marsh-the "ace trainer" person has it)
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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.
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.
The Pokemon Emerald fog problem is a bug in the software you are using. The fog bug was fixed in the next version so update the program and it will be gone.
teach a flying Pokemon defog and use it
water+air=fog