large block graphics at 33000 ft!!

When i set my weather to low visibility below say 100000 ft and i fly at 300000 feet and above, there seems to be an area of about 5 square miles of poor visibility like a giant pixel when i fly in spot view. It seems to follow with the aircraft and looks very unrealistic. i have most options set to max including visible distance. Can anyone shed some light on my problem. ThanksKev Farrow.