The sim is supposed to draw the visibility conditions based off the latest METAR report at that airport. You could check to see what that METAR was reporting to see if there should actually be reduced visibility at that airport. Unfortunately the sim draws METAR based haze out to a fixed radius from the airport, so you wind up with haze bubbles.
Unintended benefit is that you can now easily find airports. Just look from the blanket of haze!