Okay, you see something moist looking in the air out there, obscuring your view of the Mall or the next stoplight. How can you describe it? As fog? Mist? Haze? Believe it or not, science has the answer. Fog really is what people mean when they sing about "foggy London town." It is made up of water droplets, suspended in the air to the point that they reduce visibility beyond five eights of a mile. But mist is more like what Hawaiians call "liquid sunshine." It has less visually obstructive water droplets that can be condensing or falling as a very light form of precipitation. On the other hand, haze may look like fog or mist and may even contain water. But it is mostly suspended particulates such as dust, salt or smoke, and at this time of the year, it is your most likely culprit.
Mary Knapp, Weather Data Library