Visibility conditions can change daily. It may be clear one day and hazy the next. Each year, the daily results are analyzed to determine what conditions were like on the days with the best and worst visibility. Under the Regional Haze Rule, states must protect air quality to maintain the level of visibility on the clearest days and improve the visibility on the poorest days.

The air samples are analyzed for types of pollutants and sources of pollution found on the clearest and haziest days. Data show that haze-causing pollutants vary daily depending on sources of pollution, changing climatic conditions, or natural events such as fires. Scientists look to see if the types and sources of pollution are different on the clearest days as compared to the haziest days.

Visibility is measured in a unit called a "deciview," which is basically a change in visibility that the human eye can detect. In mathematical terms, one deciview is a 10 percent change in the light extinction equation used to calculate visibility. It is an inverse linear scale, so that for every change in deciviews, there is a one-to-one corresponding change in how far the eye can see. The higher the deciviews, the less a person can see into the distance.