Visibility Index Development

Executive Order 2000-3 (46 KB) directed the Governor’s Brown Cloud Summit “to establish options for a visibility standard or other method to track progress in improving visibility in the Phoenix area.” The Summit concluded that a daily visibility index for the metropolitan area should have its characteristics defined through a public survey process. This process called for a representative cross-section of residents of Area A (as described in House Bill 2538, roughly the Phoenix metropolitan area), to determine what visual air qualities are desirable, what visual range is acceptable, and how often the combination of acceptable visual range and air quality is preferred.

Through a series of meetings in 2002 and early 2003, ADEQ and the Visibility Index Oversight Committee designed the visibility survey, selected a contractor to conduct the survey, oversaw the completion of the field portion of the survey, and defined a recommended visibility index. The Visibility Index Oversight Committee Final Report was issued in early 2003 summarizing the visibility index.

Calculation of the Visibility Index

The visibility index was designed to characterize regional visibility conditions and is calculated from measurements made by a transmissometer in downtown Phoenix. The index is calculated as a 4-hour rolling average of visibility, expressed in deciviews, The index is recorded only for daylight hours. (The specific hours included in the calculation are adjusted monthly.) The index is not calculated during periods of instrument malfunction or periods of elevated relative humidity (greater than 90%). All current index values presented on this web page are considered preliminary, awaiting final validation once per year. Visibility index categories range from Excellent (14 deciviews or less) to Very Poor (29 deciviews or greater), as summarized in the table below.

The posting of current and recent visibility index values on this web site provides timely information to the public and also provides a measurement period that is useful in longer term trend analysis. The highest visibility index calculated during the daylight hours of a calendar day is used to represent overall visibility conditions for that day. Daily visibility index categories are tallied annually and used by ADEQ staff to analyze visibility trends in the Phoenix area.

Environmental Goal

As a result of pollution-fighting programs implemented by the United States Environmental Protection Agency, and programs created by HB2538 and other state legislation, it is expected that visibility in the Phoenix area will improve over the next few years. The environmental goal of the visibility index is to document continued progress through 2018 by: 1) improving visibility to move days now in the Poor/Very Poor categories up to the Fair category; and 2) moving days classified as Fair to the Good/Excellent categories. A progress assessment will be conducted every 5 years through 2018. The index will not be used to affect short term actions because other programs, such as the High Pollution Advisory Program, are currently in place in the Phoenix metropolitan area.

Current and Recent Visibility Index Values

The illustration below depicts 4-hour rolling averages in deciview for each daylight hour averaging period recorded for a 48-hour period (yesterday and today). The posted average for 10:00 am includes measurements collected from 7:00 to 10:59. The average deciview is posted at the top of the hourly bar chart. Daylight hours recorded will vary month to month.

Long Term Summary of Visibility Index Values

The illustrations below depict the annual number days in the Phoenix area (normalized to 365) which fell into each of the visibility index categories in 2004 through 2006. The environmental goal of the index is to document progress in improving visibility by showing days move from high to low categories.