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.
Visibility Index
4-Hour Daylight Averaging Period for May: 9 AM - 6 PM
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 2017.
The environmental goal of the index is to document progress in improving visibility
by showing days moving from high to low categories.