Index: Environment / Subcategory: Quality of Environment

Visibility

Date updated: 02/26/2013

Visibility is a measure of how the air 'looks' and is unique among air quality measurements because it involves human perception and judgment. Visibility impairment is primarily caused by fine particles (0.1-2.5 microns in diameter) in the air. According to the City of Fort Collins Air Quality Action Plan (2000-2003), in the Fort Collins area, these particles come from secondary aerosols (pollutants interacting with naturally-occurring substances, i.e. smog) (38%), dust and debris (18%), gasoline exhaust (18%), diesel exhaust (11%), wood burning (8%), power plant emissions (2%), and 'other' (4%).

Visibility in Fort Collins is measured by a transmissometer (an instrument measuring the amount of light blocked by particles and gasses in the air). A digital slide camera mounted near the transmissometer keeps a visual record to supplement the transmissometer's readings. The camera and transmissometer are located 15 miles northeast of Fort Collins (on top of the Anheuser Busch Brewery) and look southwest across the city. Visibility is not measured on days when humidity is 70% or higher. At 70% humidity or higher, the moisture in the air prevents accurate measurement of visibility. Colorado's visibility standard is detailed below.

In the Front Range area, wintertime temperature inversions trap cool air and pollution at ground level under a warmer layer of air. This is the most typical meteorological condition that leads to a RED air quality advisory (poor current air quality) and contributes to poor visibility (the 'Brown Cloud') in the Front Range area.

What this chart shows: Percentage of Days in Fort Collins by Compliance or Non-Compliance with State Visibility Standard, 2002-2011

Data Source: Colorado Department of Health and Environment, Air Quality Control Division - Annual Air Quality Data Reports

See data table

What the above data tell us:

For the last four years, visibility in Fort Collins has been in compliance more than 58% of the days measured.

There was a large increase in the number of days not in compliance in 2002, most likely caused by an extremely active wildfire season. In 2003, visibility improved with only 16% of days not in compliance with the State standard. This could be related to the fact that ambient carbon monoxide and particulates decreased substantially from 2002 to 2003 (Air Quality). It should be noted however, that the percentage of days that were counted as 'missing' increased from 2003 to 2005. Fort Collins Natural Resources staff reported that during 2005, the transmissometer was down for several periods due to construction at the transmissometer site and maintenance of the transmissometer itself. These periods of down-time are likely the reason for the higher-than-average percentage of 'missing' days in 2005 (37%).

Visibility improved beginning in 2006 with fewer missing or over 70% relative humidity ('Excluded') days. In 2008, the percentage of days 'In Compliance' was the highest in the last ten years.

What this chart shows: Percentage of Days in Fort Collins and Denver Not in Compliance with State Visibility Standard, 2002-2011

Data Sources: Colorado Department of Health and Environment, Air Quality Control Division - Annual Air Quality Data Reports

See data table

What the above data tell us:

For eight of the ten years shown, Denver's visibility monitoring station measured between 29% and 165% more days not in compliance with the state visibility standard than the Fort Collins visibility monitor. However in 2002, the percentage of non-compliant days recorded in Fort Collins increased to 54%, a significantly higher percentage of non-compliant days than was recorded in Denver, a much more urban area. One possible explanation for the sudden increase might be the 2002 wildfire season, when increased amounts of particulates and smoke from forest fires may have contributed more to poor visibility in Fort Collins than in Denver.

Additional Information:

Related Information on COMPASS -

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Industry Standards or Targets:

The State of Colorado has established a visibility standard that applies to all communities along the Front Range that participate in the Automobile Inspection and Readjustment (AIR) program. The standard is 0.076 per kilometer of atmospheric extinction, meaning that less than 7.6% of the light in a kilometer of air is blocked by particles. The standard applies between 8:00 a.m. and 4:00 p.m. when the relative humidity is less than 70%.

To determine relative visibility ('good', 'fair', 'poor', etc.), data from a transmissometer is converted to a Visibility Standard Index (VSI) score.

VSI Scoring:

Days that are measured in the good and fair categories are considered to be in compliance with the visibility standard. Those days that are measured in the poor and extremely poor categories are considered to be not in compliance with the standard. (See data table)

Data Tables:

Percentage of Days in Fort Collins by Compliance or Non-Compliance with State Visibility Standards

Year

% Days In Compliance

% Days Not In Compliance

% Days Missing

% Days Excluded

2002

29%

54%

7%

10%

2003

56%

16%

23%

5%

2004

53%

14%

29%

4%

2005

39%

19%

37%

5%

2006

49%

29%

17%

5%

2007

46%

28%

20%

6%

2008

64%

16%

18%

2%

2009

58%

25%

7%

10%

2010

58%

13%

26%

3%

2011

62%

9%

26%

3%

See chart

Percentage of Days in Fort Collins and Denver Not in Compliance with State Visibility Standards

Year

Fort Collins

Denver

2002

54%

39%

2003

16%

23%

2004

14%

37%

2005

19%

35%

2006

29%

19%

2007

28%

42%

2008

16%

39%

2009

25%

17%

2010

13%

17%

2011

9%

12%

See chart

Percentage of Good, Fair, Poor, and Extremely Poor Visibility Days in Fort Collins

Year

Good

Fair

Poor

Extremely Poor

Missing or Excluded

2002

4%

25%

47%

6%

17%

2003

30%

26%

16%

1%

28%

2004

25%

28%

14%

0%

33%

2005

13%

26%

19%

0%

42%

2006

21%

28%

28%

1%

22%

2007

17%

29%

27%

1%

26%

2008

32%

32%

16%

0%

20%

2009

31%

27%

24%

1%

17%

2010

24%

34%

13%

0%

29%

2011

36%

26%

9%

0%

29%