Regional Haze -- Visibility

Photo from the Acadia Hazecam looking northeast

Objects are visible because light is reflected from them to the eye of the observer - this is referred to as 'image-forming' light. When image-forming light is absorbed or scattered by particles and gases in the atmosphere we refer to it as haze.

The relationship between fine particle concentrations and the formation of regional haze is complex -- higher concentrations of fine particles do not always lead to the same amount of visibility impairment. Regional haze formation is a function of the amount of water in the atmosphere (relative humidity) and the type of fine particle that makes up the fine particle pollution.

A given concentration of sulfate particles would result in less haze at lower relative humidities (<70%), and more haze at higher relative humidities (>70%). When sulfate particles absorb water they grow larger and become better at scattering light thus resulting in more visibility impairment.

Conditions such as object/sky contrast, solar angle, sky color, cloud cover, and landscape features can affect an individual's ability to perceive changes in visual quality. Researchers have found that scenic contrast is the most important factor in people's perceptions about scenic quality, regardless of the size of the distant feature and type of lighting conditions.

The subject of visibility is very complex (involving particle size, sun angles, scenic contrast, cloud cover, etc.) and thus is beyond the scope of a single web page. For a more detailed description of how regional haze forms and reduces visibility, see the 'Introduction to Visibility'.

CAMNET --

CAMNET is a network of near real-time, high-resolution, digital scene cameras run by Northeast States for Coordinated Air Use Management (NESCAUM) that is used to demonstrate the degradation in visibility due to air pollution. It serves two primary roles: 1) public outreach & education and 2) archival documentation for later technical use.

CAMNET pictures are updated every 15 minutes; the most recent pictures and related content (air pollution and meteorological information) are at: http://hazecam.net/ The network started in the spring of 1999 with the Boston site and has grown to include more hazecams in the northeast. By the end of 2004, hazecams were located in Maine at Acadia National Park, Moosehorn National Wildlife Refuge, and Presque Isle. All images are archived and available on request from NESCAUM.

These sites include both urban and rural scenes. Visibility at rural sites is impacted primarily by regional haze. Urban areas are impacted by both regional haze and local pollution. During the colder months a 'brown cloud' sometimes can be seen hanging over urban areas. Brown clouds tend to occur on calm winter mornings during rush hour traffic when cold air is trapped near the ground by warmer air above due to temperature inversions.

The images below are from the Acadia hazecam which points northeast.

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