The National Park Service (NPS) keeps track of the visibility conditions in NPS areas, looks at the causes of haze, and works together with air regulatory agencies and partners to improve visibility. In eastern parks and wilderness areas, the average distance a visitor can see has improved from 50 miles in 2000 to 70 miles in 2015 and very clear days, now regularly occur. In western parks and wilderness areas, the average distance a visitor can see has improved from 90 miles to 120 miles over the same period. Unfortunately, the clarity of park views is still affected by air pollution in virtually all national parks across the country.
Haze is caused when sunlight encounters tiny particles in the air. The particles scatter light into and out of the sight path and absorb some light before it reaches your eyes. The more particles in the air, the more scattering and absorption of light to reduce the clarity and colors of what you see. Some types of particles scatter more light, especially when it is humid. Haze is mostly caused by air pollution from human activity including industry, power generation, transportation, and agriculture. Natural haze from dust, wildfires, and more also occurs in many parks.
Air Pollution & Visibility
On hazy days, air pollution can be visible as a plume, layered haze, or uniform haze. A plume is a column-shaped layer of air pollution coming from a point source (such as a smoke stack). Layered haze is any confined layer of pollutants that creates a contrast between that layer and either the sky or landscape behind it. Plumes and layers can mix with the surrounding atmosphere, creating a uniform haze or overall decline in air clarity.
Plumes and layered haze are more common during cold winter months when the atmosphere moves less. Uniform haze occurs most often when warm air causes atmospheric pollutants to become well mixed.
How is visibility measured?
Monitoring stations collect particles from the air on filters.
An NPS employee changes the filter packs at a visibility monitoring station.
Since the late 1980s, national parks have been monitoring air quality to better understand how air pollution affects what we can see. To do this, air samplers draw in air and collect particles on small filters about the size of a quarter. Park staff then collect these filters and mail them to a laboratory for analysis.
By identifying how many and what kind of particles were in the air, scientists can figure out how much the pollutants affected visibility and where they likely came from. For example, scientists can identify natural sources such as smoke and dust, as well as pollutants from cars, power plants, oil and gas extraction, and other human activities. From this work, we know that pollution in parks arrives from local, regional, and even international sources.
In addition to collecting particles on filters, in some parks light scattering is measured directly using instruments called nephelometers. These instruments precisely measure how air pollution is affecting what you can see every minute of the day. Similar instruments are used at airports to ensure that pilots can fly safely.
Air quality webcams at 19 parks around the country record and share long-term visibility conditions. New digital photos are uploaded to the archive website every 15 minutes. Other air quality information including visual range, current levels of ozone, particulate matter, and weather conditions is also archived where available. Check out the live webcams or visit the webcam archive to see the how clear the view is in your national parks.
Last updated: September 11, 2018