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.