A lot of things can impact the quality of the air you breathe, and they can come from places on far away or right on your block. For example, pollution spewed by factories in the Ohio River Valley does not stay there; instead, it often blows across swathes of Northeastern states. Wildfires blazing in California can send soot and gases aloft that eventually spread continent-wide. And, in the meantime, local emissions from cars and industry can foul the air and lead to smog, a sky-obscuring haze that makes it hard to breathe.

Slide Show: Tracking Air Quality

To keep track of air pollution plumes, the U.S. Environmental Protection Agency (EPA) relies on a bevy of satellites that observe Earth's atmosphere. EPA scientists then mesh these "god's-eye" views with readings from a legion of ground-based monitoring devices. "We partner with state and local agencies, and they send us their data," says Scott Jackson, an environmental scientist with the EPA's Office of Air Quality Planning & Standards. "We're like a megaphone" for these organizations, he says, weaving their threads into the always-changing tapestry of air quality.

The next step is getting this information processed and back out to those communities impacted by unhealthy air. Levels of five key pollutants monitored by the EPA, including ground-level ozone, particulate matter, carbon monoxide, nitrogen dioxide and sulfur dioxide, earn ratings on a scale called the Air Quality Index (AQI). This scale is color-coded, based on the health effects people may experience when exposed to certain levels of airborne gases and particles. Once assessed, the AQI appears on the EPA-led interagency AIRNow Web site, providing air quality forecasting and virtually real-time alerts for over 300 U.S. cities, Jackson says.

Also helping to get the word out is the "U.S. Air Quality" Weblog—often just called "The Smog Blog"—that is published daily, usually in the late afternoon, Eastern time. Like AIRNow, the blog meshes data from monitors in the sky and on the ground. "The idea is to put all that [air quality] data in one place and put a story together," says Raymond Hoff, a professor of physics at the University of Maryland, Baltimore County (U.M.B.C.) and director of the Joint Center for Earth Systems Technology. "We aim to be a one-stop shop," adds Hoff, who has run the Smog Blog with a team of current and former graduate students and other volunteers since 2003. "From a personal perspective, I thought it would be a good teaching tool, he notes, but it turned out to be much more than that."

Hoff says he cannot believe the number of visitors the Smog Blog gets each day, from amateur astronomers checking nighttime visibility before hauling out their telescopes to academic and government agency officials. "I think [the Smog Bloggers] do a good job showing how satellite data can tell a story about daily air quality," the EPA's Jackson says.