It is neither fog nor smoke, and the closer you get to it, the harder it is to see. Unlike smog and ozone, it isn't necessarily pollution.
"It's haze," said Michael Chang, of Georgia Tech's Institute for Urban and Regional Ecology. "It's a mixture of a lot of things: high humidity, water vapor in the air - the same things that make clouds visible."
Summer haze is an annual tradition in Augusta and other urban areas. Sometimes it is more visible because of tiny particles that become trapped in the suspended water vapor.
"Haze is a natural occurrence," Dr. Chang said. "But there can be pollution that occurs on top of it called particulate matter - the soot particles suspended in the atmosphere."
Particulate comes from many sources: road dust, soot from combustion engines, emissions from factories, even organic debris that becomes trapped in the atmosphere and can contribute to the density of haze.
"The smaller the particles are, the longer they stay in the atmosphere," Dr. Chang said. "Larger ones settle out by gravity, but the really tiny ones stay in the air and obscure visibility."
Particulate matter can have profound effects on the quality and variety of summer haze.
"The particles can act in different ways - they can absorb, reflect and even scatter light, so in all different scenarios, they play with the light, more or less, and can obscure visibility in different ways," Dr. Chang said.
Still, haze isn't always caused by pollution.
"Originally, the Great Smoky Mountains were smoky because of haze from naturally occurring vegetative matter," Dr. Chang said. "But in the last 20 or 25 years, visibility has decreased a lot more, due mainly to man-made issues."
Most pressing among today's air quality issues is the threat of ground-level ozone that creates unhealthy smog. Ozone is the mix of volatile organic compounds, sunlight and nitrogen oxides created by combustion.
Dr. Chang and his colleagues are in the second year of an extensive study of air quality in Augusta, Macon and Columbus - three cities already on notice that their air flunks federal health standards.This summer, Dr. Chang said, excessive ozone days have been rare statewide, and the reasons are unclear.
So far, Augusta's air has exceeded the federal ozone standard only three times this year - April 28, May 31 and July 19 - according to Georgia's Environmental Protection Division. There were six such violations last year.
"What we've seen this year are pretty mild events, too, just barely over the standard," Dr. Chang said, adding, however, that ozone season extends through the hot month of August and into September and October.
Excessive ozone days in 2000 and 2001 (to date)
N. Georgia mountains: 16------3
S. central Georgia:---6------2
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