Dear Tom,
I often hear the term acid rain. Is our rain really acid?
Phyllis Fojtik, Westchester
Dear Phyllis,
Actually all rain is naturally acidic, the result of atmospheric carbon dioxide combining with the precipitation to form a weak carbonic acid. A substance’s acidity or lack of it is measured by the logarithmic pH scale, which runs from a highly acidic 0 to a strongly basic 14. A pH value of 7 is considered neutral. Most precipitation registers a relatively harmless, slightly acidic pH reading between 5.0 and 5.6. The real problem with acid rain occurs downwind of highly industrialized areas where the rainfall’s pH can drop as low as 4.0, a value strong enough to kill fish, damage vegetation and corrode metal and stone. In recent years the pH of Chicago’s precipitation has been in the 4.7-4.8 range.
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.
Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or asktomwhy@wgntv.com (Mail volume precludes personal response.)
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
