Download the MP3 audio version of this story here, or sign up for The Explainer’s free daily podcast on iTunes.
“Black smoke rose over the city” of Haifa on Sunday morning, after Hezbollah militants fired at least 50 rockets into Israel. Meanwhile, Israeli bombs were “sending a thick column of white and black smoke skyward” over Beirut, Lebanon. And in California, firefighters watched as “plumes of gray, white and black smoke floated across the horizon.” What makes some smoke white and other smoke black?
The type of fuel and how hot it’s burning. In general, a hotter fire will convert more fuel into elemental carbon, which forms into tiny particles that absorb light and appear in the sky as black smoke. A cooler combustion—or one that doesn’t work as efficiently—yields less-pure forms of carbon. These tend to reflect light, making the smoke look white.
A wildfire can produce both colors of smoke. First, the hot, flaming combustion of dry underbrush releases little particles of black soot into the atmosphere. But the blaze also produces smoldering combustion—think of the glowing logs at the bottom of a campfire—which don’t burn quite as hot. Big branches or tree trunks that have a lot of moisture are more likely to smolder and release white smoke.
The basic by-products of a fire are carbon dioxide and water. You can’t see carbon dioxide, but water in the air might make smoke appear lighter in color. The steam produced by a wood fire can turn into a white, pyrocumulous cloud that mixes with black smoke and makes it look gray.
An oil fire tends to burn very black because most of the fuel is converted into elemental carbon. There’s also very little moisture in the oil to make the smoke look lighter. Plastic products, which are made from petroleum products, also release dark-colored smoke.
Bonus Explainer: Which color smoke is most hazardous to your health? It’s not clear. The Environmental Protection Agency cares more about the opacity of smoke than its color. A thick, opaque smoke tends to contain more polluting particles than one you can see through, whether it’s white or black. Until the 1970s, EPA officials used color as an index of opacity. They’d check what came out of a smokestack against a “Ringelmann chart,” which tells you how to convert shades of gray into percent opacities. Now they shine light through the plume of smoke to measure its opacity more directly. (Some state governments still use the Ringelmann ratings for their clean air laws.)
Got a question about today’s news? Ask the Explainer.
Explainer thanks Cathy Cahill of the University of Alaska, Fairbanks; Jeffrey Collett of Colorado State University; Mike Lunsford of Eastern Technical Associates; and Robert Yokelson of the University of Montana.
