Where does black smoke come from?

Answers to your questions about the news.
July 17 2006 6:25 PM

Black Smoke Over Beirut

Why isn't it white?

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.

Daniel Engber Daniel Engber

Daniel Engber is a columnist for Slate

Advertisement

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.

TODAY IN SLATE

Frame Game

Hard Knocks

I was hit by a teacher in an East Texas public school. It taught me nothing.

Republicans Like Scott Walker Are Building Campaigns Around Problems That Don’t Exist

Why Greenland’s “Dark Snow” Should Worry You

If You’re Outraged by the NFL, Follow This Satirical Blowhard on Twitter

The Best Way to Organize Your Fridge

The World

Iran and the U.S. Are Allies

They just aren’t ready to admit it yet.

Sports Nut

Giving Up on Goodell

How the NFL lost the trust of its most loyal reporters.

Chief Justice John Roberts Says $1,000 Can’t Buy Influence in Congress. Looks Like He’s Wrong.

How Steven Moffat Made the Best Doctor Who Episode in Years

  News & Politics
Politics
Sept. 16 2014 2:11 PM Spare the Rod What Charles Barkley gets wrong about corporal punishment and black culture.
  Business
Business Insider
Sept. 16 2014 1:23 PM Germany Has Asked Google to Reveal Its Search Algorithm, but That's Not Going to Happen
  Life
The Eye
Sept. 16 2014 12:20 PM These Outdoor Cat Shelters Have More Style Than the Average Home
  Double X
The XX Factor
Sept. 15 2014 3:31 PM My Year As an Abortion Doula
  Slate Plus
Slate Plus Video
Sept. 16 2014 2:06 PM A Farewell From Emily Bazelon The former senior editor talks about her very first Slate pitch and says goodbye to the magazine.
  Arts
Brow Beat
Sept. 16 2014 1:27 PM The Veronica Mars Spinoff Is Just Amusing Enough to Keep Me Watching
  Technology
Future Tense
Sept. 16 2014 1:48 PM Why We Need a Federal Robotics Commission
  Health & Science
Science
Sept. 16 2014 1:39 PM The Case of the Missing Cerebellum How did a Chinese woman live 24 years missing part of her brain?
  Sports
Sports Nut
Sept. 15 2014 9:05 PM Giving Up on Goodell How the NFL lost the trust of its most loyal reporters.