A lot of the airborne particles in the Earth's atmosphere come from natural sources, such as desert dust (red-orange) and sea salt (blue). But there's also soot from fires (green and yellow) and sulfur emissions (white) from burning fossil fuel. William Putman/ NASA/Goddard hide caption

itoggle caption William Putman/ NASA/Goddard

A partially frozen American Falls in sub-freezing temperatures is seen in Niagara Falls, Ontario, on Tuesday. Temperature dropped to 6 degrees Fahrenheit on Tuesday. The National Weather Service has issued a wind chill warning for western New York from midnight Wednesday to Friday. Lindsay DeDario/Reuters/Landov hide caption

itoggle caption Lindsay DeDario/Reuters/Landov

Pedestrians brave the cold during their morning commute on Friday in New York. Arctic air and bitterly cold wind is moving across the Northeast, plunging temperatures into record-low single digits, accompanied by subzero wind chills. John Minchillo/AP hide caption

itoggle caption John Minchillo/AP

An Indonesian police officer in Kumai port in Pangkalan Bun carries part of a plane found floating on the water near the site where AirAsia Flight 8501 disappeared on Sunday. Achmad Ibrahim/AP hide caption

itoggle caption Achmad Ibrahim/AP

The drought forced many citrus farmers near Orange Cove, Calif., to mulch their trees because they couldn't afford to keep them alive. Recent rain and new groundwater regulations have eased the crisis, but only slightly. Kirk Siegler/NPR hide caption

itoggle caption Kirk Siegler/NPR

Marina owner Mitzi Richards carries her granddaughter in September as they walk on their boat dock at the dried up lake bed of Huntington Lake in California, which was at only 30 percent capacity as a severe drought continued. The state was in the grip of its third year of severe drought, the worst in decades. Mark Ralston/AFP/Getty Images hide caption

itoggle caption Mark Ralston/AFP/Getty Images

A bicycle messenger struggles through the snow in downtown Cleveland on Friday. Mark Duncan/AP hide caption

itoggle caption Mark Duncan/AP

A car is abandoned on a flooded street following heavy rains and flash flooding in Bay Shore, N.Y., on Wednesday. The South Shore of Long Island along with the tri-state region saw record-setting rain that caused roads to flood. Andrew Theodorakis/Getty Images hide caption

itoggle caption Andrew Theodorakis/Getty Images