atmosphere

If you’re a frequent flyer, the script of plane travel is probably so familiar you may mumble it along with the flight attendant: “Please raise your tray tables and return your seatbacks to their full upright position. We’re beginning our descent.” The sounds of that descent are probably just as familiar: The whir of landing gear descending, the loud drone of engine power rising and falling as the plane makes a series of stair-step descents to lower and lower altitudes before landing on the runway.

It’s Friday: Is it raining where you are?

In 2009, we saw a number of changes on Earth. The average global ocean surface temperature last summer was the warmest on record. Thin seasonal ice has recently replaced thick older ice as the dominant type of ice in the Arctic. And scientists saw that despite a quiet sun with few sunspots, Earth can continue to be bombarded with a high level of solar energy.

Megacities — cities with populations equaling or greater than 10 million people — are producing an unprecedented amount of air pollution, according to scientists at the American Chemical Society's annual meeting in Washington, D.C., on Tuesday.

Predicting the weather has been central to human civilization since the Babylonians started studying cloud patterns in 650 B.C. The key to weather predictions is making correct assumptions. Today, instruments like Doppler radar that measure rainfall work under the assumption that raindrops fall at their terminal velocity. A new study, however, shows that some raindrops fall faster than they should, indicating rainfall instruments — and by extension, weather forecasts — may need some tweaking.

In 1987, nearly 200 nations signed the Montreal Protocol to restrict the use of ozone-depleting chemicals. The international treaty helped keep the ozone hole over Antarctica from growing further, preventing an increase in harmful radiation reaching Earth’s surface. Twenty years later, new research suggests that the treaty helped the planet dodge more than one bullet: The Montreal Protocol also prevented significant regional climate change.

Black carbon — fine particles of soot in the atmosphere produced from the burning of fossil fuels or biomass — has been known to be a health hazard for decades, a major contributor to the thick hazes of pollution hovering over cities around the world.