In Beijing, China banned 2.5 million cars from driving for 2 weeks to get this beautiful blue sky for a World War II commemorative parade. As soon as the parade was over, the ban was lifted, and the blue vanished within 24 hours.

Photo via CNN, where there's a longer writeup by the guys in their Beijing bureau.

It worked. On the morning of the parade, the air quality index (AQI) — an international standard for measuring the severity of air pollution — dipped to a pristine 17 out of 500, signifying very healthy air. Excited Beijingers coined the unusually blue skies "parade blue." But now the cars are back and the city is back to "Beijing gray." Friday's AQI shot up past 160 in parts of the city, rated "unhealthy".

[HT: Gabe Klein]