Both photos were taken in Beijing by Bill Bishop, who runs an excellent all-things-China e-mail newsletter called Sinocism (go subscribe). The photo on the left shows his view on a clear day. That tall building is the mammoth China World Trade Center Tower III.
On the right is a photo of the same view, taken late on Wednesday, U.S. East Coast time, or about 8 a.m. Beijing time. The 81-story skyscraper is all but invisible, shrouded by a layer of pollution so dense that even close-up objects are a blur.
Air pollution is gauged by a measurement called particles per cubic meter of air, sometimes abbreviated PM2.5 because it measures particles smaller than 2.5 micrometers wide. The higher the PM2.5 rating, the more dangerous particles are in the high, the worse the air is for your health.
To give you a sense of scale, there's a big controversy in Utah right now because the PM2.5 air pollution in Salt Lake City has sometimes hit as high as 69. That number is considered unhealthy. According to the Environmental Protection Agency, even adults should avoid all outdoor activities if the count hits 300. In Beijing and other Chinese cities, the air pollution rating can spend days hovering around 500.
The pollution has been largely sustained for several weeks now, getting so bad that even Chinese state media is starting to call for the country's leadership to reconsider its at-all-costs emphasis on economic growth.
Here's a larger version of the photo, not that you can make much out in it: