Beneath a sky that makes a locker room full of Axe Body Spray seem like a breath of fresh air, the air quality sensor at the U.S. Embassy in Beijing has measured off-the-chart levels of fine particulate pollution twice in 24 hours. The first reading showed an air quality index of 586, or simply "beyond index."

Shortly before the publication of this post, the U.S. Embassy's sensor measured another "beyond index" air quality reading at 2:00 AM in Beijing, or 1:00 PM EST.

Air quality is measured by looking at the concentration of suspended particulates of less than ten micrometers in length, which is small enough for you to breathe into your lungs. These particulates are broken into two categories: coarse particulates like dust, and fine particulates like vehicle exhaust or smoke from a fire. Fine particulates are especially dangerous to folks who have existing health problems, and in extremely high concentrations, they're dangerous to everyone.

Tens of thousands of sensors around the world measure how many of these particulates exist in one cubic meter of air, and they use the measurements to grade the air quality along a six-category index that runs from 0 to 500. Anything greater than 500—literally off-the-charts—is considered "beyond index," and that happens with some frequency in Beijing. The pollution was so bad in 2013—a 755 AQI—that one U.S. Embassy employee mistakenly spoke the truth and called it "crazy bad" on the agency's Twitter account.

For the past couple of decades, a growing number of factories across the enormous country have pumped untold amounts of pollution into the atmosphere. The country's environmental laws—which have been updated in recent years and are now considered "pretty good compared to global standards"—suffer from lackadaisical enforcement by local authorities around the country, compounding China's ever-growing problem with air pollution.

The result is air that looks like this:

The weather often plays a significant role in air pollution, and the situation in Beijing is no different.

The above chart is called a SKEW-T, and it shows the profile of the atmosphere as measured by the instruments attached to weather balloons. This particular weather balloon was launched from the Beijing Capital Airport at 00z, which is 7:00 PM EST, or 8:00 AM in Beijing, and about six hours before the U.S. Embassy recorded the first "Beyond Index" air quality reading.

The chart is very telling as to why yesterday was such an ugly pollution day in Beijing, and why the city sees so many of them to begin with. The line on the right, which I've traced in red, shows temperatures through the atmosphere as measured by the weather balloon. The diagonal lines running from bottom-left to top-right are isotherms (lines of constant temperature), in Celsius. The air temperature line spikes hard to the right immediately above ground level, and the temperature continues to rapidly climb by about 9°C as the balloon lifted few hundred feet off the ground.

This means that it was about -10°C (14°F) at ground level, but just a few hundred feet up, the air temperature was right around freezing. That's what we call a temperature or capping inversion, which occurs when a layer of warm air sits on top of a layer of much cooler air, "capping" the cooler air in place. When a capping inversion is in place close to the ground, it doesn't allow stuff like smoke and exhaust to escape into the atmosphere, keeping it at ground level and smothering area residents—to put it another way, the capping inversion forces the city to hotbox itself on pollution.

Temperature inversions occur frequently in valleys when cold air gets trapped in between (or up against) mountains, and lo and behold, look at the terrain around Beijing:

Beijing isn't exactly in a valley, but the city's airport consistently recorded winds from the south and east yesterday afternoon, pressing cold air up against the base of the mountains in a process known as cold air damming. To sum up a long story in twelve words, Beijing's pollution is awful, and the weather is making it even worse.

If you were wondering, this afternoon's AQI is 52 out of 500 (moderate) in New York, 5 out of 500 (almost perfect) in Los Angeles, and 70 out of 500 (moderate) in Chicago.

[Images: AP (showing a polluted haze in March 2014), AirNow, U. of Wyoming, Google Maps]

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