Who’s afraid of the Big Brown Cloud?

Residents of Los Angeles, Denver, Mexico City, Brisbane, and Mumbai know it all too well: The big brown cloud that consumes urban areas under certain conditions. On good days, the cloud is an eyesore and a minor annoyance, something that limits your view of the mountains. On severe days, though, individuals with respiratory issues are advised to remain indoors, commuters are requested to carpool, and households are prohibited from burning wood fires. But to truly understand how devastating these events can be, we need look only to London, England, a mere 63 years ago.

While London might automatically suggest fog-filled cobblestone alleys in our collective psyche, it’s not an especially foggy city, at least not meteorologically speaking (the cobblestones are, however, quite real). Sure, the River Thames can be misty from time to time, but no more so than other major rivers. No, the Capital’s reputation as a foggy metropolis derives from the thick “pea souper fogs” that plagued the city in the nineteenth and early twentieth centuries. Except, it wasn’t fog at all. It was smog.

As chimney upon chimney pumped the exhausts of millions of coal-fueled household fires into the London air, a toxic, yellowish blanket of smoke would settle over the city from time to time. Such events were commonplace, and Londoners faced poor air quality days with their usual plucky perseverance.

But the Great Smog of 1952 was different. From the 5th to the 9th of December, 1952, London experienced a long period of unusually calm, cold weather. Cold temperatures prompted the Capital’s residents to burn more coal, generating a hefty dose of pollution. And the lack of wind prevented the resulting smog from dispersing. Furthermore, an impressive region of high atmospheric pressure suppressed the natural buoyant forces that normally transport air from ground level to the upper reaches of the troposphere.

At least four thousand people—perhaps as many as ten thousand—died in the Great Smog of 1952. Its aftermath prompted environmental legislation to improve air quality and prevent such catastrophes from happening again. But what, aside from pollution, causes such episodes of low-level smog?

The usual culprit is the so-called temperature inversion. Normally, air temperature and pressure decrease as one ascends in elevation or altitude. It’s why your ears pop as the airplane climbs away from the airport and why you suddenly pine for a jacket when you step out of your car atop Trail Ridge Road.

A temperature inversion, however, creates a meteorological sort of “yeah, but” to the normal situation. Especially prominent in low-lying areas surrounded by mountains, an inversion occurs when a blanket of cold air accumulates in valleys and basins, and warm air slides in on top. Unable to rise above the warm air, the cold blanket simply stays in place until the inversion, well, inverts. With calm winds, there’s virtually no mechanism to move pollutants, and every chimney, car, semi truck, power plant, lawnmower, locomotive, passenger jet, and backyard fire pit contributes to an accumulated mass of smog.

We can’t control the weather, and temperature inversions are as natural as the atmospheric dynamics that create them. But we can learn from London. Following the United Kingdom’s passage of the Clean Air Acts of 1956 and 1968, air quality in London dramatically improved. The Clean Air Acts raised chimney heights, exchanged dirty fuels for cleaner varieties, and established smoke control areas.

While reducing overall pollutant levels is an obvious and desirable way to prevent the big brown cloud, smart building design, like using effective stack heights for industrial facilities and ensuring good exhaust/intake design for hospitals and labs, can play a major role in improving the quality of the air we breathe.