They're some of the most beautiful skies ever painted. They may also be some of the dirtiest. The dreamlike haze hanging over the canvases of J.M.W. Turner and Claude Monet has long been celebrated as a stylistic triumph. But recently, some atmospheric scientists have argued that these masterpiece skies may also reveal something more concrete: a record of the coal smoke that choked London and Paris during the Industrial Revolution.

In a 2023 study published in the journal PNAS, Anna Lea Albright of Sorbonne University and Peter Huybers of Harvard University tested whether the two painters' work tracked the rise of 19th-century air pollution. They analyzed 60 oil paintings by Turner (spanning 1796 to 1850), and 38 by Monet (spanning 1864 to 1901). They measured two things: contrast, or how crisp the edges are, and intensity, or how white and bright the image appears.

The physics behind the research is simple. Burning coal releases sulfur dioxide, which forms tiny sulfate particles in the air. Along with soot and other particles from coal smoke, these scatter sunlight, softening outlines and washing colors toward a pale, milky glare. Using sulfur dioxide emissions as a stand-in for overall pollution, the researchers found that as emissions rose in London and Paris, the paintings tended to lose contrast and grow paler. That pattern held even after they accounted for the year and the subject of each painting.

The strongest evidence comes from the cities' different timelines. Because London industrialized decades before Paris, Turner's later works showing London in the 1830s and 1840s are hazier than Monet's early scenes from in and around Paris in the 1860s and early 1870s. Monet's work grew far murkier when he returned to London between 1899 and 1901 to paint Waterloo Bridge, Charing Cross Bridge and the Houses of Parliament. By the study's estimates, visibility fell from about 15 miles in his early French scenes to under 4 miles in his London views. Real conditions may have been worse. The London Fog Inquiry of 1901-02 found that visibility from the city's towers never topped about 1.2 miles, even on days without fog.

Monet's letters suggest the haze was something he went looking for. Writing to his wife, Alice, he described the dismay of a clear morning: "When I got up I was terrified to see that there was no fog, not even a wisp of mist … but gradually the fires were lit and the smoke and haze came back."

The study builds on earlier research linking paintings to the atmosphere. Work led by Greek atmospheric scientist Christos Zerefos found that the red-to-green color balance in sunsets painted by European masters over several centuries correlates with independent records of volcanic haze, including ice-core data. Turner offers a vivid example. After the 1815 eruption of Indonesia's Mount Tambora, which triggered the "Year Without a Summer" that led to crop failures (and inspired Mary Shelley's "Frankenstein") in 1816, he filled a sketchbook with 65 sunset watercolors over three years. According to Albright and Huybers, those watercolors capture the volcanic reddening of the sky as it built and faded.

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It's fascinating research, but not everyone accepts the conclusions. In a formal response published in PNAS, Stanford University's Michael Marmor, who studies vision and eye disease in art, agreed that both artists "documented London haze when they encountered it." But he argued that the study went too far by linking their overall style to pollution. Impressionism deliberately moved away from sharp contrast, he noted, so edge sharpness is an unfair way to measure it. He also pointed to crisp, clear-skied works such as Turner's "Venice, the Dogana and San Giorgio Maggiore" (1834) and Monet's "Field of Poppies Near Giverny" (1890). Marmor said he would have no quarrel with the paper if its title had begun with the word "Selected." "Depiction is not equivalent to style," he wrote.

Albright and Huybers replied that they never claimed to describe all of the artists' work, only paintings in and around London and Paris as their Impressionist styles developed. They agreed that Marmor's suggested title would be more precise, and they emphasized that pollution was one influence among many. They also called it "incongruous" for Marmor to argue that the workings of the eye can shape art while the air that light passes through cannot. Recognizing that connection takes nothing away from the artists, they wrote: "Bringing forth what is real, but not before entirely seen, seems powerfully creative."

What, then, can we learn from this study, and how might it allow us to see some of the 19th century's great masterpieces in new ways? Even Albright and Huybers caution that the paintings can't yet serve as precise pollution gauges, because the artists chose their scenes, their days and their effects. Still, from an era with almost no direct measurements of city air, the canvases offer a rare glimpse of what Londoners actually saw when they looked across the Thames. Viewed alongside fog records and daily weather reports, they remind us that the haze wasn't only an artistic invention, but also something people breathed. The paintings carry a modern echo, too. The researchers note that megacities such as Beijing, New Delhi and Mexico City have recorded pollution levels similar to those of 19th-century London. Monet may have put it best. "What I want to reproduce," he once said, "is what lies between the motif and me." For Monet, that space was filled with light and air, and, some scientists argue, a great deal of coal smoke.

Lead Editor Chris DeWeese edits and/or writes The Weather Channel newsletters.