“This is a really beautiful piece of work,” says Leo Donner, a geophysicist at Princeton University who wasn’t involved in the study.
Laplace wondered to what degree the moon gravitationally squeezes the air surrounding our planet, and he set out to analyze the types of waves that might emerge as a consequence. He imagined the atmosphere as a thin fluid on a smooth sphere, and he concluded that gravity should pin one class of waves to the ground, where they would move more or less horizontally: two-dimensional undulations that hug the planet’s surface. “He was really the first guy with this picture in his mind,” says Kevin Hamilton, a professor emeritus at the University of Hawai‘i, Mānoa, and co-author of the new research. “It was an amazing insight.”
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Laplace didn’t name these waves or work out their behavior in detail, but modern atmospheric scientists now describe them as “normal modes”—waves that resonate like the ringing of a bell. The simplest mode raises the pressure in one hemisphere and lowers it in the other. More energetic modes create checkered patterns of smaller zones of high and low pressure. They race around the globe, mainly eastward and westward, at speeds exceeding those of most passenger planes.
Though Laplace started by thinking of the influence of the moon, the waves arise more from Earth’s general pandemonium: Storms rage. Winds slam into mountain ranges. Turbulence further stirs the pot. Some portion of the energy from these collective abuses fires up the normal modes, which are the only tones at which the atmosphere can respond. “It’s like a kitten walking on the keys of the piano,” Randall says—the random strikes “can show you what strings are in the piano.”
Laplace put the idea into people’s heads that such waves might exist, and his mathematics gave physicists the tools to calculate the atmosphere’s tuning. But would anybody be able to hear its notes?
Around the same time that Laplace came out with his model, explorers and naturalists, including Alexander von Humboldt, noticed that pressure in the tropics rose and fell every 12 hours. The daily timing linked the changes to heating from the sun, but theorists couldn’t explain why the effect was so large. The mystery continued to puzzle scientists for nearly a century, until Lord Kelvin guessed in 1882 that the sun’s heating cycle resonated with one of Laplace’s “free oscillations.” He thought that the sun could deliver an outsized push because it created vibrations at precisely the frequency of one of Laplace’s oscillations, much as an opera singer can shatter a wine glass with the right pitch. His proposition turned out to be wrong—researchers in the 1960s determined that a more complicated phenomenon amplifies the solar influence—but it spurred scientists to work out the quantitative details of Laplace’s theory and calculate exactly which frequencies the normal modes should have.