Fan and her co-authors ran two forecasts for the weather system that passed over the Sichuan basin during the peak of the floods: one with the thick blanket of smoke that covered the region and one with the kind of clean air that existed 40 years ago, before the Chinese economic boom. In the clean air model, moist air at Earth’s surface was heated by the daytime sun, became buoyant, and rose to great heights, triggering a convective cycle that led to storm clouds and mild daytime rainfall. But in the dirty air model, the dark veil over the plain soaked up much of the sun’s warmth high in the atmosphere, while simultaneously cooling the streets and fields below. This altered thermal structure stabilized the daytime atmosphere and suppressed rainfall. But as night fell, the moist air mass moved northward toward the Longmen Mountains, which tower some 2000 meters above the basin. The weather system that had been building energy over the plains for 12 hours was driven upward as it collided with the range’s steep contours, triggering the postponed convection. A day’s worth of rainfall from the plains was focused into a few hours over a handful of mountain valleys.

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