Well-timed Polymarket bets on the temperature in Paris saw traders walk away with more than $35,000—triggering a police complaint from France's national weather agency.

On April 6 and again on April 15, temperature readings from a sensor operated by Météo France, the French government's official weather agency, near the Paris-Charles de Gaulle airport runway spiked by more than three degrees Celsius before snapping back to normal within minutes, French broadcaster BFMTV reported. Multiple accounts appeared to have placed large bets on long-shot outcomes on the markets, according to the Financial Times.

Per BFMTV, one trader made $14,000 trading on a market linked to the sensor on April 6, with a stake of "only a few dozen dollars." Blockchain analytics platform Bubblemaps flagged another such trade made on April 15 in a Wednesday post, noting that one account purchased "NO" shares on an 18°C outcome when odds sat below 1%, spent roughly $120, and exited with around $21,000, a 180x return, within 30 minutes of the spike.

"While this trader has a long history in weather markets, this bet stands out," Bubblemaps wrote. "His average bet: $6 at 72% odds. This one: $120, 20x larger, at 0.6% odds — placed minutes before the anomaly."

Météo France has since filed a complaint for “interference with the operation of an automated data processing system” with the Roissy Air Transport Gendarmerie Brigade, “in light of physical findings on one of our instruments and the analysis of sensor data,” a spokesperson for the weather agency told Decrypt.

Single points of failure

The incidents expose a structural vulnerability in prediction markets that peg payouts to a single real-world data point.

“Basing a contract on the observations of a single weather station probably isn't a good idea,” Mark Roulston, a researcher focused on prediction markets at Lancaster University, told Decrypt.

“Even in the absence of malicious activity, individual weather stations can develop faults or return anomalous readings,” Roulston said.



He noted that national weather centres have “procedures for weeding out anomalous readings” when using station data in their models, saying how basing contracts on an average across multiple stations would make them “less easy to manipulate” and “potentially more useful” for forecasting applications such as energy demand.

The April 6 account had been created just two days before the winning bet was placed, BFMTV noted, raising questions about whether the platform, already linked to alleged insider trading tied to the Iran war and banned in France, had enabled a malicious actor to manipulate its markets.

Prediction markets under scrutiny

The episode is the second market-integrity incident to hit Polymarket in as many weeks.

Last week, UFC announcer Bruce Buffer misread a fight result live on air at UFC 327 in Miami, briefly sending one fighter's odds to 99.9% before the error was corrected, enabling a trader to turn $500 into more than $252,000.

Prediction market platforms such as Polymarket and Kalshi are facing mounting scrutiny. Last month, a bipartisan pair of U.S. senators, Adam Schiff (D-CA) and John Curtis (R-UT), introduced a bill that would ban platforms like Polymarket and Kalshi from offering sports-related wagers altogether.

Meanwhile, the Trump administration has thrown its weight behind the platforms, filing lawsuits against Illinois, Arizona, and Connecticut, saying the CFTC has exclusive jurisdiction over the sector, following actions by the states against the platforms.

Polymarket is currently in talks to raise $400 million at a roughly $15 billion valuation, following a $600 million investment from Intercontinental Exchange last month.

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