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France’s weather forecasting service has filed a police complaint after detecting anomalies in its temperature gauges at Paris-Charles de Gaulle airport, which coincided with a surge in well-timed bets on prediction market Polymarket.
Météo-France filed the complaint alleging interference with its equipment after temperatures measured at the airport spiked by several degrees Celsius in the space of a few minutes on April 6 and 15.
Users of weather forums who noticed the sudden movements said they may have been caused by people tampering with the equipment to shape the outcome of wagers on Polymarket. Multiple accounts on the platform, where traders can bet on real-world outcomes, appeared to have placed large bets on unexpected temperature rises in Paris.
Polymarket uses Météo-France data recorded at Charles de Gaulle to settle wagers on the highest temperature of the day in the city. Predicting weather patterns has become a popular activity on the platform.
On April 6, one wallet made $13,990 in profit on a stake of less than $30 after betting that the temperature in Paris would hit 21C. The account, which was opened this month, bought so-called event contracts when their price suggested the probability of a payout was just 0.2 per cent.
On April 15, when the recorded temperature jumped in a few minutes from 18C to 22C before falling back, another wallet made more than $21,000 on a stake of just $119 by betting that the temperature that day would exceed 18C, at a time when the price of the contract suggested a probability of about 0.5 per cent.
On both days, trading volume on Polymarket’s “Highest temperature in Paris” market exceeded $500,000 — more than double the typical daily volume for this market.
Hunting for unusually confident or well-timed bets has become a flashpoint for prediction market traders on social media, although some warn that lucky or smart bets can be confused with market manipulation.
While users can track traders’ individual wallets and wagers on Polymarket, the crypto-based platform does not require most accounts on its international site to provide identification documents. This means that the company itself may not know who is behind a given wager.
“In light of physical findings on one of our instruments and the analysis of sensor data, Météo-France has indeed been led to file a complaint for interference with the operation of an automated data processing system with the Air Transport Gendarmerie Brigade in Roissy,” Météo-France said, declining to comment further. The complaint was first reported by broadcaster BFMTV.
Sébastien Brana, who runs online weather forum Infoclimat, whose members track Météo-France data almost in real time, said members had not immediately suspected wrongdoing when the first anomalies occurred on April 6.
“We thought it was an issue with the sensors . . . You can have sudden temperature changes at sundown when there is a storm as well. But the weather situation didn’t explain what was happening,” Brana said. “It became clear there was something else going on when it happened again on April 15.”
Météo-France has shared readings from its sensors since 2023, Brana said.
Prediction markets, which allow customers to bet on binary outcomes of future events, have surged in popularity in recent years. However, the ability of users to make bets on highly specific real-world outcomes has triggered concerns that they are vulnerable to manipulation.
The FT reported last month that the US attack on Iran was preceded by a number of unusually large and well-timed bets, and found a similar pattern of bets in a market related to the US government’s capture of Venezuelan leader Nicolás Maduro in January.
Israel in February brought prosecutions against two reservists suspected of using classified information to bet on the country’s military operations.
Regulations around the use of prediction platforms differ between countries. In the UK, for example, the Gambling Commission considers Polymarket and its rival Kalshi, the biggest regulated platform in the US, to be unlicensed betting operators.
Polymarket, Paris police and airport police did not immediately respond to requests for comment.