Unusual Weather Bets on Polymarket Spur French Investigation

France's national weather service is investigating anomalous temperature spikes at Paris Charles de Gaulle Airport, leading to lucrative Polymarket payoffs.

Prediction markets have spurred repeated allegations of skulduggery in recent months, ranging from insider trading by politicians to suspected rigging of bets on the Ukraine war.

Now, controversy has erupted in one of their more sedate corners: wagers on the weather.

France's national weather service is investigating irregularities at a monitoring station at Paris Charles de Gaulle Airport after it reported anomalous temperature spikes. The spikes led to lucrative payoffs for some traders on Polymarket, the crypto-based betting platform.

The spikes drew the attention of local weather enthusiasts. Suspecting that traders had tampered with the data, they alerted the service, called Météo-France, which in turn notified French police.

One mystery trader, with the username "xX25Xx," made more than $21,000 in profit from a bet of just under $120 thanks to one such temperature spike on April 15, according to Bubblemaps, a blockchain analytics firm. The trader has since deleted their Polymarket username, meaning they now show up as an unlabeled anonymous account.

Every day, Polymarket lists contracts that allow its users to bet on the maximum temperature in dozens of cities worldwide. Its Paris contract is based on the reading at Charles de Gaulle airport, as reported by Weather Underground, an online weather data provider.

On April 15, the temperature in Paris had reached 18 degrees Celsius in the afternoon and was cooling down in the evening when the airport gauge showed a brief, unexplained jump, hitting 22 degrees Celsius at 9:30 p.m. local time, Weather Underground data shows. Other nearby weather stations didn't show a similar spike.

Just before the anomaly, xX25Xx placed cheap, long-shot bets on Polymarket that the maximum temperature in Paris that day wouldn't be 18 degrees Celsius, when other bettors were more than 99% sure that the day's top temperature would remain at that level.

The airport weather station also registered a temperature spike around 7 p.m. on April 6. That day, a Polymarket account with username "Hoaqin" made nearly $14,000 in profit by betting that Paris temperatures would peak at 21 degrees Celsius, Polymarket data shows. Temperatures at Charles de Gaulle had been hovering at around 18 degrees in the late afternoon, according to Weather Underground data.

Sébastien Brana was among several weather enthusiasts who noticed the unusual temperature activity and posted about it on an online weather forum run by Infoclimat, a nonprofit where Brana is a board member.

"Someone apparently won $14k with a bet," Brana wrote. He later added that his group had alerted contacts at Météo-France -- an alert that he said "was taken very seriously." Brana didn't immediately respond to a request for comment.

Météo-France confirmed that the weather service had examined one of its monitoring stations and analyzed weather-sensor data. The service has since filed a complaint with police at Charles de Gaulle airport related to tampering with an automated data system, spokeswoman Elenie Alfera said.

A Polymarket spokesperson declined to comment. The traders behind the xX25Xx and Hoaqin accounts couldn't be reached. Polymarket has a data partnership with Dow Jones, the publisher of The Wall Street Journal.

The weather trades took place on Polymarket's international platform. While the company recently launched a regulated betting platform for U.S. users, its international platform remains largely unregulated.

Traders have periodically alleged attempts to rig the outcome of Polymarket's betting contracts by tampering with the sources used to determine answers to the yes-or-no questions that its users bet on.

In November, some Polymarket users were upset by what proved to be a flawed determination that Russia had seized the Ukrainian town of Myrnohrad. Such bets are settled using an online map maintained by the Institute for the Study of War, a Washington think tank.

Just as Polymarket's betting contract was set to expire, ISW's map erroneously showed Myrnohrad to be under Russian control, even though Russia hadn't actually captured the town. The think tank later apologized and fired an employee over the incident.

In March, an Israeli journalist said he had received death threats from Polymarket bettors demanding that he revise his article about an Iranian missile strike on March 10. The details of his article were used to settle bets on whether Iran had carried out a missile, drone or airstrike that day.

Soon after the incident, Polymarket explicitly prohibited insider trading and market manipulation on its international platform for the first time. The amended rules state that Polymarket users can't trade contracts where they can influence the outcome of the underlying event.

A similar prohibition is in place in regulated prediction markets such as Polymarket's main competitor, Kalshi, as well as at Polymarket's new U.S. platform.