A hair dryer? A lighter? A lucky coincidence?
Authorities in France are investigating possible tampering with a weather monitoring device at Charles de Gaulle Airport in Paris after an unusual temperature spike was recorded around the same time a Polymarket trader cashed in.
An anonymous trader with the username "xX25Xx" has drawn scrutiny from analysts and fellow traders on Polymarket, where people can bet on things like a city's top temperature and other real-world events.
The trader bet $119 that the weather in Paris on April 15 would jump past the equivalent of 64 degrees Fahrenheit, and weather enthusiasts online noticed a sudden spike in temperature on that day. Thanks to the sudden rise in temperature, the trader netted $21,398 in profit.
When local meteorologists ruled out that the temperature anomaly had happened naturally, the Polymarket trader deleted their account.
The French weather service, Météo-France, said it filed a complaint with airport police about possible tampering with its equipment, French broadcaster BFM TV reported.
Météo-France and the airport police did not respond to NPR's request for comment.
While authorities in Paris are still investigating what happened, weather watchers and Polymarket traders on online forums have raised two possibilities for how the temperature could have been raised: a lighter or a battery-powered hairdryer.
"Love it!" wrote a Discord user who goes by Vince, who regularly trades on weather on Polymarket. "I've got to have one of those cordless hair dryers with a cord that connects to a weather station!"
Another user known as Sakuku on Discord quipped: "The good old 'blowdrier with heat setting on the publicly-accessible weather monitoring station near the airport' scam," they wrote. "It's a classic."
An analysis by the French analytics firm Bubblemaps found that no other weather station in the area recorded the temperature spike, and that the winning bet from the Polymarket trader in question was 20 times larger than their typical wager.
Polymarket did not return a request for comment.
Prediction markets like Polymarket and Kalshi allow people to place bets on every aspect of modern life, from what words President Trump will utter to the timing of military strikes in Iran and the outcome of elections.
As the sites have exploded in popularity, stories have circulated of traders going to extraordinary lengths to find an edge, like standing outside of the Super Bowl stadium to record the length of the national anthem and browsing through the software code of a musician's website to search for record sale announcements that are not yet public.
There also have been several high-profile instances of suspected insider trading, like when a trader made more than $500,000 betting on the ousting of Venezuela's President Nicolas Maduro and the exact timing an Iran ceasefire would be announced. Going back further, prediction market analysts have identified a trader who appears to have profited handsomely from having advance knowledge of the pardons former President Biden granted in his final hours in office.
In the face of what appears to be the abuse and manipulation of these prediction markets, lawmakers have proposed a flurry of actions to rein in the sites and dozens of states have launched lawsuits aiming to have the apps regulated as gambling businesses.
Right now, the Commodity Futures Trading Commission oversees the industry, with the Trump administration embracing a light-touch approach to the norm-busting companies.
The French weather bet occurred on Polymarket's unregulated overseas exchange, which is only accessible by U.S. traders with a virtual private network.
Polymarket's website shows it is no longer relying on the Charles de Gaulle weather sensor data and is instead using data from a device at the Paris–Le Bourget Airport to settle bets.
Copyright 2026 NPR
