French police are investigating suspicions that a hairdryer was used to manipulate weather sensor readings at Paris's Charles de Gaulle airport, potentially to win thousands of dollars in bets on Polymarket. Temperature readings at the airport unexpectedly spiked twice in the last month, reaching levels much higher than forecast, raising questions about the integrity of weather data used for betting markets.

On April 6 and April 15, temperatures at the weather sensor spiked late in the day to 21C and 22C respectively, despite hovering around 18C during the day. Polymarket had listed the chance of such temperatures at less than 1%, meaning early bettors would see substantial returns. One trader made $21,000 on April 15 betting against an 18C peak, with successful bets reportedly netting around $34,000 total.

Météo-France filed a police complaint after noticing physical changes to one of its sensors. The weather station's location on a public road made it easily accessible for tampering, with one possibility being a battery-powered hairdryer used to heat the sensor. The data source for Paris's hottest temperature has since been relocated to Paris-Le Bourget airport as the investigation continues.