The Trump administration appears sceptical about the climate crisis but is deeply concerned about another weather phenomenon: chemtrails.

To conspiracists, chemtrails are visible trails left by commercial airliners, lasting longer than the usual condensation trails from jets and containing unknown, sinister chemicals.

To weather scientists, chemtrails are a myth based on misidentification and a lack of understanding about how different humidity levels cause contrails to disappear quickly or linger and grow.

The US health secretary, Robert F Kennedy Jr, is determined to take action on chemtrails. During a recent TV discussion, when an audience member said chemtrails were her biggest health worry, Kennedy affirmed that material was being added to jet fuel. “I’m going to do everything in my power to stop it,” he said. “Find out who’s doing it and holding them accountable.”

Kennedy said he was trying to discover which government department was responsible. He suggested the culprits might be Darpa, the Pentagon’s advanced research arm. While there really has been military research into contrails, this was aimed at reducing them to make planes harder to spot.

Despite many claims, there is no evidence of nefarious substances being covertly added to commercial jet fuel. But to conspiracists, this only proves there must be a cover-up.