Conspiracy theorists love to talk about these markings in the sky. One popular theory says that they're chemicals sprayed by Governments in order to keep the population docile and under control. In reality, they're the contrails of ever-increasing numbers of passing aircraft, but you don't need to be a conspiracy theorist to believe that they're dangerous.
This is the sky over the River Thames in London, which has two of the busiest airports in the world in Heathrow and Gatwick. With Heathrow working on an additional runway, the skies are only going to get busier in years to come. That means more contrails like this, which represent both air pollution and greenhouse gas emissions. The planes may not be spraying chemicals onto the people down below, but they are releasing them into the sky. In the long run, that's probably even more harmful.