Strange things happened towards the end of December last year. Many TV viewers using the Freeview service over much of England and Wales saw a distorted picture or no picture at all. Freeview was not to blame for the gremlins, however – it was the result of an atmospheric phenomenon playing mischief.
A large area of high pressure had extended across much of Europe and into southern Britain bringing calm, dry weather and some welcome sunshine. Normally, the higher you go in the lower atmosphere, the colder it becomes, but high pressure can turn this pattern upside down, trapping cooler air close to the ground under a layer of warmer air above.
The phenomenon, called a temperature inversion, can bend or reflect TV and radio signals, beaming them to distant parts, sometimes for hundreds of kilometres. It can sometimes make foreign TV stations appear like ghosts on British sets, and the wayward radio signals have occasionally been known to spontaneously open or jam radio-operated garage doors.
Temperature inversions have also been revealed as a black line when there is a lot of smoke trapped under the inversion but clear air above, as happened when smoke billowed up from the huge Buncefield oil depot fire in December 2005.
