On 20 March 1941, RAF Fighter Command sounded a red alert. The Chain Home radar had detected a large number of blips moving across the Channel and it looked like a mass Luftwaffe bomber raid was on the way. But 40 miles from the south coast the blips faded out to nothing, and no aircraft were sighted.
The same thing happened several times, with fighters scrambled to intercept intruders only to fail to find any. The spurious radar tracks were whimsically known as “angels”, and radar technicians suspected they were caused by weather conditions.
Rain proved to be one source of radar interference, until engineers found they could remove the problem with polarising filters. In the 1950s, former military radar scientists used reflections from rain to develop the first weather radar to track storm systems from long range.
Temperature inversions, where a layer of warm air overlies cool air, sometimes looked like a solid mass on radar. Inversions could even bend the radar beam around the curve of the Earth and produce returns from distant objects.
However, the solution to the radar angel mystery was not found until the 1950s. Although highly seasonal, they were not directly related to the weather: the flights of angels coincided exactly with flocks of small migrating birds returning to Britain.