Allied bombing raids during the second world war inadvertently experimented on the weather by producing huge contrails over south-east England. A study of one 1944 raid offers a rare opportunity to check our models of how contrails change temperatures.
After listening to a radio programme in which an elderly woman recalled seeing a wartime sky "turn white with clouds" as fleets of bombers took off, Roger Timmis of Lancaster Environment Centre in the UK realised that the planes could have affected the weather.
Contrails are known to have several effects on climate. On the one hand, they act as a blanket, trapping heat that would otherwise escape into space. On the other, during the day they reflect incoming sunlight, cooling the Earth below more than it is warmed by the other effect. But overall, the consensus among climatologists is that they warm the planet.
In the 1940s – unlike today – there was hardly any civilian air traffic, so historical records offer an opportunity to test the daytime effects. "Pilots cared about contrails a lot," says Rob MacKenzie, formerly of Lancaster University, and now at the University of Birmingham, UK. "Aircraft were tracked using contrails and shot down. So pilots would report them."
Using operational records from the US Army Air Force and the British Royal Air Force, and archived weather data, Timmis and MacKenzie realised they could compare temperatures immediately beneath a raid's flight path to those several kilometres upwind, where there would have been no contrails.
Clear May morning
Conditions were ideal as one particular raid took off on the morning of 11 May 1944, with clear skies and enough moisture for contrails to form.
Timmis and MacKenzie found that where the aircraft circled and assembled into formation it was significantly cloudier and 0.8 °C cooler than the area upwind of the bases.
"It's innovative to use these historical records," says David Lee of Manchester Metropolitan University, UK. He says the documented cooling due to daytime contrails is "entirely consistent" with what is already known.
Field studies of contrails are rare, says David Travis of the University of Wisconsin-Whitewater. Most of our understanding of their effects is based on model studies.
Travis says studies like this MacKenzie's study could help change that. He previously found that temperatures were more variable when planes were grounded in the aftermath of 9/11, but faced criticism because the contrail effect couldn't be separated from natural variability in the weather. By comparing temperatures on the same day, but some kilometres apart, the bomber raid study was able to get around this problem.
Journal reference: International Journal of Climatology, DOI: 10.1002/joc.2392
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