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Second world war bombers changed the weather
- 00:01 08 July 2011 by Michael Marshall
- For similar stories, visit the Histories , Climate Change and Aviation Topic Guides
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
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
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