The contrails from World War II bombers were enough to change the English weather, leaving Brits shivering with cold as well as, possibly, fear.
New research led by Professor Rob MacKenzie, now at the University of Birmingham, examines the levels of Aircraft Induced Cloudiness (AIC) caused by the contrails of Allied bombers flying from England to targets in Europe.
The team focused its research on 1943 to 1945 after the United States Army Air force (USAAF) joined the air campaign, examining historical records from the Meteorological Office and the military. When the USAAF joined the Allied air campaign, it led to a huge increase in the number of planes based in East Anglia, the Midlands and the West Country.
“Witnesses to the huge bombing formations recall that the sky was turned white by aircraft contrails,” says MacKenzie. “It was apparent to us that the Allied bombing of WW2 represented an inadvertent environmental experiment on the ability of aircraft contrails to affect the energy coming into and out of the Earth at that location.”
Contrails are formed when the hot, aerosol-laden, air from aircraft engines mixes with the cold air of the upper troposphere. While some disappear quickly, others form widespread cirrus clouds. These can block sunshine from reaching the planet, and also stop energy leaving as infrared heat.
Recent research from the German Aerospace Center has shown that aircraft contrails can warm the planet by trapping heat – to such a degree that they may have more of a warming effect on the planet as a whole than their CO2 emissions.
But the Manchester research has found the opposite effect could occur in the short term, with morning temperatures in England lower because of the increased cloud cover created by the bombers.
The effect showed up strongly when raids that involved over 1,000 aircraft were compared with raid-free days with similar weather. The most extreme example was found on 11th May 1944 , when 1,444 aircraft took off from airfields across south east England into a clear sky with few clouds.
The contrails from these aircraft significantly suppressed the morning temperature increase across those areas which were heavily over flown.
“This is tantalising evidence that Second World War bombing raids can be used to help us understand processes affecting contemporary climate,” says MacKenzie.
“By looking back at a time when aviation took place almost entirely in concentrated batches for military purposes, it is easier to separate the aircraft-induced factors from all the other things that affect climate.”