Researchers from University of Reading dug into historical weather records and found that the ionosphere was affected by the massive bombing of German cities by the Allies during World War II.
By U.S. Air Force [Public domain], via Wikimedia Commons
Using the ionospheric data kept by the Radio Research Station, researchers found that the shockwaves produced by the bombs actually reached the ionosphere and heated it up. This weakened the layer of atmosphere and reduced the number of electrons in it.