I'm no scientist, but there were a lot of chemicals belching from the muzzles of both cannon + thousands of rifles. Into the atmosphere! And no I don't believe in man-made weather change of any kind. I'm just pointing out an odd coincidence (maybe)
I think that there was a connection made with heavy rain over many battlefields during WW1, a few boffins have said that all the various chemicals, smoke etc had an adverse affect on weather conditions, come to think of it, Waterloo had extremely heavy rain as did Passchendaele and the Somme but then so did Agincourt, probably just a coincidence.