[headquarters]

I am thinking about a volcanic eruption in the early 1800s..1814 ?

Anyways it occurred the same year that was dubbed "the year with no summer " -a meterological anomaly that meant that temperatures were unusally low that year , in some places meaning that there was hardly any thaw at all.

I guess that "winter " is relative , even a drop on average of a few degrees would distort growth cycles etc

as you say , no one really knows for sure - so I guess there can be found theories to support the blackened skies argument and the three year snowfall...