Thank you for your answer. The effect it small, but it's not insignificant at all. Having that said, I understand that the usual operating distance for a regular sniper doesn't exceed 500m in most cases. This would translate to about 2.5-3cm shift in the point of impact, which isn't that important at that distance. However, if you double that distance, now you're dealing with ~ 8cm. Stretch it to a mile (conventional long distance shooting), and you'll quickly start seeing more thank half a meter offset. I've personally seen a meter of offset purely due to 10 degrees temperature difference.

While I think that your method is far from scientific, it's clear to me that my experience matches yours. Higher humidity has consistantly given me a higher POI