I guess just like when it snows sometimes it becomes humid? In nyc it snows and you can't tell its cold outside because its not. When it rains himidity can kill mp soaps.
Apparently humidity is supposed to affect the length of time it takes to trace, making it longer. I used to live on Vancouver Island and we would get really humid days but I didn't notice anything different myself.
as the temps get cooler I need to adjust my insulation to get a complete gel in my soaps, and if it's really hot in the house my soaps are more prone to DOS.
But humidity itself seems not to impact my soaping or my soaps.