I did this the other day… #137 of this thread:
In my setup, I am using the approach / concept of humidity_now - humidity_average. You likely will have to tweak the statistic senser based on the behavior of how frequent your humidity sensor reporting readings to HA, the environment, etc.
Alternatively, you could also consider the trend approach to achieve this:
Obviously you also need to tweak the parameters to see what works better.
Multiple ways to skin the cat.
I ran both statistics and trend side by side for a couple of weeks, and figured statistics being more reliable (for my setup, anyways), so in the end I abandoned the method using trend
(… because I don’t want any late night sudden rain triggering my bathroom fan… but that’s just me… you might actually want to.)