I don't have all the science to explain, but in over 40 years of flying RC sailplanes, I have noticed a few things
High relative humidity tends to reduce the thermal activity ... is this due to the water vapor in the air absorbing the heat, I don't know.
Days with little change in temperature often are less active thermal days than those days with a 10-20+ degree variance between night and mid-day temperatures.
One of our club's members used to talk about the dry vs wet adiabatic lapse rate and why one day was superior in thermal conditions from another ... it sounded good but
I would suggest some reading of full scale soaring texts that get into the science and meteorology of thermal soaring ...
Have fun, learning has always been an enjoyable pasttime to me