I had to get some numbers. I would not stand behind this "science", but I had to get this out of my system. I used this site Air Density Calculator
to figure out air densities. I used this site Airplane Aircraft Wing Lift Design Equations Formulas Calculator - Force
to calculate lift of a wing. I used 1.5 for Cl for a sail.
So the jib on my Sabre 28 would theoretically have 10% more lift in 45 degree 40% relative humidity air than it would have in 85 degree 60% humidity air. That was fun.