Do you mean glider planes?

Cool/cold air is more attractive to gravity, and so it gets drawn closer to the surface of the Earth (D3 Gold). When that happens, it displaces warmed air which travels in many directions, but mostly upward where it does not have to compete with cooler air which is hugging Earth.

You have to remember that what keeps a heavier-then-air machine in the air is causing the PSI beneath the wings to be greater than the PSI above the wings. Thus, generally speaking, if the air beneath the craft is warmer than the air above the craft, without more, the craft crashes to earth. A glider (or any plane) might get some "lift" from upward welling air (both warm and cold), but most of its lift comes from the shape of the wings forcing more air beneath the wings (than above) as it moves forward thru the air.

A hot air balloon captures lighter-than-air gases and exploits gravity's desire to embrace cooler air.

Humidity is a relative term. I was reading Hyphen's TP as asking whether there needs to be an imbalance in humidity in order for a tornado-like event to occur(?). E.g., the warm air is dry and the cool air is humid, thus creating an extreme upward welling and an extreme downward welling. I am sure that water vapor lends a helping hand (D3 Gold), but I am pretty sure that it is not necessary. As noted in my example. Air which is cooled and dried by an air-conditioner on a 3rd floor travels downward thru harm air and cools a lower floor. Oftentimes the warm air which it travels thru is humid, too. If anything, the humidity should be inhibiting the downward flow of the coolness.