Physics/chemistry behind the observation
It's hard to say without seeing it, but I suggest that this phenomenon happening when you turn up or down the gas may be caused by your arm reaching out in the vicinity of the pot to turn the knob.  

The air above the pot should be saturated with water vapor at a higher temperature.  Motion nearby will create eddy currents that carry that moist air to where it can meet cooler air.  You see the moisture when it condenses to fog.