I have noticed that when I cook porridge in a saucepan, and stir it with a spatula, it only steams a little bit. But the moment I turn off the (gas) hob underneath, and keep stirring it with a spatula, suddenly a noticeably more steam starts coming out of the saucepan.
This seems counter-intuitive, because the heat source is no longer there. Is there an easy explanation for this phenomenon?
What you see of the "steam" is actually condensed water, i.e., more or less fine drops of liquid water.
Turning off the gas means that the temperature of the steam and air column above the pot decreases; therefore, more water condenses.
In reality, what you have is less and slowly rising steam—not more—so it starts to condense before it can be sucked into the aspiration fan or diffuse into the kitchen.
I wonder whether, paradoxically, there is a temporary increase in the rate of heat entry to the bulk of the porridge when you turn off the gas. The reason would be that while you are heating, a relatively poorly conducting stationary layer of solid well-cooked porridge is continually forming on the very hot base of the saucepan, despite stirring. When you turn off the gas the saucepan bottom is no longer scorching hot and the stirring is effective in removing the poorly conducting layer.
A related effect, perhaps more important, is that fatter bubbles of steam will form on the relatively smooth saucepan bottom than would have formed on the stationary layer of porridge. Thus boiling might be more dramatic when the gas is first turned off and the stationary layer dispersed.
