Try the following experiment:
The fast reduction in heat causes the water to soon stop boiling, but the immediate effect is that the cloud of vapor becomes much heavier. Is there more steam being released when heat is removed (why?) Is it just more visible steam? What’s the deal?
Steam is invisible. You are seeing condensation. Without running any numbers, you get more condensation when the energy source is removed. More water droplets, less water vapor.
condensation needs some stillness or at least less movement. you see it away from fast moving vapor.
The cloud maybe due to you shaking the pot while turning off the burner. Sort of like shaking a soda bottle.
Repeat the experiment when you keep shaking the pot even before you turn the burner off - then you wont see the cloud.
No pot shaking occurs. I’m using a heavy pot on a solidly built cooktop and the burner control is way over on the other side. And this phenomenon is hardly unique to my kitchen.
This isn’t the reason. DrFidelius has it. With the heat on, it is hotter immediately above the water surface and there is more convection carrying the water vapor away, so it takes longer to condense into visible steam* and the condensation happens over a more disperse area. With the heat off, it condenses faster and the condensation is more concentrated immediately above the water. It’s not that condensation needs stillness, it just isn’t as visible when it’s spread out.
*DrFidelius is correct that steam is invisible following the strict definition, but in lay terms “steam” almost always refers to the visible aerosol of water droplets.
After some consideration, I think this answer does make sense. Turning the heat off affects the air currents immediately, while the water takes some time to cool down. Same amount of water vapor, but it moves less, creating more condensation in the immediate area.