[kannanvijayan]

Disclaimer: I am not a physicist and this is merely semi-informed speculation by an amateur enthusiast of understanding physical phenomena.

I had assumed that those were steam bubbles, not air bubbles. At high enough temperatures water becomes a gas. In a boiling pot, the highest temperatures are at the bottom. Temperature fluctuations on the bottom would lead to more steam production in some areas than others.

Any point at the bottom of the pot that did form a steam bubble would immediately lose energy to the steam and cool down. Now we likely are thinking of metal pots in this exercise, and metal conducts heat very well, so that cooled down point in the metal would draw nearby heat from the pan towards it, decreasing the chances of bubbles nearby. This would presumably lead to bubbles forming at some reasonable distance from each other, as any bubble formation causes the nearby pot metal to cool down.

Once in steam form, the steam bubble has a harder time losing energy. The energy was gained through direct contact of water with metal, but steam is much less dense, and it seems like the high-density shell of water around the steam bubble would act more like a mirror - reflecting the kinetic energy back into the bubble. This would keep the bubble stable as it travels up.

That's been my mental model of it for a while. It would be cool if someone who studied this stuff for real opined on it though.