I was swimming in a lake in Southern Vermont yesterday, and there was several nearly stationary thunderstorms around. I noticed that in one of them there was a very bright spot inside the cloud. It looked almost like there was a flash of lightning inside the cloud that was staying bright. After staring at it for half a minute, it was very surprising when the light suddenly went out, and then came back a few seconds later.

Does anyone know what kind of lightning this is? I thought it might be ball lightning, but it was inside the cloud, not near the ground.

We see that alot in Florida--lightning just inside the clouds--here, they call it "heat lightning". I'd be interested to learn more about it, as it's really quite beautiful.

Clouds flashing; 6/19/12
the heat lightning.
I turn off my lamp.

Nathaniel, ball lightning, as you imply, follows a discharge to the ground. Yours is as per the title, one of three main types in the International Cloud Atlas, Vol I, Part III.2.4, Electrometeors, (1) Thunderstorm.

Lisa is close - heat lightning is in the same type, consisting of diffuse light flashed from distant thunderstorms, seen at horizon.

Love the haiku, Lisa.

Thank you for the information! As there are often no discernable storms in the area, I found this "heat lightning" to be rather mysterious and wondered if there were elements within the clouds, rubbing together and producing these flashing lights---almost like static electricity!
Also, thank you for your nice comment on my haiku.