I was in an aquarium one day watching the bubbles rise from a diver in scuba gear. Contrary to what the cartoons would have you believe, the bubbles did not appear as perfectly formed spheres. Instead, they appeared to be the top sections cut off from silvery spheres.
What causes this appearance? Are the air bubbles actually spheres, but we only see the top part because of the way the light reflects off them? Is it because the pressure at the top of rising air bubbles is greater? Or is it something else?
This is exactly what happens. When diving, you’ll see the bubbles get larger and larger as they ascend, until they look like a big frisbee, nearly a foot across. Then, suddenly, they’ll explode into a bunch of smaller bubbles. Repeat cycle. Kind of neat, the first time you see it, but it gets old quickly, 'specially when some turk keeps swimming below you and interfering with your picture-taking.
There’s actually quite a bit to the physics of bubbles rising through water. They have (or at least, at one time had – I haven’t been there in a while) a big tank set up at the Exploratorium in San Francisco, where you can watch them rise. In Jearl Walker’s The Flying Circus of Physics (the ultimate reference for oddball physics) he lists several odd behaviors of underwateer bubbles. Apparently, when in formation they behave very differently than when they rise on their own.