I repeated an experiment performed by Sue Gaeta which she posted to the Tesla  list.

My goal was to get some pictures of arcs on water but I noticed some interesting things happening in the gap area and got sidetracked.

My setup used a .06uf capacitor, 12kv@30ma NST, .75" brass electrode, and .25" gap between the electrode and the water.

One of the things I noticed was the physical wave created in the water each time the gap fired. At first I assumed it was due to the shockwave from the superheated air in the arc channel (which also made an acoustic "pop" each time the gap fired). Upon closer examination I noticed this was only part of what was happening.

As the voltage is increased the water under the gap is pulled up towards the electrode then dropped as the gap fires. In the second picture you can see the water as it starts to raise up (approx 3kvdc). This electrostatic effect is quite pronounced as I'm able to observe the gap distance being reduced by over half before it fires.

The arcs above are on top of the water and give a good outline of how high the water is raised before the gap fires. If you look closely It's like a little pyramid of water with a tiny plasma ball on its top (between the tip of the water and the electrode).

I had made an assumption that the breakdown voltage was simply due to the static distance between the electrode and the water's surface. But what I found was a dynamic interaction between the gap voltage and the electrostatic forces. As the voltage is increased the electrostatic forces pulled the water up closing the gap distance until it fires. With this in mind I realized the cap was probably firing with much less than a full charge so I checked and it was only charging to 6.1kv. Increasing the distance in small steps allowed me to get the charge up to 12.8kv and produced surface arcs which were twice as long.