A pilot said he saw St. Elmo's fire, what is that?

Aircraft can build up a static charge, by means such as flying through snow. The charge often manifests itself as a blue/green luminance on and around portions of the aircraft. You may see it on the cockpit windscreen, for example.

These are all correct answers but I wanted to add that if you want an example of what St. Elmo's Fire looks like, you can take a look at plasma globes you'll see in some novelty shops. These are globes that have fingers of plasma running around inside them (you can touch the globe to change the electricity's path) and exhibit a lot of similiarity with St. Elmo's Fire. Pilots report being able to touch the windcreen and play with the discharges. Get a look at one and you should have an idea of what the pilot you spoke to saw.

Here's a couple of pics for example:

http://www.teachersource.com/Images/Product/md/pls...

http://www.show.me.uk/dbimages/chunked_image/inspi...

St. Elmo's Fire is an electrical weather phenomenon in which luminous plasma is created by a coronal discharge originating from a grounded object in an atmospheric electric field (such as those generated by thunderstorms or thunderstorms created by a volcanic explosion).

St. Elmo's fire is named after St. Erasmus of Formiae (also called St. Elmo), the patron saint of sailors. The phenomenon sometimes appeared on ships at sea during thunderstorms, and was regarded by sailors with superstitious awe, accounting for the name..

Ball lightning is often erroneously identified as St. Elmo's Fire. They are separate and distinct meteorological phenomena.

St. Elmo's fire is an electrical discharge associated with a structure. Like a church stepple or even an airplane flying thru a storm.

We used to see it over the canopy and along the leading edges of the wings as we flew northern missions in the B-52. It looked like greenish - yellow continuous sparks dancing over the outside surfaces. It was a little disconcerting, because of course the wings are full of fuel. But it was harmless enough. On one such flight, we saw aurorae so bright we could turn off the cockpit lights and read the checklist by it. Very interesting..

All the scientific explanations above are correct.

But what it looks like is even cooler. It looks like the lightning that the Emperor was shooting Luke with at the end of 'Jedi.'

No joke. Its pretty sweet to see all those lightning bolts crackling over the windscreen. They are completely harmless though. You can even touch the windscreen and it doesn't affect you at all.