[chris (OP)]

The explanations for this phenomenon on the web are quite hard to understand. What is corona discharge and how does it happen?

[Soul Surfer]

In an area with a large electrical field say between two largish areas the nature of charge is to concentrate in regions of high curvature between the large areas of charge creating the field.  Think of it a being the difference between trying to push the flat back end of a pencil or the sharp point into your skin the force is greatest on the sharp point.

Now an electrical discharge can start in a neutral gas when any charged atoms or molecules in the gas can get enough energy to ionise another atom or molecule.  (there are always a few ions about due to statistical effects and cosmic rays.)

This field is strongest near points like the spiky bits at the end of lightning conductors or ships masts.  So the ability to create more ions is strongest there and fades further away from the point.  so ions tend to concentrate and accelerate near the point.

Finally when ions are created this involves knocking electrons off the atoms to create what is known as a plasma  an area of charged particles moving through the field and sometimes positive and negative particles meet and combine.  When this happens energy is released mostly in the form of light and heat radiation so the visible part of the recombination radiation is the corona.

If the overall field is weak enough this can reach a stable state and a small area of discharge is maintained around the pint but as in thunderstorms eventually the field can be so large that the corona extends right across the gap between the two charged areas and a full discharge or lightning flash happens

When corona discharge happens at the ends of ships mast in thundery weather thois is called "St Elmos fire" and it is an indication that there is a high risk of a full lightning strike.

[chris (OP)]

Brilliant answer. Thanks so much.