A bluish flame and hissing sound appeared on outer conductor
of a transmission line due to atmospheric moisture is 
termed as corono

A voilet glow, hissing noise and the production of ozone 
gas on the condustor is called CORONA.

When a high or extra high voltage transmitted in a 
transmission line, due to magnetic effect on the core of 
the conductor all most all electronics pass on the surface 
of the conductor. So the air outside the conductor surface 
becames conducive and looks bluish and due to dielectric 
breakdown of air a hissing noise comes from surrounding, 
Called as corona !

Due to corona OZONE formation on the outside surface of the 
conductor. The effect of this to the power system is called 
as corona effect.

corona appears due to the ionizing of the surronding air 
due to the electromagnetic fields surronding the cable it 
has two names dark corona and corona and it a appear as 
redd bluish glue it has adv and dis adv.

It generally occure at high voltage transmission. When 
dielectric voltage becomes more than 30v\cm . than it 
ionise the sorrounding air . So the air outside the 
conductor surface becames conducive and looks bluish and 
due to dielectric breakdown of air a hissing noise comes 
from surrounding, Called as corona !