Found this on the web...could be something your seeing...
Corona Effect
The Corona Effect is what causes neon signs to work. By inducing high voltage in the tube, the gas is ionized and thereby lights up. In the case of plug wires glowing, the Corona effect can happen when atmospheric conditions are right. High humidity and low barometric pressure combined with the high voltage of the ignition system will create an electrostatic field around the plug wires and will in effect, ionize the ~air~ around the plug wires. Thereby causing the glow. Since this is a breakdown of the surrounding air (not the wires) it will not affect ignition performance. That is of course assuming your plug wires have not been ahem, chewed on by mice ;-)
Also, since wide plug gaps require a higher voltage to fire them, this will increase the chances of the Corona Effect. Usually more visible around the plug wires rather than the plugs. Note: not to be confused with the Corona "after effect" of excessive consumption of Coronas with limes.