An overhead line carrying 13,200 Volts arced to a construction dump truck today when the operator raised the dump bed near the line. There were no injuries. The operator claims he was more than 15' feet away.
Well,I guess he doesn't lie.Quite likely HE was more than 15' away.However, his truck must have been less than 1' away from that power line when the arcing happend.
Humidity, (or moisture in the surrounding air) would work to lessen the "Flashover" distance, which has happened in this case.
Pure water is a poor conductor of electricity, however, try and find somewhere on this planet where the rain or mist water is pure.
Add impurities and the water vapour will conduct electricity with relative ease.
Variations of humidity content (1 g/m³... 30 g/m³) and temperature (-40°C...+40°C) of air affect the nonuniform air gap breakdown strengt only up to ± 40 % if compared with standard atmospheric condition . This ,according to IEC, holds for gap lenghts in the range 0.1 m < d < 10 m stressed by HV AC 50/60 Hz.
Could warm, moist, high carbon, diesel exhaust come into play here??? Sure is good you found that nail in the tire though!
Joe
Only excessive temperature could have large effect which wasn't the case.
But moist + dust,carbon and other polutions do have significant effect to surface conductivity of support and suspension insulators,and lower breakdown strenght of gaps where insulators are present.
Jimmy doesn't now that 
