“Lightning return stroke leader channel temperature in air is ~ 30,000 K”
The Lightning Discharge, Uman, Martin A., Dover Publications, 2001
As for any temperature change during a prolonged spark, I suggest you research experiments where the sparks are continuous. Maybe you could measure it yourself!
CAUTION: HEALTH HAZARD
“Exposure to an arc-producing device can pose health hazards. In a closed space such as a classroom or home, the continuous arc formation of an open-air Jacob's ladder will ionize oxygen and nitrogen, which then re-form into reactive molecules such as ozone and nitric oxide. These free radicals can be damaging to the mucous membranes of people near the spark gap. Plants are also susceptible to ozone poisoning.”
http://en.wikipedia.org/wiki/Spark_gap
Just as a possible source for your research, a study of this mechanism may bring the exact answer you are looking for:
“The Townsend discharge is a gas ionization process where free electrons, accelerated by a sufficiently strong electric field, give rise to electrical conduction through a gas by avalanche multiplication caused by the ionization of molecules by ion impact.”
http://en.wikipedia.org/wiki/Townsend_discharge
Bobbywhy