What is the temperature at which air becomes plasma?

[thepowdertoy]

Need help here to determine correct temperature when the air turned into plasma, because there is someone in other forum (not here) insisted that a 15000000 degree Celcius flame is flame, not plasma, but I think at that temperature air must already becoming plasma.

 

[PhysicoRaj]

This site may have your answer: http://www.madsci.org/posts/archives/2000-05/957446951.Ph.r.html
Which says that the temperature must be 174408K.

 

[DrZoidberg]

The higher the temperature of air is the more ions it will contain. Even at room temperature there are a few ions that are mostly generated by radiation.
A candle flame contains a lot more ions. Put a burning candle into a microwave oven and you get a nice plasma ball because the flame is so conductive it absorbs most of the microwaves. What percentage of air molecules must be ionized to call it a plasma is not clearly defined. So you can't give an exact temperature at which air turns into plasma. The hotter it is the more it is ionized.

 

[PhysicoRaj]

But without the influence of any radiation, or electric fields, etc., using only heat as the source, still can it have a vague ionization temperature????

 

[Drakkith]

At 15 million degrees not only is air a plasma, it is so hot that it emits very heavily in the extreme UV to X-ray range of the EM spectrum.
http://casswww.ucsd.edu/archive/public/tutorial/Planck.html

If you heat a mix of Dueterium and Tritium gas to 15 million degrees you actually start to get measurable fusion.
http://en.wikipedia.org/w/index.php?title=File:Fusion_rxnrate.svg&page=1

 

[Enthalpy]

At 3000K oxygen dissociates significantly into neutral individual atoms. Nitrogen needs several 1000K more. Significant ionization takes a hotter temperature. But very slight ionization happens before.

You can look for the ionization energy and divide it by R (if moles, or by k if molecules) to get a temperature where oxygen is half-way ionized.