Plasma is matter heat do a temperature so high that electrons deatach from the atoms. There is enough energy in man explosion to result in this.
Lower down in the atmosphere there is a lot of gas hot gas/plasma is limited in how far it car travel because of interaction with the rest of the atmosphere. Add to that the interaction cool down the material. At high altitude there is almost nothing there so the material can travel further and it warm longer.
You see the same effect with exhaust from the engines of a rocket. [https://www.youtube.com/watch?v=LjWpcv1vT-8](https://www.youtube.com/watch?v=LjWpcv1vT-8)
That it is a sphere is because the matter moving in all direction after a explosion and at similar speed and the result is a expanding shell.
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