• If a gas is heated to sufficiently high temperature, to the order of 10,000 K and above, many of the gas atoms or molecules acquire high velocity to cause ionization on collision with other atoms or molecules.
• The molecules excited by photon radiation also affect the ionization process.
• Thermal ionization is the principal source of ionization in flames and arcs.
• Saha derived an expression for the degree of ionization θ in terms of gas pressure p and absolute temperature T, with the assumption that under thermodynamic equilibrium conditions, the rate of new ion formation must be equal to the rate of recombination. This expression is given as follows:

where p is the pressure in Torr, W₁ the ionization energy of the gas, k Boltzmann's constant (k = 1.38 x 10^-23 J/K), θ the ratio n₁/n that is, the number of ionized to total particles, and T the absolute temperature in K.

• Thermal ionization becomes significant only at temperatures above 10,000 K , as shown in Fig. 6.4

• During electrical breakdown in gases, thermal ionization has its significance only towards the final stage of breakdown. Because of the transformation of large amount of energy in the electrically conductive channel (known as 'leader') towards the final stage of breakdown, an exceptionally high temperature rise in this channel core is possible.