Townsend Unit for reduced Electric Field (E/N):

Some rule of thumb conversions at 20*C and 1 atm:

• 1 Td = 250 V/cm
• 1kV/cm = 4 Td

N varies only slightly from 0*C (STP) to 20*C,
N(0*C) = 2.69 10^25 [m-3]
N(20*C) = 2.50 10^25 [m-3]

See: https://en.wikipedia.org/wiki/Townsend_(unit)