A comparison has been made between the Townsend primary ionization coefficient, α, for dry air and for air with humidities typical of those in the atmosphere. α is defined as the number of new electrons produced by an electron per centimeter of drift in a field. A range of field/pressure ratios,E/p 0, of 40 to 100 V (cm torr)−1 was employed. The variation of α with humidity is very small. Over the range ofE/p 0 from 50 to 100 V (cm torr)−1, the secondary ionization coefficient, γ, of a water surface has been found from sparking potential data to be typically 2×10−4. γ represents the fraction of primary ionizing collisions that ultimately result in the production of additional electrons at the surface.