See https://www.feynmanlectures.caltech.edu/II_09.html#:~:text=9%E2%80%931The%20electric%20potential,volts%2Fm%20in%20the%20air https://www.feynmanlectures.calt

See https://www.feynmanlectures.caltech.edu/II_09.html#:~:text=9%E2%80%931The%20electric%20potential,volts%2Fm%20in%20the%20air https://www.feynmanlectures.caltech.edu/II_09.html#:~:text=9.... Reference: Chalmers, J. Alan, Atmospheric Electricity, Pergamon Press, London (1957). 9–1 The electric potential gradient of the atmosphere On an ordinary day over flat desert country, or over the sea, as one goes upward from the surface of the ground the electric potential increases by about 100 volts per meter. Thus there is a vertical electric field E of 100 volts/m in the air.