Reference: Chalmers, J. Alan, Atmospheric Electricity, Pergamon Press, London (1957).
9–1The 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. The sign of the field corresponds to a negative charge on the earth’s surface. This means that outdoors the potential at the height of your nose is 200 volts higher than the potential at your feet! You might ask: “Why don’t we just stick a pair of electrodes out in the air one meter apart and use the 100 volts to power our electric lights?” Or you might wonder: “If there is really a potential difference of 200 volts between my nose and my feet, why is it I don’t get a shock when I go out into the street?”
We will answer the second question first. Your body is a relatively good conductor. If you are in contact with the ground, you and the ground will tend to make one equipotential surface. Ordinarily, the equipotentials are parallel to the surface, as shown in Fig. 9–1(a), but when you are there, the equipotentials are distorted, and the field looks somewhat as shown in Fig. 9–1(b). So you still have very nearly zero potential difference between your head and your feet. There are charges that come from the earth to your head, changing the field. Some of them may be discharged by ions collected from the air, but the current of these is very small because air is a poor conductor.