It is helpful to say we can't measure a potential, only a difference of potential between two points. We call that a voltage.
Voltage notion is interesting because:
- The work done when a charge moves through a voltage of one volt is one joule per coulomb of charge.
- Alternatively we can say the work done by an electron moving through one volt, is one electron-volt (eV), a quantity equal to 1.602 x 10-19 J.
Note earth potential is not constant, and --something at first thought unbelievable-- potential increases in atmosphere by about 100 V with each meter of altitude. Of course the voltage between hour head and our feet is null because our body is a relatively good conductor, but a plane at an altitude of 40 km is at a potential of about 400 kV. Thrilling.
Actually the potential gradient also varies a little bit. So... as everything varies, where should we put the reference?
The answer is: It doesn't matter where the reference is, but earth surface is nearly equipotential and so is a handy reference for comparing potential everywhere on it.
With this reference, the potential looks like this, without and with a person standing on earth surface:
No need to mention where the preferred path for lightning is.