The specific function of lightning rod is producing an upward stream of ionized particles pointed to clouds that will channel the eventual electrical discharge from its origin.
There is a different potential between the discharger (that has the same potential than the air around it) and both the air terminal tip and the deflection ensemble (they have the same potential than earth). This difference increases as atmospheric potential becomes higher because of the imminent lightning stroke.
Knowing the value of this difference At allows us to relate time and velocity of electrical discharge spread and, consequently, to calculate the lightning impact distance and the protection radium that offers each lightning rod model (see table below).
The knowledge of this value allows finally to select the most appropriate lightning rod model taking into account the characteristics of the structure we want to protect and the level of protection needed according the norms UNE 21.186 and NFC 17.102.