Voltage Gradient- The surge of voltage through the ground raidially outward from the lightning strike point. The voltage gradient can electrocute anyone standing on the ground close to a lightning strike. This is often how lightning injuries occur to people or animals who are near a lightning strike but are not directly hit. The reason that standing under or near a tree during a storm is dangerous is due mainly to the voltage gradient. Even if the main lightning channel flows entirely through or along the tree and does not jump over and hit whoever is standing there, the resulting huge surge of current through the ground surrounding the tree will give a nasty shock to anyone touching the ground nearby. The reason the term 'gradient' is used is because the voltage in the ground is lower with increasing distance from the strike point.
my uncle however as a child was stuck under the same tree twice within 5 years (and they say it never happens.) sadly he died a very fit man aged only 34 heartattack. wonder if it was related? he didnt smoke was a milkman and played sport.