Step potential is an electrical term for the difference in voltage between your feet. Normally there is no difference at all, but when lightning strikes it creates a potential difference over an area many metres across as the electricity seeks to go to earth. The voltage over the distance of a pace can be high enough for the current to flow though your body instead of the ground.
The US army’s Ranger School conducts lightning protection training in central Florida. The area is perfect for this activity as the hot and humid conditions give rise to frequent thunderstorms – 35 lightning strikes per square kilometre every year have led to the area being nicknamed “lightning alley”. However, the training became a little bit too realistic in August this year when lightning struck close to some trainees.
Step potential means that a large number of people can be injured in one incident: none of the soldiers were struck directly by the lightning, but all were in the area with a high step potential. Four instructors and 40 students were affected. This is not quite a record: the greatest number of people injured by a single bolt of lightning is 90 at a campground in Michigan in 1975. Large numbers of cattle or sheep huddled together have been killed in similar incidents.
In the incident in August, all 44 military personnel affected were taken to hospital and 11 remained there overnight. However, according to an army news report, all were discharged by the next day.
