leader can be several hundred meters long before the two meet. For very tall buildings, the upward leaders
begin to. form even before the downward leaders have begun to form within the cloud; such incidents are
generally described as triggered lightning. Triggered lightning is not very common for structures less than 150
meters (500 feet) in height; as the height increases above this threshold, the proportion of triggered strikes
increases rapidly (3-5).
3.4 STRIKE LIKELIHOOD.
The number of total flashes to which the structure is exposed is related principally to local thunderstorm
activity. Local thunderstorm activity can be projected from isokeraunic maps similar to those shown in
Figures 3-2 and 3-3. These maps show the number of thunderstorm days per year for various regions of the
United States and the world. Additional maps of worldwide keraunic levels can be obtained from the World
Meteorological Association (3-6).
A thunderstorm day is defined as a local calendar day on which thunder is heard irrespective of whether the
lightning flashes are nearby or at some distance away. To an observer at a specific location, the average
distance at which lightning may occur and thunder will be heard is about 10 km (6 miles) (3-5). Therefore, a
thunderstorm day means that at least one lightning discharge has occurred within an area of about 300 square
km (120 square miles) surrounding the position of the observer. The actual number of strikes in the immediate
vicinity of the observer may be considerably higher or lower than the number of thunderstorm days might
indicate, depending upon the duration and intensity of a specific storm or series of storms.
In spite of the relative inexactness of a prediction of a lightning strike to a specific object that is based on the
keraunic level, the thunderstorm day is the only parameter related to lightning incidence that has been
documented extensively over many years. Its primary value lies in the qualitative information which it
provides. This information can be used to assist in the determination of whether lightning protection should be
provided in those situations where there is serious doubt as to the relative need for such protection. For
example, a particular facility may not be essential to the safety of aircraft, but the loss of the facility may
cause traffic delay. In an area of frequent thunderstorms such as the west coast of Florida, for example, the
number of outages in areas where there was no protection could be so high as to be unacceptable; in an area of
few thunderstorms; e.g., Southern California or Alaska, the expected outage from lightning might be once every
few years (which could be significantly less than outages for routine maintenance).
The number of lightning flashes per unit earth surface area increases with the number of thunderstorm days per
year, though not linearly. Empirical evidence indicates that the number of flashes per square kilometer,
be reasonably predicted from (3-5):