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LIGHTNING ACTIVITY AND EXPOSURE CHAPTER 4: EXTERNAL GROUNDING (EARTHING)
FIGURE 4-2 LIGHTNING ACTIVITY, FLASH DENSITY (FLASHES PER SQUARE KILOMETER PER YEAR)
Table 4-1 provides a relationship between thunderstorm days per year and lightning flashes per square
kilometer per year (BS 6651:1999, table 6).
TABLE 4-1 RELATIONSHIP BETWEEN THUNDERSTORM DAYS PER YEAR AND LIGHTNING FLASHES
Thunderstorm
days per year
PER SQUARE KILOMETER PER YEAR
Flashes per square kilometer
per year
Flashes per square mile per
year
Mean Limits Mean Limits
5 0.2 0.1 to 0.5 0.5 0.25 to 1.35
10 0.5 0.15 to 1 1.29 0.38 to 2.59
20 1.1 0.3 to 3 2.84 0.77 to 7.77
30 1.9 0.6 to 5 4.92 1.55 to 13
40 2.8 0.8 to 8 7.25 2 to 20.7
50 3.7 1.2 to 10 9.58 3.1 to 25.9
60 4.7 1.8 to 12 12.17 4.66 to 31
80 6.9 3 to 17 17.87 7.8 to 44
100 9.2 4 to 20 23.82 10.36 to 51.8
NOTE: Information obtained from BS 6651:1999, Table 6.
4-4 68P81089E50-B 9/1/05