NOAA Technical Memorandum NWS SR-193, Section 6
6. Monthly and Seasonal Variations
A. National by Month
B. Regional by Month
C. National by Season
Lightning casualties and damages peak during the summer months. July has the most deaths, injuries, casualties and damage reports, as shown in Figure 26 and Table 24. Monthly percentages increase gradually through the spring, when the thunderstorm season begins for most of the country. Deaths, injuries, and damages decline more rapidly in the fall compared to the slower increase in spring. Deaths occur earlier in the year somewhat more often than injuries (Figure 26). The Maryland aircraft crash in 1963 (mentioned in section 5B) killed 81 people in December and accounts for the large number of deaths in that month.
Differences between the percentages of casualties and damages are in the lower part of Figure 26. Casualties reach a sharper maximum in July, while damage reports are spread somewhat more evenly through the year. The more concentrated casualty distribution may occur because people are exposed to lightning more often during the midsummer months. In contrast, immovable property and other objects are damaged relatively more often before and after the summer months. If this is true, damage reports may be a better representation of the distribution of cloud-to-ground flashes than casualties.
For the US, all of the following papers show a July maximum, as well as a slower increase before that month compared to the decrease after July:
*Storm Data deaths from 1959 to 1965 (Zegel, 1967).
*Storm Data deaths and injuries from 1968 to 1976 (Mogil et al., 1977).
*Storm Data casualties from 1968 to 1985 (Duclos and Sanderson, 1990).
*Storm Data casualties from 1959 to 1990 (López and Holle, 1995).
*Cloud-to-ground flashes from 1992 to 1995 (Orville and Silver, 1997).
Figure 26: Top: Monthly variations of lightning fatalities, injuries, and damage reports for the US from 1959 to 1994. Bottom: Percent casualties minus damage reports by month.
Within the US, most of the following papers show the same July peak and a slower rise before July than the subsequent decrease:
-Florida, 1978 to 1987
Fatalities peaked in August (Duclos et al., 1990).
-Michigan, 1959 to 1987 Storm Data deaths (Ferrett and Ojala, 1992).
-Central Florida, 1983 to 1990
Fatalities and property damages peaked in August, while injuries were most common
in July (Holle et al., 1992).
-Colorado, 1950 to 1991
A few more casualties in July than June. Casualties were more clustered during
midsummer in Colorado than for the US in Figure 26 (López et al., 1995).
-Colorado, Utah, and Wyoming, 1987 to 1991
Slightly more insurance claims were made in August than July (Holle et al.,
1996).
-Florida, 1996
June and July had 65% of the cases (Paxton and Morales, 1997).
Outside the United States, the following two published monthly distributions show very different results from the typical US monthly distributions:
*Singapore, 1922 to 1979 Two maxima, in November and April, were found in fatalities.
Since this location is at 1°N, the midsummer concept is not relevant. The annual
cycle of deaths was identified as similar to that of local thunderstorms (Pakiam
et al., 1981).
*Australia, 1824 to 1991 The largest number of fatalities are in January, which
had a slightly larger number of deaths than in December. Taking into account
the reversal of seasons in the Southern Hemisphere, this is the same result
as found in the US. One difference from the US data in Figure 26 is that the
buildup to midsummer is somewhat faster than the dropoff (Coates et al., 1993).
Monthly distributions of casualties and damage reports by region of the country are shown in Figures 27 and 28. Most regions show damage reports to be spread somewhat more widely through the year than casualties. Casualties and damages in the northern regions of the country tend to have narrower distributions centered on summer than regions in the southern US due to a shorter thunderstorm season in colder regions. The broadest monthly distributions are in the Southern Plains.
Figure 27 shows the differences between monthly percentage frequencies in the Northern Rockies, Northern Plains, Midwest, and Northeast, and percent frequencies in the Southern Rockies, Southern Plains, and Southeast. Northern regions have up to 4% more casualties and damages in midsummer, while southern regions have more cases in spring and, to a lesser extent, in autumn.
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Figure 27: Differences between monthly percentages of lightning casualties (solid line) and damage reports (dashed) in northern minus southern regions of the US. |
Figure 28: Monthly variations of lightning casualties and damage reports by region of the US (%).
Seasonal totals of casualties and damages reports are listed in Table 25. Maps of these two categories are also given by state in Figure 29 based on Table 26.
Spring (March, April, May)
Casualties in spring show a similar distribution to the annual pattern
in Figure 2 with one exception. The higher annual ranks in the Northeast are
much lower, most likely due to less lightning during these spring months than
elsewhere.
Damage reports show a similar distribution to the whole year in Figure
9, except for the same reduction in the Northeast shown by casualties.
Summer (June, July, August)
Casualties in the summer are almost identical in distribution to the
map for the entire year in Figure 2. This result is as expected, since Tables
25 and 26 show that most lightning casualties occur
during summer.
Damage reports for the summer are also very similar to the whole-year
results shown in Figure 9.
Autumn (September, October, November)
Casualties show a return to high rankings for states in the Southern
Plains, as for spring. West Coast states rank relatively high in casualties
during autumn, but rank very low when the entire year is considered.
Damage reports are distributed similarly in the autumn and spring.
Winter (December, January, February)
Casualties occurred in only 21 states during winter. The highest concentration
is in the Southern Plains, as shown by monthly distributions in Figure
28. West-coast states continue to rank relatively high in casualties, as
in autumn. The largest number of deaths was in Maryland (81) due to the December
1963 aircraft crash (sections 5B and 6A).
Damage reports are most common in the Southern Plains, but are scarce
in states to the north. California and Hawaii are ranked among the top 30 states,
while they rank very low in summer.
In summary, summer maps of casualties and damage reports closely follow the annual maps. During other seasons, lightning cases are more frequent in the southern regions of the United States. Frequencies in the northeast are low except during the summer, while they are high in the West Coast states during autumn and winter.
Figure 29: Seasonal variations of lightning casualties and damage reports by state.