More on lightning

B. Geerts


Lightning is an important weather hazard, although the number of lightning fatalities has declined steadily over the last few decades. In the USA about a hundred people are killed annually by lightning. At least 650 people were killed by lightning in Australia during 1824-1991, mostly people working on the land and male (1). About two-thirds of those killed outdoors (about 86% of the total) were in exposed places, not under shelter, whilst 79% of those killed while sheltered under trees. The number killed has decreased annually, especially since 1910, because fewer people work on the land or are otherwise exposed to the elements. Most of the fatal lightning strikes in Australia occurred between noon and 6pm, and from November - February, which are occasions of peak thunderstorm occurence in most places in Australia.

The Empire State Building in New York is struck about 23 times each year. New York also has about 23 thunderstorm days per year. Clearly tall objects are likely to be struck. Lightning can start bush and forest fires. These may be put out by the storm's rain, but often this is not the case, as much of the rain evaporates between the cloud base and the ground.

In preparation for the 1996 Olympic Games in Atlanta, a lightning climatology for the state of Georgia was done (2). The synoptic charts of eight days with lightning were contrasted with eight comparable days without, showing that the dewpoint at 850 hPa (~1,500m above the ground) was about 8 K higher on lightning days. Thunderstorms draw most of their energy from the lowest 1-2km above the ground. Lightning activity also correlated (positively) with atmospheric stability, notably with the convective available potential energy (CAPE), but also with the Showalter stability index and K index.

Lightning is largely a continental phenomenon. Marine cumulonimbi have fewer cloud droplets because of a shortage of cloud condensation nuclei over the oceans. Also, the convective available potential energy (and updraft speed within convective cells) is generally lower in a marine environment with thunderstorms, compared to continents. Therefore there are fewer droplets and fewer collisions between supercooled water and graupel. And therefore there is less charge separation between cloud base and cloud top. Therefore electric discharges (i.e. a lightning strike) either within marine clouds or between clouds and the sea surface are rare.

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(1) Coates, L., R. Blong and F. Siciliano 1993. Lightning fatalities in Australia, 1824-1991. Natural Hazards, 8, 217-33.

(2) Livingston, E.S., J.W. Nielsen-Gammon & R.E. Orville 1996. A climatology, synoptic assessment and thermodynamic evaluation for cloud-to-ground lightning in Georgia. Bull. Amer. Meteor. Soc., 77, 1483-95.