WRAL WeatherCenter Blog

Define "excessive lightning". Isn't one strike too much if it hits your house, and 1000 strikes irrelevant if no damage occurs? Also using the term "excessive" impies that lower amount is acceptable, right?

Posted August 23, 2008

MIKE MOSS SAYS:       Jerry,      Your point that semantics could be argued endlessly in trying to establish terminology that can clearly convey relative threats is well taken, but for the time being the National Weather Service, in an attempt to make good use of evolving research that is reaching a point where it allows better estimates of  the potential for high lightning rates in advance of storm development (based on observed or model-predicted thermodynamic profiles) or during storm cell development (based on radar observations and how echoes of certain intensity correspond to certain temperature regimes within the storm), has settled on the term "excessive" as one of its descriptors of cloud-to-ground lightning flash rates from thunderstorm cells. As currently used, this falls at the upper end of a scale with the following rough criteria:

Occasional = ~ 1 strike/minute

Frequent = ~ 2-6 strikes/minute

Continuous = ~ 6-11 strikes/minute

Excessive = ~ 12 strikes/minute and higher (corresponds to 1 or more strike every 5 seconds)

You are certainly correct that a single strike is too many if it hits your house or strikes a person. However, if two precipitation cells of roughly the same size, speed and duration pass over an area, and one produces a cloud to ground flash rate of 2-3 per minute while the other produces 20-30 per minute, the odds that SOMEONE's house will be struck are much higher with that storm, as is the relative danger associated with being caught outdoors when the higher strike rate cell crosses an area. In addition, it is not unusual for the higher rate storms to actually produce lightning for longer periods than their less active cousins, so that in addition to the rate difference, there can be an even greater difference in the absolute number of strikes produced.


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