Lightning that ignites forest fires in Finland

We studied probabilities that lightning strokes ignite a forest fire. Lightning location data from 1998 to 2002 was used. Flashes were grouped into thunderstorms based on their closeness both in time and space. Information on location, time of ignition and cause of all forest fires as recorded by chiefs of local fire brigades during this 5-year period were also used. Fires judged to be ignited by lightning were linked to strokes occurring within 10 km and 50 h of ignition. The probability that a stroke had ignited a particular fire was computed based on the distance between the two in time and space and on the number of other strokes that might have ignited the same fire. Our results show that positive and negative strokes ignite with equal probability. Increasing number of strokes per flash, hereafter referred to as “multiplicity” of flashes tended to decrease ignition probability of a stroke. Strokes in long-lasting and intense thunderstorms are less likely to cause ignition than strokes in local, small-scale thunderstorms. Our results clearly contradict those of earlier North American studies that suggest that long continuing current associated with positive flashes and negative flashes of high multiplicity are needed for ignition. Our results may find use in forecasting or nowcasting lightening-caused forest fire ignitions in Finland, and lead to the development of a similar methodology for testing and use elsewhere.