Conditions Conducive to Lightning Striking an Aircraft in a Thunderstorm

Abstract : The UHF band radar at NASA/Wallops Island Research Facility was used for guiding the NASA F106 research aircraft through the upper regions of storms with lightning activity in them so that the aircraft would be struck by lightning. The radar was used also as a tool to study nature and the characteristics of direct lightning strikes on the aircraft, and to determine the environmental conditions favorable to such strikes. The analysis of lightning echoes at the moments of the strikes strongly indicates that the aircraft itself triggers the lightning, rather than intercepting a naturally produced flash. The lightning echo characteristics of the strikes are similar to those of intracloud flashes, and led us to believe that the aircraft is most led us to believe that the aircraft is most of the time part of the lightning channel. The probability of a direct strike during a storm penetration (PDS) is defined here as the ratio of the number of direct strikes on the aircraft to the total number of flashes occurring in the radar resolution volume containing the aircraft. Correlations between the PDS and the intensity of rain and turbulence, temperature, and lightning flash rate in the storms penetrated were investigated. The trends in these parameters indicate that the highest risk of the aircraft being struck by lightning during a penetration in the upper regions of thunderstorms is when 1) there are temperatures of -40 deg C and lower, 2) there is little or light precipitation, 3) there is light or no turbulence, and 4) there is a lightning flash rate of < or = 10 -per min. (Author)