Pilots are taught to avoid thunderstorms by 20 miles or more, especially when the thunderstorm is exhibiting severe characteristics such as heavy or extreme rain, hail, high winds or tornadoes. One of the other inherent dangers of a thunderstorm is being struck by a bolt of lightning while in flight. Certainly the threat of a lightning strike increases the closer you are to the thunderstorm. So keeping your distance makes good sense. But what about aircraft-induced lightning?
There are two kinds of lightning encounters. The first one can simply be described as being in the wrong place at the wrong time. Literally the flight path of your aircraft intercepts a lightning bolt that is already in progress. While this is rare, it normally occurs down low when climbing out after takeoff as was the case of this aircraft (shown on the right) on takeoff from the Komatsu Air Force Base off the coast in the Sea of Japan.
The other more likely encounter is one induced by your aircraft. There are many observed cases of lightning strikes to aircraft inside or near clouds that had not previously produced natural lightning. Studies show that about 90-percent of the lightning strikes to aircraft are thought to be initiated by the presence of the aircraft itself. The scary statistic, however, is that 40-percent of all discharges involving airborne aircraft occurred in areas where no thunderstorms were reported. The two separate lightning strikes to Apollo 12 shortly after launch were thought to be initiated by the Saturn V rocket in a region of high instability. Other than these two lightning strikes to the rocket, there were no other lightning strikes in the region six hours prior or six hours after the launch.
While aircraft-induced lightning is still being actively researched, there are a few important characteristics to consider. From the information above, we know that it doesn't take a thunderstorm to initiate a lightning strike. The presence of the aircraft in an environment conducive to an electrical discharge is all that is necessary.
Most of the aircraft-induced lightning discharges during the warm season occur when the aircraft is at an altitude close to the melting level. As shown above, the preferred temperatures include a range of +5°C to -10°C, with the highest number of incidents occurring right at the melting level (0°C).
In general, natural lightning in deep, moist convection doesn't form until the tops of the storm build well above the melting layer. For natural lightning to form, ice crystals, graupel and supercooled liquid water need to be simultaneously present. If any one of these three ingredients is missing in sufficient quantities, lightning doesn't generally occur. So it is not a surprise that an aircraft-induced lightning strike must be within local air mass instability producing deep, moist convection that tops out well above the melting level. The conditions, however, are not conducive for natural lightning.
As mentioned above, most of the lightning strikes down low are by aircraft intercepting the lightning. Most aircraft-induced lightning strikes occur between 10,000 feet MSL and 16,000 feet MSL as seen in the chart below. In the summer the melting level in the mid-latitudes generally fluctuates between 12,000 feet and 17,000 feet.
An overwhelming number of lightning strikes occur within the cloud itself; only a very small percentage of strikes occur outside of the cloud boundary or below the cloud. A majority of the strikes occur within precipitation (rain) and in-cloud turbulence. This includes a mixture of rain, snow, ice pellets or hail.
In order to avoid an encounter with lightning, the best advice is to remain in cloud-free air whenever possible, especially when the atmosphere is unstable and capable of producing deep, moist convection. Even when thunderstorms (natural lightning) are not occurring or expected to occur, an aircraft-induced lightning strike can still be a risk. Operate outside of areas of precipitation and be especially careful operating in clouds near the melting level.
Most pilots are weatherwise, but some are otherwise™
Weather Systems Engineer
CFI & former NWS meteorologist