Lightning Characteristics

The conditions necessary for a local convective thunderstorm, an old fashion summer afternoon thunderstorm, are lots of moist air from ground level to a few thousand feet, cooler air above with little to no wind, and plenty of sun to heat both the ground and the air mass near the ground.  As the warm moist ground-air is heated, it rises quickly to heights where the temperature is below freezing, eventually forming a thundercloud.  Within the thundercloud, the constant collision among ice particles as driven by the turmoil of rising air causes a static charge to build-up.  Eventually the static charge becomes sufficiently large to cause the electrical breakdown of the air — a lightning strike.

The average thunderstorm is approximately six-miles wide and has an approximate 25 MPH rate of travel.  The anvil shape of the cloud is due to a combination of significant thermal layer (tropopause) and upper high velocity rising winds that cause the top of the cloud to mushroom and be pushed forward.  The area of immanent danger is the area up to ten miles in front of the leading edge of the cloud. 

The conditions necessary for a local convective thunderstorm, an old fashion summer afternoon thunderstorm, are lots of moist air from ground level to a few thousand feet, cooler air above with little to no wind, and plenty of sun to heat both the ground and the air mass near the ground.  As the warm moist ground-air is heated, it rises quickly to heights where the temperature is below freezing, eventually forming a thundercloud.  Within the thundercloud, the constant collision among ice particles as driven by the turmoil of rising air causes a static charge to build-up.  Eventually the static charge becomes sufficiently large to cause the electrical breakdown of the air — a lightning strike.

The average thunderstorm is approximately six-miles wide and has an approximate 25 MPH rate of travel.  The anvil shape of the cloud is due to a combination of significant thermal layer (tropopause) and upper high velocity rising winds that cause the top of the cloud to mushroom and be pushed forward.  The area of immanent danger is the area up to ten miles in front of the leading edge of the cloud. 

When a lightning strike does occur, the return stroke rapidly deposits several large pulses of energy along the leader channel.  That channel is heated by the energy to above 50,000ºF.  This heating of a short section (~30 feet) of the channel takes only a microsecond and hence the channel section has no time to expand while it is being heated.  The high-pressure channel rapidly expands into the surrounding air and compresses it.  This disturbance of the air propagates outward in all directions.  For the first ten yards or so it propagates as a shock wave (faster than the speed of sound) and after that as an ordinary sound wave.  The thunder we hear is the pressure variations induced in the air by the expansion of each part of the lightning channel due to its initial high pressure.

When a lightning strike does occur, the return stroke rapidly deposits several large pulses of energy along the leader channel.  That channel is heated by the energy to above 50,000ºF.  This heating of a short section (~30 feet) of the channel takes only a microsecond and hence the channel section has no time to expand while it is being heated.  The high-pressure channel rapidly expands into the surrounding air and compresses it.  This disturbance of the air propagates outward in all directions.  For the first ten yards or so it propagates as a shock wave (faster than the speed of sound) and after that as an ordinary sound wave.  The thunder we hear is the pressure variations induced in the air by the expansion of each part of the lightning channel due to its initial high pressure.

With sound traveling at approximately 1090 feet per second at sea level or .021 (rounded) miles per second, we can approximate the distance between the lightning event and ourselves.  If we start measuring time from the initial observation of the flash and stop measuring when the initial thunder is heard, we have a flash-to-bang time in seconds.  If this time is multiplied by the speed of sound you have an approximation of the distance between the observer and the beginning of the heated channel.  (Five seconds is approximately one mile.)

It should come as no great surprise that the lightning event experienced by your equipment consists of several huge impulses of energy.  The majority of the energy is pulsed dc.  Riding on top of the pulse is a substantial amount of RF energy.  The RF energy is derived from the fast rise time of the pulses.  A typical lightning strike rise time is 1.8uS.  That translates into an RF signal at 2.2MHz.  The rise times can vary from a very fast of 0.25uS to a very slow of 12uS, thus yielding an RF range from 16 MHz down to 333 KHz.  The RF nature of the energy will have a major affect on the design of the protection plan.  In addition to the pulses, the antenna and coax feed line form a tuned circuit that will ring when the fast rise times pulses hit.  This is much like a struck tuning fork in that it adds additional ringing energy to the radiated lightning RF energy captured by the antenna.

Average peak current for the first strike is approximately 18kA (the 98 percentile range is 3kA to 140kA).  For the second and subsequent impulses, the current will be about half the initial peak.  Yes, there is usually more than one impulse.  The reason that we perceive a lightning strike to flicker is that it is composed of an average 3 to 4 (98 percentile range is 1 to 20+) impulses per lightning strike.  The typical interval between impulses is approximately 50mS.