What is Lightning?

Much of the following was taken or adapted from NOAA Technical Memorandum ERL NSSL -102 by Holle and Lopez.
 

Lightning is a transient discharge of static electricity that serves to re-establish electrostatic equilibrium within a storm environment.

Any pilot will tell you that the average thunderstorm has a very turbulent environment.  Strong updrafts and down drafts occur with regularity, even within small thunderstorms.  The updrafts transport water droplets up into the cloud, while ice particles descend from the frozen upper regions of the cloud.  As they do, they bump and collide with each other.  Through this process, electrons shear off of the ascending water droplets and collect on the descending ice particles.  (A similar effect occurs when you rub your feet across a carpet before touching a door knob.)  This generates an electric field within the cloud, with the top having a positive charge, and the bottom having a negative charge.  An electric field is also generated between the bottom of the cloud and the surface of the earth, though not nearly as strong as the field within the cloud.  As a result, most lightning (~75-80%) occurs within the cloud itself.

In the world of electricity, opposites attract and insulators inhibit.  In a developing storm cloud such as the example above, there is an electric attraction (i.e. electric field) between its top and bottom.  As the charges to separate, the field strength grows.  The greater the magnitude of separation, the stronger the field, and the stronger the attraction between the positively charged top and the negatively charged bottom.  However, the atmosphere is a very good insulator, so a TREMENDOUS amount of charge has to build up before lightning can occur.  When that threshold is reached, the strength of the electric field overpowers the atmosphere's insulative properties, and lightning results.
 

Lightning discharges can be divided into two types:

Cloud to ground (CG) discharges, which have at least one channel connecting the cloud to the ground.

Cloud discharges that have no channel to ground.  These cloud discharges are classified as in-cloud (IC), cloud to air (CA), or cloud to cloud (CC).
 

This discussion will highlight CG type flashes, though IC, CC, and CA flashes behave similarly.

(Very slow motion animation.  The entire process usually occurs in less than a half-second.)

While lightning occurs instantaneously, it also takes place over several steps:

STEP 1)   A CG lightning discharge typically initiates inside the thundercloud.  When enough electrons collect in the bottom of the cloud, a very faint, negatively charged channel, called the stepped leader, emerges from the base of the cloud.  Under the influences of the electric field established between the cloud and the ground, the leader propagates towards the ground in a series of luminous steps about 50 meters in length and 1 microsecond (10^-6 seconds) in duration, in what can be loosely described as an "avalanche of electrons".  Between steps there is a pause of about 50 microseconds, during which time the stepped leader "looks" around for an object to strike.  If none is "seen", it takes another step, and repeats the process until it "finds" a target.  It takes the stepped leader on the order of 50 milliseconds (0.050 seconds) to reach its full length, though this number varies depending on the tortuosity of its path.  (Studies of individual strikes have shown that a single leader can be comprised of more than 10,000 steps!)  As the stepped leader's channel approaches the ground, it carries about 5 Coulombs of negative charge, and has a VERY strong electric potential of about 100 million volts with respect to the ground (though this can be as high as a BILLION volts).

STEP 2)  When the stepped leader approaches the ground, its strong, negative electric field repels all negative charge in the surrounding ground, while attracting all positive charge.  This induces an upward moving positive charge from the ground and/or objects on the ground.  When this positive charge collects into a high enough concentration, they form bolts of ground-to-air lightning known as streamers.  When one of these positively charged streamers contacts the tip of a negatively charged leader, (anywhere from 30 to 100 meters above the surface), the following three steps occur:

STEP 3) The leader channel's electric potential is connected to the ground

STEP 4) All other branches of the leader channel cease further propogation toward the ground, and all negative charge within these branches starts flowing to the ground through the newly established ground/cloud connection

STEP 5) An electric current wave then propagates up the channel as a bright pulse

This discharge process takes less than 100 microseconds and is called the return stroke.  It produces almost all of the luminosity and charge transfer in most cloud-to-ground strokes.  The lightning is actually traveling FROM the ground INTO the cloud, but because the process takes place so quickly, to the unaided eye is appears that the opposite is true.  (In photographs, it may APPEAR that lightning is descending from the cloud to the ground, but in reality, the return stroke is so brilliant that as it travels up the channel, it illuminates all of the leader's branches that did not connect with a streamer.)   Electric charge flows up the channel behind the wave front and produces a ground level current.  This current has a peak value of about 30,000 amperes, though it can be as high as 300,000 amperes.  It takes about 1 microsecond for the current to reach its peak value, and about 50 microseconds to decay to half that value.

(Side Note:  As the leader charge flows down the channel to the ground, electric and magnetic field changes are produced that propagate outward from the entire length of the channel.  These field changes have rapid variations that follow the channel of the stepped leader.  The field changes have  electrostatic, inductive, and radiative components, and each of the components has fluctuations of different frequencies that have different attenuation characteristics as the fields propagate from the lightning channel.  Therefore, the shapes of the field changes are strong functions of the radial distance from the channel.  The detailed structure of the first several microseconds of the electric and magnetic field changes produced by the return stroke is of fundamental importance in cloud to ground lightning detection systems.)

STEP 6)  After the current has ceased flowing up the leader channel, there is a pause of about 20 to 50 milliseconds.  After that, if additional charge is made available at the top of the leader channel (through breakdown mechanisms known as K and J processes), another leader can propagate down the established channel.  This leader is called a dart leader because it is continuous instead of stepped.  Dart leaders are what give lightning its flickering appearance.  Not every lightning flash will produce a dart leader, as sufficient charge to produce one must be made available within about 100 milliseconds of the initial stepped leader.

The dart leader deposits about one coulomb of charge along the channel and carries additional electric potential to the ground.  The negatively charged dart leader then will induce a new, positively charged return stroke from the ground.  The peak amplitude of the current usually decreases as additional dart leaders are produced.  As a consequence, the induced field changes are also smaller in amplitude and have a shorter duration than those of the first return stroke.  Dart leaders and their subsequent return strokes are not normally branched like the initial stepped leader and return stroke.

The combination of each leader (stepped and dart) and the subsequent return stroke is known collectively as a stroke.  All strokes that use the same cloud-to-ground channel constitute a single cloud-to-ground flash.  A flash can be made up of a single stroke, or as many as tens of strokes.  (The highest number of strokes in a single cloud-to-ground flash ever recorded is 47.)

CG lightning can also be initiated by stepped leaders that are positively charged.  The resulting return stroke carries a negative charge and transfers positive charge from the cloud to the ground.  The combination of the leader and the return stroke is then called a positive flash.  Usually there are no subsequent dart leaders down the existing channel, so only one stroke makes up a positive flash.

Positive flashes constitute less than 10 percent of all CG flashes, and most often occur on the periphery of a thunderstorm away from the central rain shaft.  However, the peak current of their return strokes is often much larger than the peak current of the negative return strokes. Thus, they are more lethal, and can cause greater damage than negative flashes.  It is believed that a large percentage of forest fires and power line damage is caused by positive flashes.

There is an extremely small percentage of flashes that are initiated from the tops of buildings and towers, as well as those triggered by rockets attached to ground by wire.  Their leaders move up to the cloud and their channels branch upward.
 

THUNDER

Thunder is the acoustic shock wave caused by the extreme heat generated by a lightning flash.  When a lightning bolt occurs, the air surrounding its channel is instantaneously heated to as much as 50,000 F (~28,000 C), a temperature that is five times the surface of the sun!  Like all gases, when air molecules are heated, they expand.  The faster they are heated, the faster their rate of expansion.  But when air is heated to 50,000 F in a fraction of a second, its expansion rate exceeds the speed of sound, and a sonic boom (thunder) results.  In short, the air literally explodes.

When lightning strikes very close by, the thunder will sound like a loud bang, crack or snap, and its duration will be very short.  As the shock wave (thunder) propagates away from the strike center, it stretches and becomes elongated.  At large distances from the center, the shock wave (thunder) is often many miles across.  As it passes through different points, a continuous rumble results that may last for several seconds, depending on your distance from the strike.

Thunder can typically be heard up to 10 miles (16 kilometers) away.  During heavy rain and wind this distance will be less, but on cool, calm, and quiet nights when a storm is many miles away, thunder can be heard beyond 10 miles.

DEFINITIONS
 

Coulomb -  The current U.S. legal standard of a COULOMB is the amount of charge transferred in 1 second by a current of 1 AMPERE; i.e., it is 1 ampere second.  It is equal in magnitude to the charge of 6.28 X 10¹⁸ electrons.