A Lightning Primer

The conditions if not the mechanics that generate lightning are now well known. In essence, this atmospheric fire is started by rubbing particles together. But there is still no agreement on which processes ignite lightning. Current hypotheses focus on the separation of electric charge and generation of an electric field within a thunderstorm. Recent studies further suggest that lightning initiation requires ice, hail, and semifrozen water droplets, called "graupel.” Storms that do not pro­duce large quantities of ice usually do not develop lightning.[116] Graupel forms when super-cooled water droplets condense around a snowflake nucleus into a sphere of rime, from 2 to 5 millimeters across. Scientific debate continues as experts grapple with the mysteries of graupel, but the stages of lightning creation in thunderstorms are clear, as outlined by the National Weather Service of the National Oceanic and Atmospheric Administration (NOAA).

First comes charge separation. Thunderstorms are turbulent, with strong updrafts and downdrafts regularly occurring close to one another. The updrafts lift water droplets from warmer lower layers to heights between 35,000 and 70,000 feet, miles above the freezing level. Simultaneously, downdrafts drag hail and ice from colder upper layers. When the opposing air currents meet, water droplets freeze, releasing heat, which keeps hail and ice surfaces slightly warmer than the sur­rounding environment, so that graupel, a "soft hail,” forms.

Electrons carry a negative charge. As newly formed graupel collides with more water droplets and ice particles, electrons are sheared off the ascending particles, charging them positively. The stripped electrons col­lect on descending bits, charging them negatively. The process results in a storm cloud with a negatively charged base and positively charged top.

Once that charge separation has been established, the second step is generation of an electrical field within the cloud and, somewhat like a mirror image, an electrical field below the storm cloud. Electrical opposites attract, and insulators inhibit current flow. The separation of positive and negative charges within a thundercloud generates an electric field between its top and base. This field strengthens with fur­ther separation of these charges into positive and negative pools. But the atmosphere acts as an insulator, inhibiting electric flow, so an enor­mous charge must build up before lightning can occur. When that high charge threshold is finally crossed, the strength of the electric field over­powers atmospheric insulation, unleashing lightning. Another electrical field develops with Earth’s surface below negatively charged storm base, where positively charged particles begin to pool on land or sea. Whither the storm goes, the positively charged field—responsible for cloud – to-ground lightning—will follow it. Because the electric field within the storm is much stronger than the shadowing positive charge pool, most lightning (about 75 to 80 percent) remains within the clouds and is thus not attracted groundward.

The third phase is the building of the initial stroke that shoots between the cloud and the ground. As a thunderstorm moves, the pool of positively charged particles traveling with it along the ground gath­ers strength. The difference in charge between the base of the clouds and ground grows, leading positively charged particles to climb up taller objects like houses, trees, and telephone poles. Eventually a "stepped leader,” a channel of negative charge, descends from the bottom of the storm toward the ground. Invisible to humans, it shoots to the ground in a series of rapid steps, each happening quicker than the blink of an eye. While this negative leader works its way toward Earth, a positive charge collects in the ground and in objects resting upon it. This accumulation of positive charge "reaches out” to the approaching negative charge with its own channel, called a "streamer.” When these channels connect, the resulting electrical transfer appears to the observer as lightning.

Finally, a return stroke of lightning flows along a charge channel about 0.39 inches wide between the ground and the cloud. After the ini­tial lightning stroke, if enough charge is left over, additional strokes will flow along the same channel, giving the bolt its flickering appearance.

Land struck by a bolt may reach more than 3,300 °F, hot enough to almost instantly melt the silica in conductive soil or sand, fusing the grains together. Within about a second, the fused grains cool into ful­gurites, or normally hollow glass tubes that can extend some distance into the ground, showing the path of the lightning and its dispersion over the surface.

The tops of trees, skyscrapers, and mountains lie closer to the base of storm clouds than does low-lying ground, so such objects are commonly struck by lightning. The less atmospheric insulation that lightning must burn through, the easier falls its strike. The tallest object beneath a storm will not necessarily suffer a hit, however, because the opposite charges may not accumulate around the highest local point or in the clouds above it. Lightning can strike an open field rather than a nearby line of trees.

Lightning leader development depends not only upon the electrical breakdown of air, which requires about 3 million volts per meter, but on prior channel carving. Ambient electric fields required for lightning leader propagation can be one or two orders of magnitude less than the electrical breakdown strength. The potential gradient inside a developed return stroke channel is on the order of hundreds of volts per meter because of intense channel ionization, resulting in a power output on the order of a megawatt per meter for a vigorous return stroke current of 100,000 amperes (100 kiloamperes, kA).