“What makes a lightning strike from ground up? Jock McTavish retired avionics tech, lifelong experimenter
Let’s clear up the words up and down, and the direction of electricity. Electricity is the flow of electrons. Electrons have a negative charge. They move towards a positive charge, which can be relative or just less negative. But lightning is the second phase of discharge. Its direction is clear, from cloud to ground, as the electrons seek the earth. But there is a first phase of lightning, before the major discharge, only discovered in the last century with modern high speed cameras. Lightning strikes in quite a few directions. And it is a bit complicated. It gathers charge and it depends upon random field ionizing to pick a path of discharge. It mostly balances charge internal to the cloud. It can strike the ground, and it strikes into the troposphere. Lightning stems from accumulating charges that reach millions of volts as the violent wind in storm clouds literally rubs electrons free from colliding water drops. So regions of the cloud, according to the directions of these wind currents, become charged. In general the lower regions of the storm cloud take up a negative charge, and the top of the cloud correspondingly becomes positive. But the ground situation is different. As the electrons accumulate in the lower cloud regions below the updraft areas, the cloud and ground below become a very large capacitor. The negative millions of volts above drives away electrons in the conducting ground below, creating a positive charge. People report feeling the hair rising on their necks due to the rising electric field when they are beneath such a cloud. And sometimes lightning rises from the ground. For air is an insulator and only carries electricity when it becomes ionized or glowing. It takes 10–15,000 volts per cm before this glowing sparking begins with air. Like the small sparks and shocks from removing a wool sweater. But a cloud even charged with millions of volts, can’t ionize the air sufficiently to discharge over the great distances involved. Fields of only half the potential to ionize are found typical of storm clouds. This has long been a riddle. That actual storm field measurements aren’t sufficient for ionization of the air into lightning, yet lightning is surely there. Current research and high speed cameras have helped us further understand lightning. Two wondrous things happen to make possible a lightning strike. “Leaders” and Cosmic Rays. Extremely high speed cameras and geiger tubes have found a relationship between lightning formation and cosmic rays. As cosmic rays constantly shower our earth, they create small paths of ionized air particles, which in the regions of a storm cloud’s extremely high voltage electric fields, triggers the formation of plasma, or ionized atmospheric gas, somewhat like the insides of a glowing neon sign, and triggers the strong electric fields into ionization. Once started, these small ribbons of zig zag ionized and conducting glowing air, begin to reach towards their opposite charge. They are called “leaders”. With extremely fast cameras, we can now photograph these leaders reaching erratically from the cloud towards the ground, and much more weakly, reaching from the ground towards the cloud. And when a leader hits ground, or a pair of leaders touch each other, a path of conduction, sky to earth is made and the capacitor that is the cloud discharges with a lightning bolt of millions of amperes. So does the lightning strike down, or up? Well both actually. This describes the common storm cloud, but positively charged storm clouds have been documented. And lightning is observed discharging upwards from the charged regions of the cloud in the troposphere. These discharges into space are a new subject of study, and are called sprites.”
