Lightning.. ................

Lightning is a flash or several flashes of very bright light in the sky caused by electricity. In other words lightning is an electric discharge of electricity when some part of atmosphere gets enough electrical charge to overcome air resistance. Lightning can be seen in the form of electric spark. Sometimes during thunderstorm we can see flashes and spark of lightning within clouds. It is usually associated with thunder clouds but may occur in snowstorms and dust storms too and sometime in the dust and gases emitted during volcanic eruption. Generally science describes that a lightning is actually a potential difference between cloud and ground that may be sometime hundred million volts. This causes high voltage difference in the clouds and lightning occurs. The temperature in the lightning is of order 30,000 K or 50,000 0F. During the lightning of cloud to ground, it releases two strokes- a pale leader stroke that strikes the ground and a highly luminous return stroke. The leader stroke reaches the ground in 20 milliseconds and return stroke reaches cloud in about 70 microseconds. The spark or flash we see actually is a rapid heating of air by thunder along the length of lightning channel of two strokes while striking ground and returning back to clouds. The heated air thus expands at supersonic speeds. The shock waves decay within a metre or two into sound wave which is thus modified by the intervening air and topography and produces a series of rumbles and claps.
Generally lightning occurs during thunder storms because of vertical air motion and interaction between clods because’s positive and negative charges and produces potential differences. According to the direction of propagation and polarity of the initial leader, lightning between cloud and ground are classified. Let us take an example- during cloud to ground lightning a negative discharge is initiated by downward propagating leader. In such case, the total discharge will effectively lower negative charge to ground and positive charge will get deposited in the cloud. A discharge can also be initiated by downward propagating positive leader. Positive discharge occurs frequently than negative ones.
The other type of lightning is ground to cloud discharge that begins with positive leader propagating upward. It rarely occurs and is usually initiated by high mountains and trees. The rarest lightning is discharge that begins with negative leader propagating upward. During direct strikes, the electric currents that flow in return strokes can be measured in instrumented towers. The peak current in a negative first stroke is typically 30 kilo amperes, with a zero-to-peak rise time of just a few microseconds. This current decreases to half-peak value in about 50 microseconds, and then low-level currents of hundreds of amperes may flow for a few to hundreds of milliseconds. The long continuing currents cause fires and produce charge transfer in the order of ten of coulombs. The peak currents of subsequent return strokes are of 10-15 kA. Five percent of the negative discharges to ground generate peak currents that exceed 80 kA, and 5% of the positive discharges exceed 250 kA. Positive flashes frequently produce very large charge transfers, with 50% exceeding 80 coulombs and 5% exceeding 350 coulombs. Red sprites, elves, and blue jets are upper atmospheric optical phenomena associated with thunderstorms and have only recently been documented using low-light-level television technology. Sprites are massive but weak luminous flashes appearing directly above active thunderstorms coincident with cloud-to-ground lightning. They extend from the cloud tops to about 95 km (59 mi) and are predominantly red. High-speed photometer measurements show that the duration of sprites is only a few milliseconds. Their brightness is comparable to a moderately bright auroral arc. Elves are associated with sprites. They are optical emissions of approximately 1milliseconds, with a fast lateral, horizontal expansion that emits more red than blue light. They occur at altitudes of 75–95 km (47–59 mi). Blue jets are optical ejections from the top of the electrically active core regions of thunderstorms. Following the emergence from the top of the thunder cloud, they typically propagate upward in narrow cones of about 15° full width at vertical speeds of roughly 100 km/s (60 mi/s), fanning out and disappearing at heights of about 40–50 km (25–30 mi). All things we discussed above are some facts about the lightning. It seems just a flash but the mechanism and causes cannot be written and described shortly….