3. Explain the phenomena Lightning.
Answers
Answer:
From the base of a cloud, usually negatively charged, a pre-discharge known as a stepped leader escapes. This stepped leader is barely luminous and its progression towards the ground is by leaps of several tens of meters; in reality, it is a succession of discharges travelling along the same ionized path at intervals of 40 to 100 µs (average progression speed of the order of 0.5 to 1 m/µs).
As it approaches the ground, the extremity of the highly charged stepped leader causes a significant increase of the average electric field strength along its length. This electric field can reach values of 400 to 500 kV/m. When the atmospheric air ionization threshold is reached (30 kV/cm) at high points, which are the preferred impact points for lightning (tree tops, chimneys, lightning rods, etc.), corona discharges occur. Locally, for higher electric field values, these corona discharges are transformed into positive rising discharges called “upward leaders”. The corona discharges with the best initiation characteristics or those progressing the most rapidly will join the downward stepped leader. The electrical connection between the thunder cloud and the ground is established by an ionized channel. Charge is then returned from the ground to the cloud (return stroke), which has a tendency to neutralize its own charge.
Several restrikes may be exchanged in the space of 0.2 to 1 second. Their progression is continuous and their propagation speed very high.
Mechanism of a lightning stroke
It is impossible to discern each particular phase of a lightning stroke by simple visual observation. This can only be done with high-speed cameras. Most lightning bolts exhibit the following phenomena: a leader leaves a point in the cloud and travels about 50 m at a very high speed of around 50 000 km/s. A second leader then leaves the same point, follows the previous path at comparable speed, goes beyond the final point of the first leader by an approximately identical distance, then disappears in turn.
The process is repeated until the tip of the last leader reaches a point a few dozen metres, or even just a few metres above ground level.
The ascending jets then converge, producing a return stroke from the ground towards the cloud (the upward streamer) during which the electric current circulates: The convergence of these two phenomena produces the main discharge, which may be followed by a series of secondary discharges, passing unbroken along the channel ionised by the main discharge.
In an average negative lightning stroke, the maximum current is around 35 000 A.
Lightning phenomenon
Storms or Thunder cloud
The presence of unstable, moist and warm air masses gives rise to the formation of cumulonimbus storm clouds. This type of cloud is very extensive, both horizontally (about 10 km in diameter) and vertically (up to 15 km). Its highly characteristic shape is often compared with the profile of an anvil of which it displays the upper and lower horizontal planes. The existence of extreme temperature gradients in a cumulonimbus (the temperature can drop to -65 °C at the top) generates very rapid ascending air currents, and results in the electrical energization of the water particles.
In a typical storm cloud, the upper part, consisting of ice crystals, is normally positively charged, whilst the lower part, consisting of water droplets, is negatively charged. Consequently, the lower part of the cloud causes the development of electrically opposite charges (i.e. positive over the part of the ground nearby).
Thus the cumulonimbus formation constitutes a sort of huge plate /ground capacitor whose median distance can often reach 1 to 2 km. The atmospheric electrical field on the ground, about 600 V/m in fine weather is reversed and can reach an absolute value of 15 to 20 kV/m when a ground discharge is imminent (the lightning stroke). Before and during the appearance of the lightning stroke, discharges can be seen both within the cloud and between clouds.
Explanation:
Explanation:
Lightning is a naturally occurring electrostatic discharge during which two electrically charged regions in the atmosphere or ground temporarily equalize themselves, causing the instantaneous release of as much as one gigajoule of energy.
