Question No.  19485 :
How are lightning and thunder generated? How does lightning travel through air?

Everything around us is made up of atoms. Although atoms are normally electrically neutral, they may become positive or negative if they lose or gain electrons. Positive and negative charges are attracted to one another. When they move through air toward one another, they form an electric current that causes a spark. Lightning is a form of visible electric discharge between rain clouds or between a rain cloud and the Earth. The discharge is seen as a brilliant arc. Most rain clouds are negatively charged at the base and positively charged at the top. Scientists are uncertain exactly how rain clouds become electrically charged. When the electrical potential between two clouds or between a cloud and the Earth reaches a sufficiently high value, the air becomes ionised along a narrow path and lightning flash results. Lightning that strikes the Earth consists of one or more electrical discharges called strokes. The bright light that we see in a flash of lightning is called a return stroke. Return strokes travel at about the speed of light, which is 299,792 kilometres per second. A typical lightning flash involves a potential difference between cloud and ground of several hundred million volts, with peak currents on the order of 20,000 amperes. Temperatures in the channel are on the order of 30,000 K. Air is heated instantly when an electrical charge of lightning passes through it. The heat causes the molecules of air to expand, or fly out, in all directions. As the molecules seek more room, they collide violently with layers of cool air, and set up a great air wave that has the sound of thunder. The sound of thunder reaches us after we have seen the lightning. That is because light travels at 299,792 kilometres per second, while sound travels at a rate of only about 335 metres per second. The number of seconds between seeing the lightning flash and hearing the thunder, divided by three, yields the distance of the lightning from the observer in kilometres. The flash of cloud-to-ground lightning is initiated by the neutralization of the small net-positive charge in the lowest region of the cloud. A cloud-to-ground flash comprises at least two strokes: a leader stroke and a return stroke. A leader stroke carrying a negative charge passes from cloud to ground. Occasionally, however, the leader stroke is from ground to cloud - especially with very high structures such as church steeples, multistory buildings, or tall trees.) The leader stroke is not very bright and is often stepped and has many branches extending out from the main channel. As it nears the ground, it induces an opposite charge, concentrated at the point to be struck, and a return stroke carrying a positive charge from ground to cloud is generated through the channel. The two strokes generally meet about 50 m above the ground. At this juncture, the cloud is short-circuited to the ground and a highly luminous return stroke of high current passes through the channel to the cloud. The entire process is very rapid; the leader stroke reaches the juncture point or the ground in about 20 milliseconds, and the return stroke reaches the cloud in about 70 microseconds. Question Asked By:

Name: STUDENT

Age Group: 6 to 12

Occupation Type: Student

Education Level: Primary