All forms of Lightning are produced when electrical charge passes between positively and negatively charged areas. In forked lightning, an initial stroke (known as a leader) travels to the ground at a speed of around 100km per second (62 miles per second), creating a path of electrically charged air. A return stroke that travels immediately back along the path is what we see. Lightning also flashes between points within the cloud itself and between the cloud and the air, lighting up the sky. If the flash of lightning is hidden by cloud cover, it appears to make the cloud glow. This is called sheet lightning.
Any lightning that you can see is a potential danger. Lightning is a life-threatening event and should never be taken lightly.
Forked lightning is quite visible as in the image above. Lightning comes out of a cloud and sometimes it forks, sometimes not, and strikes an object on the ground (cloud to ground) or another cloud (cloud to cloud) or even another part of the same cloud (in-cloud.)
Sheet lightning can light up a entire cloud in a brief but spectacular display. It’s almost as cool looking as a long-lasting multi-forked cloud to ground strike.
Far away sheet lightning seems to produce no sound and is sometimes called heat lightning. If you were closer, you would hear the thunder.
They are all dangerous. Sheet lightning is a bolt that you can’t see because it is hidden by the cloud that it lights up. Most often, it is cloud to cloud or in-cloud (from one part of a cloud to another part of the same cloud.)
Lightning can strike “out of the blue” on a clear day. Sometimes called “clear air lightning,” it originates within a cloud that is near or beyond the horizon—just a few miles on relatively flat ground. It starts as a cloud to cloud or in-cloud bolt, then exits the cloud and follows a horizontal path for some distance until it turns toward the ground. And that is why sheet lightning is just as dangerous. You can’t predict where it will go.
In a thunderstorm, the strong upward draft of air in the center of the cell carries ice crystals and super-cooled (-10 F) water droplets up toward the top of the cell. Small hail, which is heavier, is suspended or falls slowly in the moving air. As the water and ice crystals travel up though the hail, the hail gathers positive charges from the water and crystals which leaves them negatively charged. (explaining it simplistically here)
This leaves the mid and lower sections of the cell negatively charged and the top, or anvil, positively charged. As water condenses at the very bottom of the cloud, it too becomes positively charged in warmer air and falling rain.
As the thunderstorm moves, the electrical charges in the cloud induce a smaller, opposite charge on the earth. As the cloud comes closer, the charge on the earth increases.
We all know that in electricity and magnetism, opposites attract. A plasma channel of ionized air opens between the negative charges in the cloud and the positive charges on earth. The charges in the cloud follow this “leader” and reach for the earth in steps. Likewise, the charges on the ground also reach toward the cloud. The result is a bolt of lightning.
Negative charges in one cloud can form leaders to the positive charges in another cloud, or even within the same cloud. A strong cell can produce a lightning strike every three seconds.
Picture Credit : Google