Presentation on theme: "Lightning. Facts about lightning 80% of lightning is from cloud-to-cloud. Only 20% strikes the ground. Primarily in mature cumulonimbus clouds. More frequent."— Presentation transcript:
Facts about lightning 80% of lightning is from cloud-to-cloud. Only 20% strikes the ground. Primarily in mature cumulonimbus clouds. More frequent at higher altitudes. Lightning always produces thunder-however you can’t always here it. Thunder arrives after 5 seconds for each mile.
Why is there lightning? As we all know positive and negative charges attract one another while like charges repel. In a storm cloud most of the positive charges are pushed towards the top of the cloud, while the negative charges are at the bottom. This is especially true when positive charges are sticking up from the ground. These positive charges want to jump up from the ground to the negative cloud base, but they can’t due to the fact that the air is an electrical insulator.
Stages of a Lightning Strike 1.Stepped Leader The strong attraction between the cloud and the ground produces a large electrical field. Near the cloud, the air molecules can break apart into positive and negative ions, allowing electricity to flow more freely. These regions of “ionization” finger down from the clouds in a branching structure in rapid jumps, or steps. This is called the “stepped leader,” and has the structure of an inverted tree. You can’t see the lightning yet…it’s invisible.
Stages of a Lightning Strike 1.Stepped Leader As the stepped leader gets closer to the ground, it draws the positive charge even stronger to its location. If you are the positive charge then you may feel your hair standing on end. Eventually, a positive ionization leader leaves the ground and meets the stepped leader a few hundred feet above the ground.
Stages of a Lightning Strike 2.Return Stroke When the ionization channel reaches the ground, it has “broken the dam” and charge rushes between the cloud and the ground. This is the lightning stroke you see. The electric flow heats the air to 30,000 K, and you see a bright white or blue glow. The heat expands the air. This 100-fold rapid expansion and subsequent collapse creates the shock wave called thunder. Note that the whole branched structure lights up, not just the direct path from the ground to the cloud.
Stages of a Lightning Strike 3.Dart Leader After the stroke has finished, charge within the cloud may redistribute, putting negative charge back near the old stroke. This negative charge finds the old channel, and rapidly creates a new leader, now called a “dart leader.”
Stages of a Lightning Strike 4.Multiple Return Stroke When the dart leader hits the ground, you get a new return stroke, usually less bright. This time, only the direct path lights up, since the other paths dissipated and weren’t reestablished in the dart leader. Many strokes may occur in one “strike” within the first second.