Lightning

Upward connecting discharge

As the leader channel approaches the ground strong electrical attraction develops between negative charge in the leader channel and positive charge on the surface of the ground. Several positively charged sparks develop and move upward toward the stepped leader. One of these will intercept the stepped leader and close the connection between negative charge in the cloud and positive charge on the ground.

Collisions between ________________________ produce the electrical charge needed for lightning

Collisions between PRECIPITATION PARTICLES produce the electrical charge needed for lightning

Lightning is most commonly produced by __________________

Lightning is most commonly produced by THUNDERSTORMS

Lightning hazards/safety (cars)

Stay away from tall isolated objects during a lightning storm. You can be hurt or killed just by being close to a lightning strike even if you're not struck directly. An automobile with a metal roof and body provides good protection from lightning. The lightning current will travel through the metal and around the passengers inside. The rubber tires really don't play any role at all.

The ______________________ is where precipitation forms and an electrical charge for lightning is created

THE MIDDLE OF THE CLOUD. A large part of the middle of the cloud is found at below freezing temperatures and contains a mixture of super cooled water droplets and ice crystals. This is where precipitation forms and is also where electrical charge is created

fulgurite

When lightning strikes the ground it will often melt the soil (especially sandy soil) and leave behind a rootlike structure called a fulgurite. A fulgurite is just a narrow (1/2 to 1 inch across) segment of melted sand (glass).

When temperatures are colder than -_________, graupel becomes negatively charged after colliding with a snow crystal.

When temperatures are colder than -15 C, graupel becomes negatively charged after colliding with a snow crystal. At temperatures warmer than -15 (but still below freezing), the polarities are reversed

Cloud-to-ground lightning

bout 1/3 rd of all lightning flashes strike the ground. These are called cloud-to-ground discharges (actually negative cloud-to- ground lightning).

Multi-stroke lightning (subsequent stroke)

A downward dart leader travels from the cloud to the ground. The dart leader doesn't step but travels smoothly and follows the channel created by the stepped leader (avoiding the branches). It is followed by a slightly less powerful subsequent return stroke that travels back up the channel to the cloud. This second stroke might be followed by a third, a fourth, and so on.

The return stroke

A powerful current travels back up the channel from the ground toward the cloud. This is the return stroke. Large currents (typically 30,000 amps in the first return stroke) heat the air to around 30,000K (5 times hotter than the surface of the sun) which causes the air to explode. When you hear thunder, you are hearing the sound produced by this explosion.

Lightning formation

Collisions between precipitation particles produce the electrical charge needed for lightning. When temperatures are colder than -15 C, graupel becomes negatively charged after colliding with a snow crystal. The snow crystal is positively charged and is carried up toward the top of the cloud by the updraft winds. At temperatures warmer than -15 (but still below freezing), the polarities are reversed. Large positive and negative charge centers begin to build up inside the cloud. When the electrical attractive forces between these charge centers gets high enough lightning occurs.

Function of lightning rods

Houses with and without lightning rods are shown above. When lightning strikes the house without a lightning rod the powerful return stroke travels into the house destroying the TV nd possibly starting the house on fire. A lightning rod is supposed to intercept the stepped leader and safely carry the lightning current around the house and into the ground. A lightning rod doesn't really attract lightning. The lightning doesn't know where it is going to strike until it gets to within 100 meters or so of the ground. The connection between the stepped leader and the upward discharge creates a "short circuit" between the charge in the cloud and the charge in the ground.

Upward "triggered" lightning

Lightning sometimes starts at the ground and travels upward. Upward lightning is generally only initiated by mountains and tall objects such as a skyscraper or a tower of some kind. Note the discharge is different in another way also. These discharges are initiated by an upward leader. This is followed by not by a return stroke but by a more normal downward leader. Once the 2nd leader reaches the ground, an upward return stroke travels back up the channel to the cloud.

stepped leader

Most cloud to ground discharges begin with a negatively-charged downward-moving stepped leader. A developoing channel makes its way down toward the cloud in 50 m jumps that occur every 50 millionths of a second or so. Every jump produces a short flash of light (think of a strobe light dropped from an airplane that flashes periodically as it falls toward the ground). The sketch below shows what you'd see if you were able to photograph the stepped leader on moving film. Every 50 microseconds or so you'd get a new picture of a slightly longer channel displaced slightly on the film. The bottom segment, highlighted in yellow, produces a bright flash of light (the remainder of the channel is often weakly illuminated).

Intracloud lightning

Most lightning (2/3rds) stays inside the cloud and travels between the main positive charge center near the top of the cloud and a large layer of negative charge in the middle of the cloud; this is intracloud lightning. A

Positive lightning

Occasionally a lightning stroke will travel from the positive charge region in the top of the thunderstorm cloud to ground. These types of strikes are more common at the ends of storms and in winter storms. This is probably because the top part of the cloud gets pushed sideways away from the middle and bottom portions of the cloud. Positive strokes are very powerful. They sometimes produce an unusually loud and long lasting clap of thunder.

The 30/30 rule

People should seek shelter if the delay between a lightning flash and its thunder is 30 seconds or less. People should remain under cover until 30 minutes after the final clap of thunder.

Rocket triggered lightning

The fact that lightning could begin with an upward discharge that begins at the ground lead (French) scientists to develop a technique to trigger lightning by firing a small rocket up toward a thunderstorm. The rocket is connected by a thin wire to the ground. When the rocket gets 50 to 100 m above the ground upward lightning will develop off of the top of the wire. Scientists are able to take closeup photographs and make measurements of lightning currents using triggered lightning. Triggered lightning can also be used to test the operation of lightning protection devices.

Determining the distance to a lightning strike

To estimate the distance to a lightning strike count the number of seconds between the flash of light and when you first hear the thunder. Divide this by 5 to get the distance in miles.