Mon., Nov. 27 notes

Monday Nov. 27, 2006

In order to save some time after a busy Thanksgiving break, the lightning notes from the Section 3 class have been moved over to the Section 2 web page.

We will spend the whole period on lightning today.

A summer thunderstorm in Tucson. Remember that even in the summer a large part of the middle 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 the ice crystal process of precipitation formation operatures and is also where electrical charge is created.

Collisions between precipitation particles produces the charge needed for lightning. When temperatures are below -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 temperature warmer than -15 (but still below freezing), the charging is reversed. Large positive and negative charge centers begin to build up inside the cloud. When the electrical attrative forces between these charge centers gets high enough lightning occurs. Most lightning (2/3) 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. About 1/3 of all lightning flashes strike the ground. These are called cloud-to-ground discharges.

Most cloud to ground discharges begin with a negatively charged downward moving stepped leader. It 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. Another view of the stepped leader is shown below.

Imagine dropping a strobe light from the cloud and watching it as it fell from the cloud toward the ground. You'd periodically see a flash of light as the strobe flashed on and off.

The picture above shows you what you would see on a photograph made on moving film. Every time the channel jumps and produces a flash of light the channel is photographed on a slighly different position on the film. With a telephoto lens you'd see the short 50 m channel segments that are illluminated during the jump.

As the stepped leader nears the ground, the electrical attraction between negative charge in the tip of the stepped leader and positive charge in the ground causes several positively charged upward discharge to develop. One of these will intercept and connect with the stepped leader.

Several positively charged sparks begin to travel upward from the ground. One of these will eventually intercept the stepped leader.
This is what determines what will be struck by the lightning. Lightning doesn't really know what it will strike until it gets close to the ground. Lightning rods take advantage of this principle.

Houses with and without lightning rods are shown below. When lightning strikes the house without a lightning rod the powerful return stroke travels into the house destroying the TV and 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.

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. 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.

Stepped leader - upward connecting discharge - return stroke animation

In about 50% of cloud to ground discharges, the stepped leader-upward discharge-return stroke sequence repeats itself with a few subtle differences. 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.

A normal still photograph would capture the separate return strokes superimposed on each other. If you bumped or moved the camera during the photograph the separate return strokes would be spread out on the image.

The image above shows a multiple stroke flash consisting of 4 separate return strokes.
There is enough time between separate return strokes (around 1/10 th second) that your eye can separate the individual flashes of light.
When lightning appears to flicker you are seeing the separate return strokes in a multiple stroke flash. The whole flash usually lasts 0.5 to 1 second.

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.

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. These discharges are initiated by an upward leader. This is followed by a more normal downward leader and an upward return stroke.

Scientists are able 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. A short video showing rocket triggered lightning experiments was shown in class.

Near the end of the tape you will some cases where the lightning didn't follow the wire all the way to the ground (this is one reason why you need to be very careful doing experiments of this type). When the lightning strikes the sandy soil (instead of striking instruments on the ground) it sometimes will leave behind a fulgurite (note spelling in the figure below is incorrect)

This is a drawing of a science fair project. If 10 to 20 Amps will cook a hotdog,
imagine what the 10,000 to 30,000 Amps in a lightning return stroke can do. We wrapped up class with some lightning safety information.

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 people in Florida that were triggering lightning were inside a metal trailer and were perfectly safe).

You shouldn't use a corded phone and electrical appliances during a lightning storm because lightning currents can follow wires into your home.

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.

If there is less than 30 seconds delay between the light and the thunder then the lightning is close enough to present a risk to you. You should wait 30 minutes after the last lightning before assuming that a storm has dissipated or moved out of your area.