Tue., Nov. 30 notes

Most of the class period today was devoted to lightning. Here's a little introductory information that wasn't covered in class.

Lightning kills just under 100 people every year in the United States (more than tornadoes or hurricanes but less than flooding, summer heat and winter cold) and is the cause of about 30% of all power outages. In the western United States, lightning starts about half of all forest fires.

Lighning is most commonly produced by thunderstorms (it has also be observed in dust storms and volcanic eruptions such as the one that occurred earlier this year in Iceland). Lightning caused fires are a particular problem at the beginning of the thunderstorm season in Arizona. At this time the air underneath thunderstorms is still relatively dry. Rain falling from a thunderstorm will often evaporate before reaching the ground. Lightning then strikes dry ground, starts a fire, and there isn't any rain to put out or at least slow the spread of the fire. This is so called dry lightning.

A typical summer thunderstorm in Tucson is shown in the figure above. Even on the hottest day in Tucson in the summer 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. Doesn't it seem a little unusual that electricity can be created in such a cold and wet environment?

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 temperature 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 attrative forces between these charge centers gets high enough lightning occurs. Most lightning (2/3 rds) 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 rd of all lightning flashes strike the ground. These are called cloud-to-ground discharges (actually negative cloud-to- ground lightning). We spent most of the rest of the class learning about this particular type of lightning.

A couple of interesting things that can happen at the ground when the electrical forces get high enough. Attraction between positive charge in the ground and the layer of negative charge in the cloud can become strong enough that a person's hair will literally stand on end (see photo below). This is incidentally a dangerous situation to be in as lightning might be about to strike. St. Elmo's fire is a faint electrical discharge that sometimes develops at the tops of elevated objects during thundestorms. It was first observed coming from the tall masts of sailing ships at sea (St. Elmo is the patron saint of sailors).

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.

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.

Lightning rods take advantage of this principle.

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 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 and into 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.

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.

The figure below shows what we've learned so far in simplified form

Many cloud-to-ground flashes end at this point. In about 50% of cloud to ground discharges, the stepped leader-upward discharge-return stroke sequence repeats itself (multiple times) 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. This second stroke might be followed by a third, a fourth, and so on.

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.

Here's a stepped leader-upward connecting discharge-return stroke animation.
We also watched a fast time resolved video of an actual stepped leader that is on YouTube

Here are some unusual types of 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.

Here's an even rarer form of 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.

Here's a slow motion video showing upward 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. A short video showing rocket triggered lightning experiments being conducted in northern Florida was shown in class.

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). The video showed archaeology students excavating around the lightning triggering site after the summer's experiments. They were able to uncover and reveal a very long (perhaps world record length) fulgurite.

Lightning is a serious weather hazard and kills just under 100 people every year in the United States. We discussed some lightning safety rules that you should keep in mind during thundery weather.

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 people in Florida that were triggering lightning with rockets (shown on a video last week) were inside a metal trailer and were perfectly safe. All of the connections made to equipment outside the trailer were done using fiber optics, there were no metal wires entering or leaving the trailer.

You shouldn't use a corded phone or electrical appliances during a lightning storm because lightning currents can follow wires into your home. Cordless phones and cell phones are safe. It is also a good idea to stay away from plumbing as much as possible (don't take a shower during a lightning storm, for example). Vent pipes that are connected to the plumbing go up to the roof of the house which puts them in a perfect location to be struck.

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. For example, a delay of 15 seconds between the flash of light and the sound of thunder would mean the discharge was 3 miles away.

The latest lightning safety recommendation is the 30/30 Rule.

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.

It is very hard to actually measure the speed of the rotating winds in a tornado. Researchers usually survey the damage caused by the tornado to come up with a Fujita Scale rating. Here are some representative photographs of tornado damage.

Roof damage is typical of an F1 tornado. The buildings on the left suffered light roof damage. The barn roof at right was more heavily damaged. Barns present a larger crossection to the wind and often aren't built as sturdily as a house.

More severe damage to what appears to be a well built house roof.

F1 tornado winds can tip over a mobile home if it is not tied down (the caption states that an F1 tornado could blow a moving car off a highway). F2 level winds (bottom photo above) can roll and completely destroy a mobile home.

Trees, if not uprooted, can suffer serious damage from F1 or F2 tornado winds.

F2 level winds have completely removed the roof from this building. The outside walls of the building are still standing.

The roof is gone and the outer walls of this house were knocked down.
This is characteristic of F3 level damage. In a house without a basement or storm cellar it would be best to seek shelter in an interior closet or bathroom (plumbing might help somewhat to keep the walls intact).

In some tornado prone areas, people construct a small closet or room inside their home made of reinforced concrete.
A better solution might be to have a storm cellar located underground.

All of the walls were knocked down in the top photo but the debris is left nearby. This is characteristic of F4 level damage. All of the sheet metal in the car body has been removed in the bottom photo and the car chasis has been bent around a tree. The tree has been stripped of all but the largest branches.

An F5 tornado completely destroyed the home in the photo above and removed most of the debris. Only bricks and a few pieces of lumber are left.

Several levels of damage (F1-F3) are visible in the photograph above. It was puzzling initially how some homes could be nearly destroyed while a home nearby or in between was left with only light damage. One possible explanation is shown below (from the bottom of p. 164 in the photocopied ClassNotes.

Some big strong tornadoes may have smaller more intense "suction vortices" that spin around the center of the tornado. Tornado researchers have actually seen the pattern shown at right scratched into the ground by the multiple vortices in a strong tornado.

The sketch above shows a tornado located SW of a neighborhood. As the tornado sweeps through the neighborhood, the suction vortex will rotate around the core of the tornado.

The homes marked in red would be damaged severely. The others would receive less damage. Remember that there are multiple suction vortices in the tornado, but the tornado diameter is probably larger than shown here.

At this point we watched the last of the tornado video tapes. It showed a tornado that occurred in Pampa, Texas (here are a couple of videos that I found on YouTube: video 1, video 2, they're missing the commentary that was on the video shown in class). Near the end of the segment, video photography showed several vehicles (pick up trucks and a van) that had been lifted 100 feet or so off the ground that were being thrown around at 80 or 90 MPH by the tornado winds. Winds speeds of about 250 MPH were estimated from the video photography.