Tue., Apr. 24 notes

Tue., Apr. 24, 2007

The Experiment #4 report revisions and the Optional Assignment were collected today. A handout with answers to the Optional Assignment was distributed in class.

Quiz #4 is Thursday this week. Reviews are scheduled for Tue. and Wed. afternoons. A copy of the Quiz #4 Study Guide will be handed out in class. The Quiz #4 Study Guide is also available online.

We have some tornado material to finish up and then will learn a little about lightning.

A sketch showing some of the characteristic features of supercell thunderstorms. Supercells are first of all much larger than ordinary air mass thunderstorms. In an ordinary thunderstorm the updraft is unable to penetrate into the very stable air in the stratosphere. The upward moving air just flattens out and forms an anvil. In a supercell the rotating updraft (shown in orange above) is strong enough to penetrate into the stratosphere a little ways. This produces the overshooting top or dome feature above (yellow). A wall cloud and a tornado are shown at the bottom of the mesocyclone. The flanking line is a line of new cells trying to form alongside the supercell thunderstorm.

A little more detailed sketch is shown below

A photograph of a distant supercell thunderstorm was shown in the next video tape. A computer simulation of the air motions inside a supercell thunderstorm was also shown. Researchers understand the development of a supercell pretty well. The exact process that initiates tornado development is still unknown.

A radar picture of a supercell thunderstorm will often have a characteristic hook shape (outlined in black above). The hook is caused by spinning motions inside the thunderstorm The large orange shaded area is the thunderstorm updraft, the mesoscylone. Smaller regions of rising air are shown along a gust front.

Blue shaded areas shown where precipitation falls out of the cloud. The flanking line of new cells is forming along the gust front produced when cold downdraft air from the thunderstorm (purple arrows) collides with prexisting winds (green arrows). Weak tornadoes can sometimes form along the gust front. The largest and strongest tornadoes come from the mesocylone and wall cloud.

Before viewing the last (and perhaps the best) tornado video it would be worthwhile to learn more about the Fujita Scale that is used to rate tornado strength or severity. The Fujita Scale runs from F0 (weakest) to F5 (strongest, though there are a very few tornadoes with winds over 300 MPH that have been given an F6 rating).

The scale below is an "easy to remember" version (you can compare this with the scale in Table 10.2 (p. 281) in the textbook). About 2/3 rds of all tornadoes are F0 or F1 tornadoes and have spinning winds of 100 MPH or less. Only a few percent of tornadoes develop into F4 or F5 tornadoes, but those account for about 2/3 of all tornado deaths.

Simplified, Easy-to-Remember version of the Fujita Scale
winds < 100 MPH

F0
F1	roof damage, mobile home tipped over

microburst winds can cause this degree of damage

winds 100 to 200 MPH

F2	roof gone, outside walls still standing
F3	outside walls gone, inside walls intact

winds 200 to 300 MPH

F4	home destroyed, debris nearby
F5	home destroyed, debris carried away

Some representative examples of F1 through F5 level damage were shown in class

The buildings on the left suffered light roof damage. The barn roof at right was more heavily damaged.

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) can roll and destroy the 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 building walls 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.

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. Note 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 are visible in this photograph. 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

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 scouring pattern shown at right in the figure above that the multiple vortices can leave behind.

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, however that there would probably be multiple suction vortices in the tornado).

The following material wasn't coveredin class.
Air motions inside tornadoes are complex and difficult (dangerous) to study. Researchers resort to laboratory simulations and computer models.

Winds thought to be found in a weak tornado.
Friction probably causes the closed circulation near the bottom center of the tornado.

Winds in a somewhat stronger tornado.

Downward moving air is found in the core of this tornado.
This tornado would also have a larger diameter than the weaker tornadoes above.

Vortex breakdown has reached the ground.
This may lead to the formation of multiple vortices.

Lighning is usually produced by thunderstorms.

A typical 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 electrical 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.

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 (a dangerous situation to be in). St. Elmo's fire is a faint electrical discharge that sometimes develops at the tops of elevated objects during thundestorms.

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. An upward discharge is initiated when the stepped leader nears the ground. A powerful return stroke travels back up the channel (and out into all the branches) once the upward discharge and the stepped leader meet. These three steps are shown in additional detail below.

A sequence of stepped leader jumps.

Several positively charged upward discharges 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 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.

Stepped leader - upward connecting discharge - return stroke animation

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 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.

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.

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 (some of the comments on the figure above apply to the video)

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.

The correct spelling is fulgurite.

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). The rubber tires really don't play any role at all.

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

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.

The latest lightning safety recommendation is the 30/30 Rule. You 'll see that explained below.

The 30/30 rule
30/30 Rule graphic

Any lightning safety plan should incorporate the 30/30 Rule. The 30/30 Rule states that people should seek shelter if the "Flash-To-Bang" delay (length of time in seconds between a lightning flash and its subsequent thunder), is 30 seconds or less, and that they remain under cover until 30 minutes after the final clap of thunder.

A 30 second lead time is necessary prior to a storm's arrival because of the possibility of distant strikes. A 30 minute wait after the last thunder is heard is necessary because the trailing storm clouds still carry a lingering charge. This charge can and does occasionally produce lightning on the back edge of a storm, several minutes after the rain has ended.

Studies have shown most people struck by lightning are struck not at the height of a thunderstorm, but before and after the storm has peaked. This shows many people are unaware of how far lightning can strike from its parent thunderstorm. DO NOT wait for the rain to start before seeking shelter, and do not leave shelter just because the rain has ended.
(from Jetstream An Online School for Weather )