Thursday Apr. 17

The Quiz #4 Study Guide is now available in very preliminary form.

Here are some of the pictures that people drew on the back side of the Optional Assignment handed out in the MWF class on Monday  together with some comments.  I hope you enjoy them, I did.  And here is a sample of some of the pictures from the T Th class (I didn't include all of them, there were too many)
The sprint finish at the end of the final stage of the 2007 Tour de France was shown before the start of class.  I don't know for sure how fast the racers are riding at the finish, 40 MPH maybe more.  It takes a lot of effort to ride your bicycle that fast.  If you stopped pedaling you would quickly slow down.  That is because the force of friction is always acting against you.

At speeds of 40 MPH friction is due primarily to wind resistance (imagine what you would feel standing outside in a 40 MPH wind).
An ordinary air mass thunderstorm in its mature stage is shown below.

The downdraft will spread throughout the interior of the cloud and eventually interfere with the updraft.  That will cause the storm to weaken and dissipate.  Next we will see how one small change in atmospheric conditions can create a storm that will last longer, and grow bigger and stronger.



The winds are increasing in speed with increasing altitude in the figure above.  This is vertical wind shear (changing wind direction with altitude is also wind shear).

The thunderstorm will move to the right more rapidly than the air in the thunderstorm updraft which originates at the ground.  Rising air that is situated at the front bottom edge of the thunderstorm will find itself at the back edge of the storm when it reaches the top of the cloud.  This produces a tilted updraft. 

Remember that an ordinary air mass thunderstorm will begin to dissipate when the downdraft grows horizontally and cuts off the updraft.  In a severe storm the updraft is continually moving to the right and staying out of the downdraft's way.  Severe thunderstorms can get bigger, stronger, and last longer than ordinary air mass thunderstorms.  The strong updraft winds can keep hailstones in the cloud longer which will allow them to grow larger.

We will find that sometimes the tilted updraft will begin to rotate.  A rotating updraft is called a mesocyclone.  Low pressure in the core of the mesocyclone creates an inward pointing pressure difference (pressure gradient) force that keeps the updraft winds spinning in circular path (just as low pressure keeps winds spinning in a tornado).  The cloud that extends below the cloud base and surrounds the mesocyclone is called a wall cloud.  The largest and strongest tornadoes will generally come from the wall cloud.



Sketches showing some of the characteristic features of supercell thunderstorms.  Supercells are first of all much larger than ordinary air mass thunderstorms (a purple air mass T-storm is superimposed on the top figure for comparison).  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. This produces the overshooting top or dome feature above.  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 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, however.

A radar picture of a supercell thunderstorm will often have a characteristic hook shape (outlined in brown 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 show 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.  The two tornado formation regions are shaded yellow in the figure.



Actual radar display with 4 thunderstorms with hook echoes.  The hook echo feature is not always easy to spot.  The "Xenia cell" produced a large tornado.

The last video featured a tornado observed in Pampa, Texas and was shown at this point.  At one point the tornado winds just above the ground were estimated at 250 MPH.  Several vehicles (pickups and a van) were seen on the video being thrown from the tornado cloud at a height of about 100 feet at speeds of 80 to 90 MPH.  Imagine something like that coming down in your backyard.
The Fujita Scale is used to rate tornado strength or severity.  We spent the next part of the class looking at photographs of tornado damage.

 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

Here are some photographs of tornado damage


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.


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.  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 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


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).

At this point we watched the last of the tornado video tapes.  It showed a tornado that occurred in Pampa, Texas.  Near the end of the segment, video photography showed several vehicles (pick up trucks and a van) 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.
At this point we moved into the section on lightning, the next topic in class.  Lightning kills around 100 people every year in the United States.  The next page (now about 10 years out of date) lists some of the economic costs of lightning.


Lightning is the cost of about 30% of all power outages (yesterday's power failure was a reminder of how inconvenient that can be).  In the western United States, lightning starts about half of all forest fires.  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.

Lighning is most commonly produced by thunderstorms (it has also be observed in dust storms and volcanic eruptions).


A typical summer thunderstorm in Tucson (found on p. 165 in the photocopied Classnotes).  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.  Doesn't that seem a little unusual that electricity can come from such a cold and wet environment?

Here's a story (written during the Spring 2006 semester when the course instructor thought he might be on the verge of a nervous breakdown - something he is a little worried about this year) about how a seemingly unrelated series of minor events could lead to a potentially deadly result.
Jack and the Lightning Strike

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 steps.  Note each of the channels in the drawing should actualy be superimposed on each other.  There is just a single channel that every 50 microseconds of so gets 50 meters longer.


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.

We had time at the end of class to talk about 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.


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 will be shown in class next Tuesday.