Thu., Apr. 22 notes

Thursday Apr. 22, 2010
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A couple of songs that I heard last weekend down at The Hut on 4th Ave. ("No Sugar Tonight/New Mother Nature" from The Guess Who and "Darkness, Darkness" from The Youngbloods) before class today.

Note the 2nd 1S1P topic added to Assignment #3. This will be the last of the 1S1P topics. Here's a link to some incredible photographs that relate to the topic (sent to me by a student in this class). Check to be sure your name isn't on the 45 pts list before writing any more 1S1P pts.

The Quiz #4 Study Guide Pt. 2 is now online. Please read the warning at the end of the study guide.

A few details, not discussed in class, were added to the sketch of a severe thunderstorm drawn in class on Tuesday. The tilted updraft will sometimes begin to rotate. A rotating updraft is called a mesocyclone (pt. 4). Meso refers to medium size (thunderstorm size) and cyclone means winds spinning around low pressure. Low pressure in the core of the mesocyclone creates an inward pointing pressure gradient force needed to keep the updraft winds spinning in circular path (low pressure also keeps winds spinning in a tornado). The cloud that extends below the cloud base and surrounds the mesocyclone is called a wall cloud (pt. 5). Later in class today we'll see why this extra bit of cloud is able to form below the base of the rest of the thunderstorm. The largest and strongest tornadoes will generally come from the wall cloud.

Here's a pretty nice photograph of a wall cloud and what is probably a relatively weak tornado (from the University Corporation for Atmospheric Research)

This seemed like a good place to briefly discuss supercell thunderstorms.

Here is a relatively simple drawing showing some of the key features on a supercell thunderstorm. In a supercell the rotating updraft (shown in orange above) has enough upward momentum 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. In an ordinary air mass thunderstorm the updraft is unable to penetrate into the very stable air in the stratosphere and the upward moving air just flattens out and forms an anvil. The flanking line is a line of new cells trying to form alongside the supercell thunderstorm (existing winds converge with winds from the thunderstorm downdraft along a gust front).

Here is a second slightly more complicated drawing of a supercell thunderstorm. A typical air mass thunderstorm (purple) has been drawn in for comparison.

Thunderstorms with rotating updrafts often have a distinctive radar signature.
We haven't discussed weather radar in this class. In some ways a radar image of a thunderstorm is like an X-ray photograph of a human body. The Xrays pass through the flesh but are partially absorbed by bone.

The radio signals emitted by radar pass through the cloud itself but are reflected by the much larger precipitation particles. The intensity of the reflected signal (the echo) is color coded. Red means an intense reflected signal and lots of large precipitation particles. The edge of the cloud isn't normally seen on the radar signal.

Thunderstorms with rotating updrafts produce a very characteristic hook shaped echo (hook echo) on radar. Here are a couple of examples:

This is the radar image of a thunderstorm that produced a very strong tornado that hit Oklahoma City in May 1999
( http://www.spc.noaa.gov/faq/tornado/radscel.htm ). The hook echo is visible near the lower left hand corner of the picture. Winds in the tornado may have exceeded 300 MPH.

A short segment of video was shown at this point. The video first showed some good quality video of a close tornado. This was followed by photographs of a distant supercell thunderstorm and photographs of the bases of nearby supercell thunderstorms. Here you could see the spectacular wall cloud that often forms at the base of these storms. Finally a computer simluation showed some of the complex motions that form inside supercell thunderstorms, particularly the tilted rotating updraft.

Tornadoes begin in and descend from a thunderstorm. You might see a funnel cloud dropping from the base of the thunderstorm. Spinning winds will probably be present between the cloud and ground before the tornado cloud becomes visible. The spinning winds can stir up dust at ground level. The spinning winds might also be strong enough at this point to produce some minor damage. We saw an example of this in a video tape of a tornado in Oklahoma.

In Stage 2, moist air moves horizontally toward the low pressure in the core of the tornado. This sideways moving air will expand and cool just as rising air does (see figure below). Once the air cools enough (to the dew point temperature) a cloud will form.

Tornadoes can go from Stage 2 to Stage 3 (this is what the strongest tornadoes do) or directly from stage 2 to stage 5. Note a strong tornado is usually vertical and thick as shown in Stage 3. "Wedge tornadoes" actually appear wider than they are tall.

The thunderstorm and the top of the tornado will move faster than the surface winds and the bottom of the tornado. This will tilt and stretch the tornado. The rope like appearance in Stage 5 is usually a sign of a weakening (though still a dangerous) tornado.

The tornado cloud forms when moist air moves into lower pressure in the core of the tornado. The air expands and cools to the dew point and a cloud forms. This is just like the cloud that forms when air rises (and moves into lower pressure and expands). I made an offer to students in the T Th class. If they tattoo the figure above somewhere (especially the left side) on their body they can use it on next week's quiz.

At about this point we watched a short video segment that illustrated well the first 3 steps in the formation of a tornado (dust swirl stage up to mature stage). The tornado was photographed near Luverne Oklahoma in May 1991. It was eventually rated an F3 tornado.

The Fujita Scale is used to rate tornado strength and damage potential. 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. Above is a simplified, easy to remember, version of the Fujita Scale.

This was followed by another short video, a potpourri of images of several different tornadoes. Descriptions of the tornadoes are given in the table below (the numbers in the left most column were used on the video to identify each tornado)

54a	F3	Grand Island, NE	Mar. 13, 1990	tornado cloud is pretty thick and vertical
61f	F3	McConnell AFB KS	Apr. 26, 1991	this is about as close to a tornado as you're ever likely to get. Try to judge the diameter of the tornado cloud. What direction are the tornado winds spinning?
52	F5	Hesston KS	Mar. 13, 1990	Watch closely, you may see a tree or two uprooted by the tornado winds
51	F3	North Platte NE	Jun. 25, 1989	Trees uprooted and buildings lifted by the tornado winds
65	F1	Brainard MN	Jul. 5, 1991	It's a good thing this was only an F1 tornado
57	F2	Darlington IN	Jun. 1, 1990	Tornado cloud without much dust
62b	F2	Kansas Turnpike	Apr. 26, 1991	It's sometimes hard to run away from a tornado. Watch closely you'll see a van blown off the road and rolled by the tornado. The driver of the van was killed!
47	F2	Minneapolis MN	Jul. 18, 1986	Tornado cloud appears and disappears.

Here are some representative photographs of tornado damage. You'll find some additional descriptions on p. 164 in the photocopied ClassNotes.

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 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 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 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 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.