Severe storms are more likely to form when there is vertical wind shear (the picture above is on p. 154a in the ClassNotes).  Wind shear (pt 1) is changing wind direction and/or wind speed with distance.  In the case shown above, the wind speed is increasing with increasing altitude, this is vertical wind shear.

A thunderstorm that forms in this kind of an environment will move at an average of the speeds at the top and bottom of the cloud (pt. 2).  The thunderstorm will move to the right more rapidly than the air at the ground which is where the updraft begins.  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 (pt. 3).  The downdraft is situated at the back of the ground.  The updraft is continually moving to the right and staying away from the downdraft.  The updraft and downdraft coexist and do not "get in each others way."  If you remember in air mass thunderstorms, the downdraft gets in the way of the updraft and leads to dissipation of the storm.

Sometimes the tilted updraft will 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 (tornadoes are sometimes called cyclones).  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.

The cloud that extends below the cloud base and surrounds the mesocyclone is called a wall cloud (pt. 5).  The largest and strongest tornadoes will generally come from the wall cloud.  We'll see some pretty dramatic videos of wall clouds on Friday when we finish this section on tornadoes.

Note (pt. 6) that a tilted updraft also provides a way of keeping growing hailstones inside the cloud.  Hailstones get carried up toward the top of the cloud where they begin to fall.  But they then fall back into the strong core of the updraft and get carried back up toward the top of the cloud.

A wall cloud can form a little bit below the rest of the base of the thunderstorm.   Clouds normally form when air rises, expands, and cools as shown above at left.  The rising air expands because it is moving into lower pressure surroundings at higher altitude.  Only when the air has risen high enough, moved into low enough pressure, expanded and cooled enough will a cloud form.

At right the air doesn't have to rise to as high an altitude to experience the same amount of expansion and cooling.  This is because it is moving into the core of the rotating updraft where the pressure is a little lower than normal for this altitude.  Cloud formation is a little bit closer to the ground.

Here's a picture of a portion of the bottom of a thunderstorm with a wall cloud and, what appears to be, a relatively weak tornado (narrow diameter and almost transparent).

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The United States has roughly 1000 tornadoes in an average year.  That is more than any other country in the world .

A year's worth of tornado activity plotted on a world map.  Part of the reason why the central US has some many tornadoes is just a consequence of geography. 

Without any mountains in the way, cold dry air can move in the spring all the way from Canada to the Gulf Coast.  There it collides with warm moist air from the Gulf of Mexico to form strong cold fronts and thunderstorms.  There are some other meteorological conditions that come into play that make these storms capable of producing tornado

The following map (p. 161 in the ClassNotes) wasn't shown in class.

Tornadoes have been observed in every state in the US, but tornadoes are most frequent in the Central Plains, a region referred to as "Tornado Alley" (highlighted in red, orange, and yellow above).  

Here are some basic tornado characteristics (the figure above is also on p. 161)

1.  About 2/3rds of tornadoes are F0 or F1 tornadoes (this is referring to the Fujita Scale, we'll learn more about the Fujita scale used to rate tornado intensity next Tuesday) and have spinning winds of about 100 MPH or less.  Microburst winds can also reach 100 MPH.  Microbursts are much more common in Tucson in the summer than tornadoes and can inflict the same level of damage. 

2.  A very strong inwardly directed pressure gradient force is needed to keep winds spinning in a circular path.  The pressure in the center core of a tornado can be 100 mb less than the pressure in the air outside the tornado.  This is a very large pressure difference in such a short distance.  The PGF is much stronger than the Coriolis Force (CF) and the CF can be neglected.

3.  Because the Coriolis force doesn't play a role, tornadoes can spin clockwise or counterclockwise, though counterclockwise rotation is more common.  This might be because larger scale motions in the cloud (where the CF is important, might determine the direction of spin in a tornado).

4, 5, 6.  Tornadoes usually last only a few minutes, leave a path on the ground that is a few miles long, and move at a few 10s of MPH.  There are exceptions, we'll look at one shortly.

7, 8.  Most tornadoes move from the SW toward the NE.  This is because tornado-producing thunderstorms are often found just ahead of a cold front where winds often blow from the SW.   Most tornadoes have diameters of tens to a few hundred yards but tornadoes with diameters over a mile have been observed.

9, 10.  Tornadoes are most frequent in the Spring.  The strongest tornadoes also occur at that time of year.  Tornadoes are most common in the late afternoon when the atmosphere is most unstable.

A little more infomation on p. 161 that wasn't mentioned in class.  At the present time about 75 people are killed every year in the United States by tornadoes.  This is about a factor of ten less than a century ago due to improved methods of detecting tornadoes and severe thunderstorms.  Modern day communications also make easier to warm people of dangerous weather situations.  Lightning and flash floods (floods are the most serious severe weather hazard) kill slightly more people than tornadoes.  Hurricanes kill fewer people on average than tornadoes.  The increase in the number of tornadoes observed per year is probably more due to there being more people in locations that are able to observe and report a tornado rather than a true increase in tornado activity. 

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This figure traces out the path of the 1925 "Tri-State Tornado" .  The tornado path (note the SW to NE orientation) was 219 miles long, the tornado lasted about 3.5 hours and killed 695 people.  The tornado was traveling over 60 MPH over much of its path. It is still today the deadliest single tornado ever in the United States.  The Joplin Missouri tornado (May 22, 2011) killed 162 people making it the deadliest since 1947 and the 7th deadliest tornado in US history.

Tornadoes often occur in "outbreaks."  The paths of 148 tornadoes during the April 3-4, 1974 "Jumbo Tornado Outbreak" are shown above.  Note the first tornadoes were located in the upper left corner of the map and all of the tornado paths are oriented from SW to NE.  The storm system responsible for the outbreak is shown below.

The April 25-28, 2011 outbreak is now apparently the largest tornado outbreak in US history (358 tornadoes, 346 people killed)

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As we learn more about tornadoes I'm hoping you'll look at video with a more critical eye than you would have otherwise.  So we took a moment, at this point,  to have a look at some tornadoes caught on video.  If you click on the links you'll see the same or a similiar video that I found online.

54a	F3	Grand Isle, 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.

The Kansas turnpike video also has a warning that a highway underpass is actually a very dangerous place to take shelter from a tornado.  Here is some additional information from the Norman OK office of the National Weather Service.  Slide 6 lists some of the reasons why underpasses are so dangerous.

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We looked at one more short video segment with photographs of supercell thunderstorms and wall clouds.  The tape also showed a computer simulation of the complex motions inside a supercell thunderstorm.  As we'll see next Tuesday thunderstorms with rotating updrafts (mesocyclones) and supercells produce a unique signature on radar called a hook echo. We'll also look at the Fujita Scale used to rate tornado strength and intensity and look at some of the kinds of damage that tornadoes can produce.