Tornado season this spring (2011) has been particularly destructive and deadly.  In this lecture we'll look at some of the kinds of damage tornadoes can do and we'll introduce the Fujita Scale used to rate tornado strength or intensity.

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 and assign a Fujita Scale rating.  The original scale, introduced in 1971, has recently been revised because the estimated wind speeds were probably too high.  The newer scale is called the Enhanced Fujita Scale and became operational in 2007.  The chart below compares the two scales.



The original Fujita Scale actually goes up to F12.  An F12 tornado would have winds of about 740 MPH, the speed of sound.  Roughly 3/4 of all tornadoes are EF0 or EF1 tornadoes and have winds that are less than 100 MPH.  EF4 and EF5 tornadoes are rare but cause the majority of tornado deaths. 

The EF scale considers 28 different "damage indicators," that is, types of structures or vegetation that could be damaged by a tornado.  Examples include:
Damage Indicator
 Description
2
 1 or 2 family residential home
3
 Mobile home (single wide)
10
 Strip mall
13
Automobile showroom
22
Service station canopy
26
 Free standing light pole
27
 Tree (softwood)


Then for each indicator is a standardized list of "degrees of damage" that an investigator can look at to estimate the intensity of the tornado.  For a 1 or 2 family home for example
degree of damage
 description
 approximate
wind speed (MPH)
1
 visible damage
 65
2
 loss of roof covering material
 80
3
 broken glass in doors & windows
 95
4
 lifting of roof deck, loss of more than 20% of roof material, collapse of chimney, garage doors collapse inward, destruction of porch roof or carport
 100
5
 house slides off foundation
 120
6
 large sections of roof removed, most walls still standing
 120
7
 exterior walls collapse (top story)
 130
8
 most interior walls collapse (top story)
 150
9
 most walls in bottom floor collapse except small interior rooms
 150
10
 total destruction of entire building
 170

You'll find the entire set of damage indicators and lists of degrees of damage here.
EF2 Damage
roof is gone, but all walls still standing
 EF4 Damage
only the strong reinforced concrete basement walls are left standing.
 EF5 Damage
complete destruction of the structure




The photos above show examples of damage caused by EF2, EF4, and EF5 tornadoes.

Several levels of damage (EF1 to about EF3) 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 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.

Air motions inside tornadoes are complex and difficult to study directly.  Researchers resort to laboratory simulations and computer models.  The figures below show some of the air motions thought to occur in tornadoes.


Wind motions in a fairly weak tornado.  The winds would also be spinning in addition moving upward as shown here.

This tornado is a little stronger.



This tornado is even stronger.  The air in the center has started to sink (this is called vortex breakdown), but the sinking air doesn't reach the ground.  The diameter of the tornado has also grown.


It is when the sinking air in the middle reaches the ground that multiple vortices may form. 

You'll find lots of tornado videos online.  One that we often show in the classroom version of this course is a tornado that occurred in Pampa, Texas (here are a couple of videos on YouTube: video 1, video 2).  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 and 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 (the wind speeds were measured above the ground and might not have extended all the way to the ground).