March 31, 2008

 

Plan for today is to finish the tornado material, then take questions about homework #5 and quiz #4 material.

 

Tornadoes (Continued)

n     Tornado Statistics and Characteristics for the United States

o      Tornadoes kill an average of about 60 people each year in the US.  But there is great variability from year to year.  The deadliest tornado days are often associated with a large area tornado outbreak.  These occur when conditions are highly favorable for tornado development over a rather large area.  One of the most violent occurred on April 3 and 4, 1974. During a 16 hour period, 148 tornadoes cut through parts of 13 states, killing 307 people, injuring more than 6000, and causing an estimated $600 million in damage.

 

o      In the late 1960s, the late Dr. T. Theodore Fujita, a noted authority on tornadoes at the University of Chicago, proposed a scale (called the Fujita scale for classifying tornadoes according to their rotational wind speed based on the damage done by the storm.  Starting February 1, 2007, the National Weather Service revised the original Fujita scale linking tornado damage to windspeed. The new scale is called the enhanced Fujita scale (EF scale). There are still 6 categories for tornadoes (now EF-0 through EF-5). The new EF scale was developed to improve the estimation of windspeed based on the damage by better considering the structural integrity of different building types.

§       Go over the link to the EF scale.

 

o      Below is a table summarizing some of the characteristics of tornadoes.  You should be familiar with the material presented in this table

 

Characteristic

Most Common

Extreme / Possible

Location on Earth

United States

Almost anywhere

Time of year (US)

March – July (figure 10.29)

Anytime of year

Time of day

Late afternoon (4 – 6 PM)

Anytime of day

Size (diameter)

50 yards

> 1 mile

Movement-Speed (US)

30 mph

0 – 70 mph

Movement-Direction (US)

Toward the northeast

Any direction

Length of ground path

< 2 miles

> 300 miles

Time on Ground

< 5 minutes

> 6 hours

Wind speed

< 100 mph (EF0, EF1)

> 300 mph

 

 

 

 

 

 

 

n     Tornado Safety

o      Most tornado-related deaths and injuries are caused by flying debris, so the most important consideration is to shelter yourself from flying debris. If possible go to a sturdy structure. Your best bet is to move into a basement or underground storm shelter. If these are not available go to an interior closet away from windows. Cars and mobile homes are not sturdy structures and are dangerous locations to wait out a tornado. If you are caught outside, you want to get as low to the ground as possible. Your best bet is to lie flat in a ditch or depression in the ground.

§       In recent years about 45% of all US tornado fatalities happened in mobile homes.

 

o      When meteorologists at the National Storm Prediction Center determine that conditions are favorable for tornado development (i.e., unstable atmosphere, wind shear profiles, etc.), a tornado watch will be issued.  The tornado watch is meant to alert people to be on the lookout for tornadoes.

 

o      Tornadoes come and go so quickly and are so small that predicting where and when one will hit more than 15 to 30 minutes ahead of time is not possible. The average warning time for a tornado with winds of 158 miles an hour or faster - the type that accounts for most deaths - is 18 minutes.  Tornado warnings are issued by local weather offices once a tornado is spotted visually (by a trained spotter) or detected on a RADAR screen. 

§       Recall that a hook echo on a conventional RADAR screen often indicates that where a tornado is about to form.

 

§       Tornadoes can be detected with Doppler RADAR and when they are, tornado warnings are issued. All weather radars work by measuring the reflected (or more correctly backscattered) radiation coming from large particles such as raindrops and hail. Doppler radars can also detect whether the reflecting particles are moving toward or away from the radar site. Thus, the rotating winds around a tornado can show up very well on Doppler Radars. Sometimes the rotation can be observed at cloud level, and a tornado warning issued, before the tornado circulation touches down on the ground.

 

n     Another quick look at tornado alley

o      Time permitting, we will briefly discuss the information provided on the assigned reading page (tornado alley) and the embedded “convection cap” link to point out a few more reasons why this area is the most favored area on Earth for the development of monster tornadoes.

 

End of Quiz 4 Material

Quiz #4 will consist of about 42 multiple choice questions, of which about 16 will only have two answer choices.

 

Tentative breakdown of quiz questions by topic (subject to minor changes):

 

Topic

# Questions

Cloud formation

8

Fog

1

Cloud types

1

Lifting parcels – using table

4

Lifting parcels – general

4

T’storms (where they occur)

3

T’storms (stability, how they work)

2

T’storms (life cycle, how they work)

3

T’storms (severe)

2

Lightning (general)

4

Lightning (safety)

3

Tornado (characteristics)

3

Tornado (life cycle)

2

Tornado (formation)

1

Other

1