Severe Weather

Tornadoes

A Tornado is a violently rotating column of air which descends from a thunderstorm. No other weather phenomenon can match the fury and destructive power of tornadoes. They can destroy large buildings, leaving only the bare concrete foundation., lift 20-ton railroad cars from their tracks, or drive straw and blades of grass into trees and telephone poles. Tornado damage to man-made structures is the result of high wind velocity and the associated windblown debris. Tornadic winds have been measured well in excess of 300 mph (480 km/h). When viewed from above, the majority of tornadoes rotate counterclockwise. About 1% of tornadoes rotate clockwise. A funnel cloud is a tornado whose circulation remains above the ground.

Each year, tornadoes take the lives of many people. The yearly average is about 100 (60 in the United States), although over 100 may die in a single day. The deadliest tornadoes are those that occur in families; that is, different tornadoes all spawned within a region with favorable atmospheric conditions for tornado development.

When a large number of tornadoes (typically 6 or more) forms over a particular region, this constitutes a tornado outbreak. 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.

Tornado Occurrence

Tornadoes occur in many places of the world, but no country experiences more tornadoes than the United States, which averages more than 1000 annually and experienced a record 1424 tornadoes during 1998. Surprisingly, central Florida experiences the greatest number of tornadoes. However, most of the tornadoes that occur in Florida are weak and cause relatively minor damage and few deaths.

While tornadoes have occurred in every state the greatest number of strong tornadoes, those that cause extensive damage and are responsible for the largest share of deaths, occur in the tornado belt or tornado alley of the Central Plains, which streches from central Texas northward through Oklahoma and Kansas to Nebraska and includes parts of Colorado, Iowa, Illinois, Missouri, and Arkansas.

About three-fourths of all tornadoes in the United States develop from March to July. Although tornadoes have occurred at all times of the day and night, they are most frequent in the late afternoon, when the surface air is most unstable.

Tornado days per year

Significant (F2 or larger Tornado days per century

Violent (F4 or larger) Tornado days per millennium

Tornado Winds

The strong winds of a tornado can destroy buildings, uproot trees, and hurl all sorts of lethal missiles into the air. Our earlier knowledge of the furious winds of a tornado came mainly from observations of the damage done and the analysis of motion pictures.

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.

Fujita Scale for Damaging Wind

Scale	Category	Mi/Hr	Expected damage
F0	Weak	40-72	light: tree branches broken; sign boards damaged
F1		73-112	moderate: trees snapped, windows broken
F2	Strong	113-157	considerable: large trees uprooted, weak structures destroyed
F3		158-206	severe: trees leveled, cars overturned, walls removed from buildings
F4	Violent	207-260	devastating: frame houses destroyed
F5		261-276	incredible: structures the size of autos moved over 100 meters, steel reinforced structures highly damaged

Tornado Characteristics

Summary of Tornado Characteristics

Characteristic	Most Common	Extreme / Possible
Location on Earth	United States	Nearly Anywhere
Time of year (USA)	March - July	Any month
Time of day	4 - 6 PM	Any time
Size (diameter)	50 yards	> 1 mile
Movement-speed	30 mph	0 - 70 mph
Movement-direction (USA)	Toward Northeast	Any Direction
Length of Ground Path	< 2 miles	> 300 miles
Time on Ground	< 5 minutes	> 6 hours
Wind Speed	< 100 mph (F0,F1)	> 300 mph (F5)

Tornado Safety

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.

Tornado Formation

Ingredients necessary for tornado formation:

1. An unstable atmosphere that is capable of producing strong thunderstorm updrafts and downdrafts.

2. A lifting force. The most common lifting forces are heating of air near the surface and weather fronts.

3. Vertical wind shear to provide rotation.

The exact sequence of events that lead to tornadoes are not fully understood. Although meteorologists can locate regions where the conditions are more favorable for tornado development (based on the ingredients above), the exact location and time cannot be predicted accurately. When favorable conditions are present over a particular region, a tornado watch may be issued.

The leading theory of tornado formation involves rotating thunderstorm updrafts. The two pages listed below will also be distributed as part of an in-class handout. ( formation page 1, formation page 2).

Tornado Warning

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. Of course if the tornado forms right on top of you, then in some cases no warning can be given. 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.

Tornado Alley

Why is the favored region for strong tornado formation in the central plains states (often referred to as TORNADO ALLELY)? Severe tornadoes often form when three very different types of air come together in a particular way. Near the ground surface southerly or southeasterly transport warm, humid air from the Gulf of Mexico into the Plains. As you go up to the middle layers of the atmosphere above the humid air, the winds veer to the southwest and transport hot, dry air from the Mexico highlands and deserts over the Plains. This forms a layer of hot, dry air in which the temperature often increases with height. The hot, dry layer is very stable and inhibits any convection that tries to develop. The winds continue to veer as you go up into the higher layers of the atmosphere. At the high layers of the atmosphere, westerly winds transport cool, moist air from the Pacific Ocean over the Rocky Mountains and into the Plains above the convective cap. This sets up a scenario that has warm, humid air near the surface with hot dry air above it and cool, moist air at the higher levels above the dry air. These three air streams also arrive from different directions, which provides the necessary wind shear. This scenario highly favors severe weather, including supercells and tornadoes.

The existence of the mid-level convective cap, which will be referred to as an inversion layer, is a key ingredient. An inversion layer is a layer in the atmosphere where the temperature increases with increasing height. Recall that typically temperature decreases with increasing height. An inversion layer is extremely stable for rising motion. At first an inversion layer will inhibit thunderstorm formation because rising parcels from the surface will not be able to penetrate through this layer. However, as the day progresses and the atmosphere gradually warms from below the inversion layer erodes away, and surface parcels become warm enough to "break through" the old inversion layer and reach the unstable atmosphere above. All day the sun's energy was used to heat the lower atmosphere and increase the water vapor content by evaporation essentially adding more energy to the lower atmosphere. When parcels are finally able to "break through", it is like blowing the lid off of a pressure cooker. This is depicted graphically on this convection cap page from USA Today An illustrative diagram of a convective cap will be drawn in lecture.

Other Tornado Links

A very comprehensive explanation behind the formation of tornadoes is given in the NOAA web page: tornadoes... Nature's Most Violent Storms A PREPAREDNESS GUIDE Including Safety Information for Schools U.S. DEPARTMENT OF COMMERCE National Oceanic and Atmospheric Administration National Weather Service September 1992 (NOAA, FEMA, The American Red Cross).

Links to the Historical Tornado Data Archive hosted by the STORM PREDICTION CENTER