[Home] [Lectures] [Previous] [Next]


Tornado with condensation
A Tornado is a violently rotating column of air that is associated with 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 225 mph (375 km/h). When viewed from above, the majority of tornadoes rotate counterclockwise (at least in the Northern Hemisphere). About 1% of tornadoes in the Northern Hemisphere rotate clockwise. The center of a tornado is characterized by low pressure, which is typically 10-20 percent lower than the surrounding air pressure. This pressure differential occurs over a very short distance, resulting in a large pressure gradient force that generates high wind speeds. As air spirals into the low pressure center, it expands and cools. If the air cools enough condensation occurs and the characteristic condensation funnel can be seen. If condensation does not take place, the only visual sign of the tornado is circulating dust and debris lifted by the strong winds. There is a common misconception that the strong winds are only found inside the funnel, however, in reality, the strong winds can extend a considerable distance away from the visible funnel. A funnel cloud is a tornado whose circulation remains above the ground.

Tornado with debris funnel

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.

An extreme tornado outbreak occured in the southeastern United States from April 25 - 28, 2011. There were 359 confirmed tornadoes in 21 states and 322 tornado deaths in six states. A record number of 208 tornadoes were observed on April 27 alone and four of those have been classified as EF-5 tornadoes, the strongest category for tornadoes. About one month later on May 22, the devastating EF5 Joplin tornado struck Joplin, Missouri, killing 155 people. This was the deadliest single tornado to strike the US since 1947. Throughout the year there were 1897 tornadoes reported in the United States, the second highest annual total since records began in 1953. Thus, 2011 will go down as one of the worst tornado years in US history.

Many in the media have tried to connect the strong 2011 tornado season in the United States with global warming or climate change. Some predicted even more tornadoes in 2012 and beyond. However, a quick study of the historical tornado data in the United States does not indicate a trend torward more or stronger tornadoes in recent decades or years (see figure below). There is a lot of variability from year to year though. 2012 was one of the quietest tornado seasons in US history. This was followed by 2013, which had the lowest annual number of US tornadoes (943), since counting officially began in 1953. The next year, 2014, was another quiet tornado season with only 1055 tornadoes reported in the US, which is the second fewest since 2005. Last year, 2015, was in the middle of the pack with respect to the number of US tornadoes over the last 10 years. While these recent years are not proof that climate change will not result in more future tornadoes, it does cast some doubt on predictions of increasing tornado activity as a result of human-caused climate change.


Tornado Occurrence

Tornadoes occur in many places of the world, but no country experiences more tornadoes than the United States, which averages more than 1200 annually. Roughly 75% of all tornadoes reported worldwide occur in the United States. Canada is a distant second, with around 100 per year. Other locations that experience frequent tornado occurrences include northern Europe, western Asia, Bangladesh, Japan, Australia, New Zealand, China, South Africa, and Argentina. Surprisingly, within the United States 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, are more likely to 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. In this region warm, humid tropical surface air from the Gulf of Mexico interacts with much colder and drier air masses moving down from Canada. This area is unique on Earth. A warm ocean surface lies to the south and a large continental land mass to the north with no mountainous barriers to prevent the clash of warm, humid air masses from the south and very cold, dry air masses from the north (See figure below on left). This also explains why this region experiences high numbers of severe thunderstorms as well.

[Conditions leading to severe thunderstorms in central U.S.] [all tornadoes] [significant tornadoes] [violent
Conditions favorable for tornadoes
in the central US
Tornado days per year
(All tornadoes)

Click on thumbnail to view
Significant (EF-2 or stronger)
Tornado days per century

Click on thumbnail to view
Violent (EF-4 or larger)
Tornado days per millennium

Click on thumbnail to view

The three figures above on the right display where tornadoes of differing strengths are most common in the United States. Click on the thumbnails to see larger image. The right-most figure shows where the extremely violent tornadoes occur most often. Notice the bullseye over central Oklahoma, extending outward throughout tornado alley. The EF scale is defined in the next section.

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

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. The way this scale has been used is that a tornado's windspeed is estimated based on the damage caused by the tornado (after the fact). The Fujita scale has six categories for tornadoes, labeled F0, F1, F2, F3, F4, and F5, where F0 is the weakest category and F5 the strongest category. Later research has shown that the Fujita scale overestimates the actual windspeeds in tornadoes. For example, it was once believed that the strongest tornadoes produced winds of 300 mph or greater. New research estimates that the strongest tornadoes produce winds of around 225 mph.

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.

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 (EF0,EF1) > 200 mph (EF5)

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.

Interesting and Easy to Understand article on tornado characteristics and safety from the Storm Prediction Center in Norman, OK.

You Tube video of cars tossed around by a tornado in Alabama

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.

There seem to be several mechanisms that can lead to tornado formation, none of which are fully understood. One proposed mechanism of tornado formation involves rotating thunderstorm updrafts. A simple description is provied in the following two linked pages: ( formation page 1, formation page 2). The mechanism described on the pages above is probably not the mechanism typically responsible for tornado formation in supercell thunderstorms, though. The proposed explanations for tornado formation in supercells is too complex to cover in this class.

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 (come from a different direction) 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. Note that high level air just needs to be cold, so that surface parcels lifted upward will be warmer than the surrounding air and thus unstable. Sometimes cold, dry air at high levels coming from the northwest (Arctic air) can act to make the atmosphere more unstable as the first clouds that form will evaporate, which cools this air even more. Either way this sets up a scenario that has warm, humid air near the surface and cold air at the higher levels. This is highly unstable, which is one component necessary for severe thunderstorms. In addition, the three air streams arrive from different directions, which provides the second necessary component, vertical 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, can be another key ingredient in the formation of strong supercell thunderstorms. 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. Why? Rising parcels expand and cool as they rise upward. For a rising parcel to become unstable it must become warmer than the environmental air surrounding the parcel. In a temperature inversion, the environmental air is getting warmer as you move upward, while the rising air parcel is cooling. Therefore, rising parcels will remain colder than the surrounding air when rising through a temperature inversion and will not become unstable, which is required for thunderstorm formation.

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 and how the environmental air temperature changes during the day is shown in following diagram: Click here to view the diagram.

Other Tornado Links

A comprehensive web page covering the formation and occurence of tornadoes is given in the NOAA web page: Tornado Basics

Another informative guide (updated in Jan 2015), which includes answers to freuqently asked questions about tornadoes and includes links to historical tornadoes, is hosted by the US STORM PREDICTION CENTER is available fom The Online Tornado FAQ.

[Home] [Lectures] [Previous] [Next]