Tornadoes are a fascinating and destructive weather phenomenon that captivate the imagination of people around the world. However, despite the global fascination with tornadoes, there is one surprising fact about them – the vast majority of tornadoes occur only in one country, the United States.
What is a tornado?
A tornado is a violently rotating column of air that extends from a thunderstorm cloud down to the ground. Winds inside a tornado can reach speeds of over 300 mph making them one of nature’s most violent storms. Tornadoes form when warm, moist air meets cool, dry air and creates an instability in the atmosphere. This instability causes thunderstorm “supercells” to develop and spawn tornadoes.
Tornadoes can appear as a visible funnel extending from the cloud base to the ground or they can be nearly invisible with only swirling debris at ground level indicating their presence. Tornado paths average 4 miles long but some can extend for over 50 miles. The width of a tornado ranges from a few yards to over 1 mile wide. They can last from several seconds to over an hour.
Tornadoes are rated on the Enhanced Fujita Scale from EF0 to EF5 based on wind speed and damage. An EF0 tornado has winds of 65-85 mph while an EF5 has winds over 200 mph and can level well-built homes and throw vehicles hundreds of yards.
Tornado Alley in the US
So why do most of the world’s tornadoes form in the United States? The reason has to do with geography and weather patterns.
The central United States sits in the area known as “Tornado Alley” where most tornadoes form. Tornado Alley extends from Texas in the south to Nebraska in the north and from eastern Colorado in the west to western Ohio in the east. This area sees frequent collisions of warm, humid air from the Gulf of Mexico and cold, dry air from Canada to create the perfect conditions for tornadoes to form.
During springtime, the jet stream dips southward over the Rocky Mountains in the west and then turns northeastward across the Plains. The jet stream brings cold, dry air down from Canada which meets warm, moist air flowing north from the Gulf of Mexico. This boundary is called the “dryline” and is a hot spot for tornado development.
Image source: National Oceanic and Atmospheric Administration
Why Tornado Alley?
There are four key ingredients that make Tornado Alley the ideal location for tornadoes:
- Jet stream – As mentioned above, the southward dip of the jet stream brings cold, dry air southward across the Plains. This cold air is a key ingredient in tornado formation.
- Moisture – Warm, humid air from the Gulf of Mexico provides moisture to the atmosphere.
- Flat land – The flat terrain allows cold and warm fronts to advance and collide unimpeded.
- Dryline – The boundary between moist and dry air focuses instability in one concentrated area in the Plains.
The most tornadoes form where these factors come together in Tornado Alley during springtime. The region sees an average of 271 tornadoes per year.
Tornado Hotspots Outside Tornado Alley
While Tornado Alley sees the highest concentration of tornadoes, other areas of the United States also see a high frequency of tornado development:
- Dixie Alley – The southern US from Mississippi to Georgia sees more tornadoes per square mile than Tornado Alley.
- Hoosier Alley – Indiana and parts of Ohio see around 30 tornadoes per year.
- Carolina Alley – North and South Carolina average 31 tornadoes per year.
- Florida – The warm climate and tropical systems create frequent tornadoes.
Annual US Tornado Statistics
On average, around 1200 tornadoes are reported in the United States annually. Here are some key tornado statistics for the US:
| Statistic | Number |
|---|---|
| Avg. # of tornadoes per year | 1200 |
| Peak months | May – June |
| Peak time of day | 3-9 pm |
| States with most tornadoes per year | Texas (139) Kansas (96) Florida (66) |
| Deaths per year | 55 |
| Injuries per year | 1500 |
| Cost of damage per year | $1.1 billion |
As the table shows, Texas has the highest frequency of tornadoes at 139 per year on average followed by Kansas and Florida. Tornadoes cause billions in damage and dozens of fatalities in the US annually.
Why tornadoes rarely form outside the US
So why isn’t Europe or China inundated with tornadoes like the United States? There are a few key reasons:
1. Unique geography
As mentioned earlier, the geography of the central United States makes it primed for tornado formation. The positioning of the Rocky Mountains and the Gulf of Mexico funnels warm and cold air masses together across the Plains. The flat terrain provides an ideal environment for these air masses to collide and spawn tornadoes. No other area in the world has such ideal tornado-spawning geography.
2. Global wind patterns
The jet stream over North America contributes to steering cold air from the poles down into the US forming a boundary with warm Gulf air. The upper level wind patterns over Europe, Asia, and other parts of the world do not channel cold and warm air together as efficiently. This limits instability and tornado formation.
3. Water temperature differences
The Gulf of Mexico provides a source of hot, humid air to fuel violent thunderstorms. Cooler water bodies near Europe and Asia do not contribute as significantly to regional temperature contrasts and instability.
4. Mountain barriers
Mountain ranges like the Alps in Europe and the Himalayas in Asia block or divert air masses and inhibit tornado-favorable weather patterns. North America has fewer barriers to allow cold and warm fronts to collide.
5. Global Storm Tracks
Upper level wind patterns carry and steer storm systems. North America sits in a prime global storm track which favors frequent cyclogenesis and storms. Other continents are less ideally positioned in the global atmospheric circulation pattern for frequent tornado formation.
Notable Tornado Outbreaks Outside the US
While the US dominates global tornado frequency, other countries can still see destructive tornadoes on rare occasion:
- Russell, Manitoba, Canada (2013) – An EF4 tornado caused major damage.
- Grandchester, Queensland, Australia (2015) – An EF3 tornado lasted nearly 1 hour.
- Xilinhot, Inner Mongolia, China (2016)– An EF4 killed 98 people.
- Northwestern Europe (2018) – A small outbreak spawned several tornadoes.
These events prove tornadoes can occur elsewhere, but factors limit their frequency and intensity compared to those in the US.
The Future of US Tornadoes
Despite year-to-year variability, the 12-month tornado running total has remained relatively steady at around 1,200 tornadoes per year since modern records began in the 1950s. However, some studies suggest climate change could impact future tornado activity.
There is still considerable uncertainty, but a warming world could potentially cause two changes:
- The zone conducive to tornadoes may shift northward.
- There will be an increase in weather pattern variability leading to more extreme outbreaks.
More research is needed, but it is likely tornado alley will still remain centered over the Great Plains as geography permits. But we may see more variability in the number of tornadoes from year to year with a higher frequency of extreme outbreaks resulting in deadlier storms.
Conclusion
In summary, the United States is uniquely situated geographically to spawn frequent and intense tornadoes. The positioning of air masses guided by mountains and water sources like the Rocky Mountains and Gulf of Mexico create ideal tornado conditions across the Great Plains. Europe, Asia, and the rest of the world lack these geographic advantages. While deadly tornadoes can still occasionally occur outside the US, the vast majority will continue developing over the Great Plains in an area known as Tornado Alley.
