Tornado Basics

What is a tornado?

A tornado is a narrow, violently rotating column of air that extends from a thunderstorm to the ground. Because wind is invisible, it can be hard to see a tornado unless it forms a condensation funnel. A condensation funnel is made up of water droplets, dust, and debris.

Where do tornadoes occur?

Tornadoes occur in many parts of the world, including the United States (U.S.), United Kingdom (UK), Europe, Asia, and South America. New Zealand reports approximately 20 Tornadoes every year. Two of the highest concentrations of tornadoes outside the U.S. are Argentina and Bangladesh. It’s fair to say the U.S. has the highest concentration of tornadoes worldwide.

How many tornadoes occur in the U.S. every year?

Approximately 1,200 tornadoes hit the U.S. annually. Because official tornado records only date back to 1950, the actual average number of tornadoes that occur each year is unknown. Plus, tornado spotting and reporting methods have changed a lot over the last several decades, which means that more tornadoes are being observed. 

Where is tornado alley located?

Tornado alley is a label invented by the media to refer to a broad area of relatively high tornado occurrence in the central U.S. Various tornado alley maps look different because tornado occurrence can be measured in many ways: by all tornadoes, tornado county-segments, strong and violent tornadoes only, and databases with different time periods. 

However, the idea of tornado alley can be misleading. The U.S. tornado threat shifts from the southeast in the cooler months of the year, toward the southern and central Plains in May and June, and the northern Plains and Midwest during early summer. Tornadoes can occur and have been reported in all 50 states. As a reminder, violent tornadoes can occur outside tornado alley every year. 

When are tornadoes most likely to occur? 

Tornado season usually refers to the time of the year the U.S. sees the most tornadoes. The peak tornado season for the southern Plains is from May into early June. On the Gulf Coast, it’s earlier in the spring. In the northern Plains and upper Midwest, tornado season is in June or July. However, tornadoes can happen at any time of the year. Tornadoes can also happen at any time of the day or night. Most tornadoes occur between 4:00pm to 9:00pm. 

What is the difference between a tornado watch and a tornado warning?

A tornado watch is issued by the NOAA Storm Prediction Center (SPC) meteorologists who watch the weather 24/7 across the entire U.S. for weather conditions that are favourable for tornadoes. A tornado watch can cover parts of a state or several states. This means you should watch and prepare for severe weather (tornadoes in this particular case). 

A tornado warning is issued by NOAA National Weather Service Forecast Office meteorologists who watch the weather 24/7 over a specific area. This means a tornado has either been reported by spotters or indicated by radar and there is a serious threat to life and property to those in the path of the tornado. Essentially, a tornado warning suggests that you should find appropriate shelter straight away!

How is the strength of a tornado rated?

To figure out the strength of a tornado, experts examine the damage it caused. From this information, experts can estimate the wind speeds. Back in 2007, an Enhanced Fujita Scale (EF-Scale) was implemented by the National Weather Service to rate tornadoes in a more consistent and accurate manner. 

The EF-Scale takes into account more variables than the original Fujita Scale (F-Scale) when assigning a wind speed rating to a tornado, incorporating 28 damage indicators such as building type, structures and trees. For each damage indicator, there 8 degrees of damage ranging from the start of visible damage to complete destruction of the damage indicator. 

The original F-scale didn’t take the above details into account. The original F-Scale historical database will never change. An F5 tornado rated years ago is still an F5, but the wind speed associated with the tornado may be somewhat less than previously estimated. 

An association between the original F-Scale and the EF-Scale has been developed. Essentially, this makes it possible to express ratings in terms of one scale to the other, preserving a historical database. 

How do tornadoes form? 

Well, experts to this day still don’t fully understand the process. The most powerful tornadoes occur from supercells, which are rotating thunderstorms with a well-defined radar circulation called a mesocyclone. 

Supercells can also produce damaging hail, severe winds, frequent lightning, and flash floods. The formation of a Tornado is believed to be dictated mainly by the things which happen in and around the mesocyclone. 

With all that being said, an excellent list of questions and answers about tornadoes can be found here.