As the days grow shorter and the weather turns colder, many people think they're in the clear and safe from tornadoes. However, the changing weather specifically makes us more vulnerable to tornadoes. Today's severe weather is a hard reminder that we're in the second peak of tornado season.

Tornado outbreaks are synonymous with the spring. The simple mention of "April 27" to many people from the southeastern United States will strike-up a conversation about where they were and people they know who were affected by the tornadoes that day in 2011. That's not to mention that almost all of the record-breaking tornadoes in this country's history have occurred during the month of May.

Spring marks the shift from brutal cold to relentless heat, and as such the United States is often sharply divided between warm and humid weather in the south and cold and dry conditions to the north. The steep temperature gradient allows the jet stream to dip farther and farther south, creating more opportunities for severe weather outbreaks to unfold.

[The image above shows strong winds at 500 millibars, or around 18,000 feet up in the atmosphere, on the morning of the May 20, 2013 EF-5 tornado in Moore, Oklahoma.]

As the country becomes a uniformly hot bucket of sweat during the summer months, the relative lack of a temperature gradient usually pushes the jet stream far enough north into Canada that it only tends to affect the northern part of the United States. The retreat of the jet stream allows the tornado season to calm down—not completely, of course—but usually prevents intense outbreaks like we would see in April and May.

When fall rolls around and the country again starts to experience wild swings in temperatures, the jet stream dips south once more and allows weather systems to develop and interact with the warmth and humidity. This results in spring-like severe weather outbreaks, complete with a second spike in tornado activity.

The above chart shows monthly tornado reports between January 2000 and September 2014. A line graph of the data does an excellent job showing the dual tornado seasons. While total tornado activity varies widely from year to year, every spring produces a pronounced spike in tornado activity, giving us our primary tornado season.

Highlighted in the red ovals is each fall between 2000 and 2013, showing a clear (but again, variable) secondary spike in tornado activity each autumn. While some of these fall twisters were the result of landfalling hurricanes (the reason behind the abnormally high numbers in 2004, for instance), many of these outbreaks are the result of the same processes we see during spring severe weather events.

Some of these fall tornadoes can even be particularly strong. One of the most gut-wrenching and memorable tornado videos from the past year came out of Washington, Illinois last November, when an early morning EF-4 tornado swept through the town and a man recorded the ordeal as the tornado struck the home in which he and his daughter took shelter.

Tornadoes are possible at any time of the year just about anywhere in the United States, but the southeast usually takes the brunt of the twisters no matter when they form. Always keep an eye on the forecasts, and don't let your guard down just because the pumpkins are out and the air has a chill.

[Images: NASA, TwisterData, author | Video: Mark Wells/YouTube]

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