This is Why 'Tornado Season' Technically Doesn't Exist

Tornadoes Can and Do Happen Year Round

double tornado
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Question: I always hear meteorologists on TV talk about "tornado season." What does this mean?

Answer: The phrase "tornado season" can be very misleading. It suggests there are specific dates during which tornadoes occur and outside of which they do not occur (much like the June 1 to November 30 Atlantic hurricane season). But this isn't true! Unlike hurricanes, tornadoes aren't limited in when they can occur.

A tornado can happen at any time of the year -- winter, spring, summer, or fall. This is because

(Related: When is hurricane season?)

That being said, there are certain times of the year when tornadoes are more likely to occur. In the United States, this happens to be during the spring (March, April, May) and in the fall (September, October, November). So, when you hear "tornado season" being used, it refers to one of these two times of year when tornado activity across the U.S. tends to peak.

Spring Is the Primary Season for U.S. Tornadoes

Spring tornadoes occur most often across the Southern Plains and Southeastern regions of the United States.

There are two primary reasons why Northern Hemisphere spring favors tornado development. For one, sunlight strikes the Earth's surface more directly and for longer periods of time (due to the lengthening of daylight hours) in springtime. This means that more heating is available to fuel severe weather.

Spring's jet stream also contributes to storminess by steering storms over certain parts of the nation as it makes its retreat northward toward its summer position over Canada.

Tornado Activity Peaks Again in Late Fall (Not Summer, as You Might Expect)

Fall tornadoes are especially common across the Gulf Coast states and the Ohio Valley regions of the United States.

With heat being a severe weather trigger in spring, you might think that summer's endless warmth would fuel tornadic activity during the summer, but this isn't the case. While it's true that heat triggers convective instability, the key is in how this heated air interacts with its surrounding air. By summer, warm air has had a chance to accumulate in the upper atmosphere, so as surface air warms, it rises and simply mixes with already warmed air aloft (which does little to encourage instability). This is why tornado outbreaks in summer are somewhat rare. However, after the summer solstice occurs, the amount of heat energy entering the atmosphere gradually lessens. As summer transitions into fall, the jet stream starts to dip back down into the lower U.S., allowing cooler air from Canada to pool into the upper levels of the atmosphere. This cooler air, which has a tendency to sink, begins to take up residence in the upper atmosphere. As it does so, it mixes with the warm, moist air at lower levels resulting in the occasional late-season tornado outbreak.