While tornadoes are a severe weather phenomenon which can be found everywhere in the world except Antarctica, nearly all tornadoes occur in the middle latitudes of the world. In particular, roughly ninety percent of the world's tornadoes occur in the United States and to a lesser extent in Canada, mostly within a large quadrant stretching roughly from the continental divide in the west, through parts of Texas and down to the Gulf of Mexico, north through Florida and Georgia, inland to the Appalachian mountains, and north as far as the northern edge of Lake Superior.
Within this quadrant, tornadoes are mostly found in four separate belts known as 'tornado alleys'. These consist of:
1. The midwestern belt, reaching from as far north as Edmonton, Canada to Nebraska, Minnesota, Wisconsin, Illinois, Iowa, and the headwaters of Lake Superior;
2. The Great Lakes belt, a relatively narrow stretch of land between Lake Michigan, Lake Huron / Lake St. Clair, and Lake Ontario which includes Michigan, Indiana, and southwestern Ontario;
3. The Gulf Coast belt, stretching from eastern Texas through Louisiana to Georgia and Florida;
4. The southern plains belt, starting from northern Texas, Oklahoma, and Kansas, passing through Iowa and Missouri, to end in the Appalachian mountains of Kentucky and Tennessee.
Of these, the region which most frequently gives rise to the most violent tornadoes, as well as tornado clusters and even tornado outbreaks consisting of dozens of tornadoes over a two or day period, is the southern plains belt, where approximately 70% of all North American tornadoes occur.* It is to this region that the term 'Tornado Alley' is most commonly applied.
What all four tornado alleys have in common is a geography which allows frequent interaction between cold dry air and warm moist air in such a way that the interacting air masses produce wind shear and consequently cloud rotation, resulting in a tornadic storm system capable of giving rise to tornadoes. In particular, the exceptionally violent tornadoes of the southern plains belt draw their energy from the collision of dry, cold mountain air with warm, humid Gulf of Mexico air. This interaction is made possible by geographical features which tend to run north-south, such as the Rocky Mountains, the prairies, and the Mississippi river valley, and thus draw together air masses which might otherwise be mostly blocked from each other (as is the case with the European Alps and the Caucasus Mountains).
Thus tornado formation is closely tied to seasonal air mass patterns, which in turn are tied to the exact path of the jet stream. Winter tornadoes are most common across the Gulf Coast tornado belt, which also tends to see some of the longest-lived storms. Midwestern and Great Lakes tornadoes occur most often during summer and early fall, when the unstable mixture of Gulf moisture and dry plains air moves further north; and may also be tied in some parts to the interaction of hurricane remnants with dry, cold air. Uniquely, southern plains tornadoes are increasingly beginning to strike outside the traditional high frequency period of late spring and summer to become almost year-round events.
Again because of the necessary clash between cold dry air and warm moist air, most tornadoes occur during the afternoon and evening, when the surface air will be warmest and the contrast of temperatures highest.
* Exact statistics are difficult to obtain, since there is disagreement about whether multiple touchdowns by a single funnel constitute one tornado or several. As well, a tornado which crosses state lines, as many do, is often counted separately by each state.