In order to understand why so many tornadoes occur in the United States, take a look at a topographical map of the North American continent, and especially of the United States. The Rocky Mountain range in the west, and the Appalachian Mountain range in the east, act like the sides of a gigantic natural funnel, allowing huge amounts of warm, moist air from the Gulf of Mexico, cold, dry air from Canada and the Arctic, and cool, dry air spilling over the Rockies from the Pacific (losing most of its moisture in the process) to come into contact--and often, conflict! When these three thermally unbalanced air masses meet, the reaction is almost always violent, and, in many cases, downright catastrophic!
The primary field of conflict, so to speak, is an area generally known as "Tornado Alley." It stretches from Texas all the way up to Minnesota. Better than 95% of the more than 1,000 tornadoes that occur in the United States each year (on average) occur in this region. Because of this, most of the research into severe weather and severe weather safety are concentrated in this area. In fact, the National Weather Service maintains two major facilities for this purpose in Norman, Oklahoma, often referred referred to as "the heart of Tornado Alley." The first of these, the National Severe Storms Laboratory, is devoted primarily to severe weather research, while its counterpart, the Storm Prediction Center, is dedicated to actively predicted to forecasting severe weather, and alerting the public when it actually develops.
In recent years, a second area, commonly referred to as "Dixie Alley," has been identified. It includes the states of Mississippi, Louisiana, Alabama, the upper half of Georgia, and the western 2/3 of Tennessee. A number of the most destructive tornadoes in recent years have occurred, many of them at night, and more than a few of them in what would normally be considered the "dead of winter." The reason for this is that the "Dixie Alley" region is significantly closer to the Gulf of Mexico than the traditional "Tornado Alley." This creates a sort of "thermal flywheel" or "thermal storage battery" effect, allowing severe weather to develop much later in the day, and in the year, than might otherwise be expected.
In most areas of the United States, there are two periods when tornadoes, and the severe thunder-storms that spawn them, are most likely to develop. Both of these periods are times of transition from one season to the next. The primary season includes the months of March, April, and May, which is the transitional period between winter and summer. The secondary period includes late October, November, and early December, the seasonal transition between summer and winter. This especially holds true in the region known as "Dixie Alley." Also, and especially in "Dixie Alley," most tornadoes tend to develop late in the day, when the heat and humidity reach their daily peak. This is also the reason so many tornadoes occur at dusk, or even in the early nighttime hours. It should also be noted that tornadoes which occur at night are all but impossible to see, which makes them even more dangerous than their daytime counterparts.
In my next posting, we'll begin covering the basic safety measures to protect you, and those you love, against the dangers of severe weather, and especially tornadoes.