What is Tornado Alley?
Tornado Alley is the area in the United States in which tornadoes are most frequent. It encompasses the great lowland areas of the Mississippi, the Ohio, and lower Missouri River Valleys. The ten states that are involved are: Alabama, Arkansas, Florida, Iowa, Kansas, Mississippi, Missouri, Nebraska, Oklahoma, and Texas. In the southern states the peak time for tornadoes occurs between March and May and again in November. In the northern statesthey peak during April and June.
There are three types of air needed to cause a tornado. The plains of the United States are uniquely suited to bring all these ingredients together. The main factors are Rocky Mountains to the west, the Gulf of Mexico to the south, and a terrain that slopes downward from west to east.
Tornadoes form under a certain set of weather conditions where three different types of air come together in a certain way. Near the ground lies a layer of warm and humid air along with strong south winds. Colder air along with strong west or southwest winds line the upper atmosphere. Temperature and moisture differences between the surface and the upper levels create instability. A third layer, of very warm dry air, forms between the warm and moist air at low levels and the cool dry air above. The warm layer acts as a cap and allows the atmosphere to warm further, making the air even more unstable.
the storm system above moves east it begins to lift the various layers. Through
this lifting process the cap is removed, setting the stage for explosive
thunderstorm development as strong updrafts develop. Interactions
between the updraft and the surrounding winds at storm level and near the
surface may cause the updraft to begin rotating and a tornado is born.
The warm moist air, called tropical maritime air, is swept up from the Gulf of Mexico and the Caribbean Sea due to lack of mountain barriers. During the spring months the Earth begins to warm, which adds to the layer of warm moist air which is close to the ground. While this is occurring, cool dry air masses, called maritime polar, often sweep in from the north or northeast. The cool air is trapped by the Rocky Mountains and rides close to 10,000 feet above the warmer air below. Cool air over warm air is an unstable condition. The hot middle layer, coming from the west often acts as a “cap” on the low-level warm, moist air. Only the strongest areas of heating near the ground can penetrate the cap. But when they do, the bottled-up, low-level moist air feeds into the break from miles around. The shifting winds then twist theses updrafts forming supercell thunderstorms. A breaking cap, with the help of an upper level jetstream, can cause convection to grow explosively, with storms rapidly becoming severe and tornadic.
Components of Tornado Alley:
Gulf of Mexico
amount of cold air
amount of warm and humid air
amount of warm dry air
lifting (of the cap)
seasonal changes in heating
mixing of air masses
The combinations of conditions that cause tornadoes are common across the southern USA in the early spring. As the season goes on, tornadoes are likely farther and farther north on the Plains and in the Midwest, but in April and May tornadoes are common in both the South and on the Plains and in the Midwest. Often, a large storm system can create tornado conditions for several days in a row.
Tornadoes mostly move from the southwest to the northeast. Tornadoes can also come from other directions, like from a loop or they can be stationary. Most of the time tornadoes move thirty five miles per hour. Sometimes tornadoes will move up to seventy miles per hour. No matter how fast a tornado is moving it can cause a lot of damage.
Most tornadoes will last about fifteen minutes, but there have been tornadoes that have lasted up to seven hours. About two percent of the tornadoes are rated violent. A tornado can travel for three hundred miles. A tornado can be three miles in width.