why do tornadoes occur in the us

Tornado occurrence is highly dependent on the time of day. Austria, Finland, Germany, and the United States'
peak hour of occurrence is 5Ppm, with roughly half of all tornado occurrence between 3 p. m. and 7 p. m. local time, due to this being the time of peak atmospheric heating, and thus the maximum available energy for storms; some researchers, including of the, have referred to this phenomenon as "five o'clock magic. " Despite this, there are several morning tornadoes reported, like the, Texas one in April 1980. The time of year is a big factor of the intensity and frequency of tornadoes. On average, in the United States as a whole, the month with the most tornadoes is May, followed by the months June, April, and July. There is no "tornado season" though, as tornadoes, including violent tornadoes and major outbreaks, can and do occur anywhere at any time of year if favorable conditions develop. Major tornado outbreaks have occurred in every month of the year. July is the peak month in Austria, Finland, and Germany. On average, there are around 294 tornadoes throughout the United States during the month of May, and as many as 543 tornadoes have been reported in the month of May alone (in 2003). The months with the fewest tornadoes are usually December and January, although major tornado outbreaks can and sometimes do occur even in those months. In general, in the Midwestern and Plains states, springtime (especially the month of May) is the most active season for tornadoes, while in the far northern states (like and ), the peak tornado season is usually in the summer months (June and July).

In the colder late autumn and winter months (from early December to late February), tornado activity is generally limited to the southern states, where it is possible for warm Gulf of Mexico air to penetrate. The reason for the peak period for tornado formation being in the spring has much to do with temperature patterns in the U. S. Tornadoes often form when cool, polar air traveling southeastward from the Rockies overrides warm, moist, unstable air in the eastern states. Tornadoes therefore tend to be commonly found in front of a cold front, along with heavy rains, hail, and damaging winds. Since both warm and cold weather are common during the springtime, the conflict between these two air masses tends to be most common in the spring. As the weather warms across the country, the occurrence of tornadoes spreads northward. Tornadoes are also common in the summer and early fall because they can also be triggered by hurricanes, although the tornadoes caused by hurricanes are often much weaker and harder to spot. Winter is the least common time for tornadoes to occur, since hurricane activity is virtually non-existent at this time, and it is more difficult for warm, moist maritime tropical air to take over the frigid Arctic air from Canada, occurrences are found mostly in the Gulf states and Florida during winter (although there have been some notable exceptions).

Interestingly, there is a second active tornado season of the year, late October to mid-November. Autumn, like spring, is a time of the year when warm weather alternates with cold weather frequently, especially in the Midwest, but the season is not as active as it is during the springtime and tornado frequencies are higher along the Atlantic Coastal plain as opposed to the Midwest. They usually appear in late summer. Oklahoma s devastating tornado, which at least 24 people and injured more than 200 others, is drawing comparisons to past U. S. twisters today, including the that hit the same region in 1999. And the United States has plenty of examples to draw from. According to the (NOAA), the United States, in averaging more than 1,000 tornadoes each year, is by far the global leader when it comes to number of twisters recorded. Canada finishes a distant second with roughly 100 per year. Here s NOAA s map of the regions of the world that are most likely to experience tornadoes. In addition to the United States and Canada, the organization highlights many European countries and parts of other nations including Argentina, South Africa, Bangladesh, and Japan (click on the image below to expand): So why is the United States so disproportionately prone to tornadoes?

According to a explainer on the subject, the distinction is a result of climatology, geography, and topography (the NOAA image at the top of this post shows this week s storm system over Moore, Oklahoma): [T]he United States has an abundance of flat, low-lying geographic regions, and it also has a climate that is conducive to intense thunderstorms, and tornadoes tend to form during thunderstorms. Turning for a moment from topography to geography, the United States has a few places that might be called tornado hotspots. Most prominent among them, of course, is Tornado Alley, a slice of America s mid-section running horizontally from Texas up to North Dakota taking in portions of Oklahoma, New Mexico, Colorado, Kansas and Nebraska. Tornado alley s tornadoes usually happen later in the spring time and sometimes into the fall. The region is considered a prime breeding ground for supercell thunderstorms, which tend to produce the strongest tornadoes. Supercell thunderstorms contain something called a mesocyclone, which has a rotating updraft they re very dangerous but also, when identified as supercells, can provide a good heads-up that the extreme weather they can produce, like tornadoes, is possible. Florida, too, has lots of tornadoes. That s because the state has many thunderstorms on a daily basis, and it s also a pit stop for many tropical storms or hurricanes (the tropical storms and hurricanes don t tend to produce the kind of killer tornadoes that come about during non-tropical storms).

While the United States leads the world when it comes to sheer volume of tornadoes, the ranking changes when you apply other filters. The United Kingdom, for example, has more tornadoes relative to its land area than (a fact one expert to the country s position on the Atlantic seaboard, at the nexus of polar air from the North Pole and tropical air from the Equator). And factors such as high population density, ineffective warning systems, and shoddy infrastructure mean tornadoes can be particularly deadly in countries like Bangladesh, which experienced a tornado that killed in 1989. Writing for, Peter Tyson points out that America s tornado tally may be so high relative to the rest of the world in part because other countries aren t as diligent about recording twisters. And he adds that all nations that experience tornadoes have something in common: They lie 20` to 50` on either side of the equator, in the mid-latitudes. You could probably get a tornado anywhere on the planet, but there are places where they are far less frequent, says John Snow, a tornado expert at the University of Oklahoma. For good meteorological reasons, these tend to be in the tropics and the very high latitudes. The only continent where twisters have yet to strike? Antarctica.

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