tim marshall: now there are thousands of people who do this.ur groups nowadays, and you can get in a van and go with a group of fellow enthusiasts and drive out onto the plains and see the majestic skies. allison chinchar: in the united states, we average more than 1,200 tornadoes per year. that is number one in the world by a long shot. the second closest is canada, which only gets 100 per year. you could actually add canada, australia, and all european countries combined, and we still see more than they do. tornadoes are most common in the midwest. max olson: we also have another area that's really prone to tornadoes in our southeastern states. harold brooks: the tornado season depends on where you are in the country. in the plains, tornado season is typically over a relatively short period of time in the springtime. in the southeastern united states, tornadoes are more likely to occur at any time of the year. tornadoes actually can and do strike all over the country. tim marshall: tornadoes have been known to be in every state of the us. the

tim marshall: tornadoes have been known to be in every state of the us. they have been in the highest of mountains, beaches, and offshore. harold brooks: over the last 50 years, what we've seen has been an increase in the number of tornadoes in what we think of as the mid-south. it looks like it's associated with physical changes in the atmosphere, but we don't have a real underlying reason of why that's occurred. reporter: a rare sight in the skies over japan. a powerful twister ripping across the eastern part of the nation. tornadoes occur worldwide. jeff piotrowski: europe has tornadoes. south america has tornadoes. australia, japan. trent okerson: but the united states has a unique combination of geographical features that can lend itself to a lot of tornadoes. the gulf of mexico, that provides the very warm, humid air. then you also get cooler, drier air that's either coming down from canada or off the rockies. there's really no other place in the world that has the exact combination of ingredients like we do here in the united states. storm chaser: