Windshear will cause any precipitation particles that form to grow to the side ofthe updraft, so that the resulting downdraft of cold air mixed with precipitation is separated from the updraft. This will keep the updraft alive. As the updraft slowly rotates around, it may wrap part of the downdraft around it, producing a second, small, downdraft on the other side of the updraft. The wrapping of the precipitation outflow around the updraft is visible on radar as a precipitation echo in the shape of a hook. Therefore, such a hook-echo is an indication of rotation, and possibly the generation of a tornado.
Tornadoes can occur almost anywhere on earth, but in most places, their frequency is very low.
For instance, in Holland, we get only a few F0 tornadoes per year, and F3 to F4 tornadoes only once every 30 years or so on average. However, in North America, tornadoes occur with a relatively high frequency, as the conditions there in spring and summer are often just right for supercell storms to develop. Down the middle of North America runs a strip of land, roughly from Arkansas to Colorado and all the way from Texas to North Dakota, which is called Tornado Alley, due to the high number of tornadoes striking there. As said, a vortex or so-called funnel cloud is a tornado as soon as it reaches the surface. However, many more vortices never make to the ground.