I am going to ask and answer this question since I played in one for the first time in almost 40 years yesterday, and because I find it surprising that no one has asked this previously.
So, "what is a "tornado" in chess?"
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I do not know if this term is used in other countries, but in the U.S., a “tornado” is a swiss chess tournament with four rounds in one day, typically at a sudden death in 60 minutes time control. It was probably given that nickname because your mind is spinning from so much chess in one day.
I played in the “Queen City Tornado” yesterday, and there were rounds at 10 a.m., 1 p.m., 3:30 p.m., and 6 p.m.. It makes for a very long day.