The Tornado class will not start races if it's below 6 knots or above 25. A race will be abandoned if the wind drops below 3 knots for 15 minutes, or if the wind shifts significantly and precludes fair sailing.

Ref: http://www.tornado.org/html/rules.asp , Appendix C.

I think these are sensible rules.. (I dont know if a protest on a significant wind shift that precludes fair sailing have won yet)