The greatest incidence of tornadoes is in North America, and especially just west of the Mississippi Valley in an area labeled "Tornado Alley." In actual numbers observed, Australia ranks second to the United States. The United States is notable for the incidence of severe tornadoes of scale F4 or F5. Within the United States, Texas records the greatest number, usually 15 to 20 percent of the nation's annual total of about 1,000. On an areal basis, however, Texas ranks ninth, far behind Oklahoma. A rather steady increase in the annual total has been observed, probably as a result of the improving reporting system.