<P> Tornado occurrence is highly dependent on the time of day, because of solar heating . Worldwide, most tornadoes occur in the late afternoon, between 3 pm and 7 pm local time, with a peak near 5 pm . Destructive tornadoes can occur at any time of day . The Gainesville Tornado of 1936, one of the deadliest tornadoes in history, occurred at 8: 30 am local time . </P> <P> The United Kingdom has the highest incidence of tornadoes, measured by unit area of land, than any other country in the world . Unsettled conditions and weather fronts transverse the Islands at all times of the years and are responsible for spawning the tornadoes, which consequently form at all times of the year . The United Kingdom has at least 34 tornadoes per year and possibly as many as 50, more than any other country in the world relative to its land area . Most tornadoes in the United Kingdom are weak, but they are occasionally destructive . For example, the Birmingham tornado of 2005 and the London tornado of 2006 . Both registered F2 on the Fujita scale and both caused significant damage and injury . </P> <P> Associations with various climate and environmental trends exist . For example, an increase in the sea surface temperature of a source region (e.g. Gulf of Mexico and Mediterranean Sea) increases atmospheric moisture content . Increased moisture can fuel an increase in severe weather and tornado activity, particularly in the cool season . </P> <P> Some evidence does suggest that the Southern Oscillation is weakly correlated with changes in tornado activity, which vary by season and region, as well as whether the ENSO phase is that of El Niño or La Niña . Research has found that fewer tornadoes and hailstorms occur in winter and spring in the U.S. central and southern plains during El Niño, and more occur during La Niña, than in years when temperatures in the Pacific are relatively stable . Ocean conditions could be used to forecast extreme spring storm events several months in advance . </P>

How strong do winds have to be for a tornado