Ranking tornadoes began in 1971 with the introduction of the Fujita Scale.
Since then, the National Weather Service has modified the scale based on research and field expertise of trained meteorologists and engineers.
The current scale is called the Enhanced Fujita Scale.
The current scale has 28 damage indicators based on elements of modern day structures. The damage indicators then have a degree of damage. It also provides an upper and lower boundary for wind damage estimates.
The threshold for winds in the Enhanced Fujita Scale starts at 65 miles per hour. The lowest ranking is an EF0 with wind speeds of 65-85 miles per hour. This is where most California tornadoes rank.
The three tornadoes that hit Glenn County on Wednesday were ranked as EF1s, which is above the average California tornado.
The strongest tornadoes are ranked as EF5s with wind speeds greater than 200 miles per hour. EF5 tornadoes cause complete destruction. The tornado that hit Moore, Okla. on May 20, 2013 was ranked as an EF5 with peak winds estimated at 210 miles per hour.