Given the following passage  "Oklahoma City has a very active severe weather season from March through June, especially during April and May. Being in the center of what is colloquially referred to as Tornado Alley, it is prone to especially frequent and severe tornadoes, as well as very severe hailstorms and occasional derechoes. Tornadoes have occurred in every month of the year and a secondary smaller peak also occurs during autumn, especially October. The Oklahoma City metropolitan area is one of the most tornado-prone major cities in the world, with about 150 tornadoes striking within the city limits since 1890. Since the time weather records have been kept, Oklahoma City has been struck by thirteen violent tornadoes, eleven F/EF4s and two F/EF5. On May 3, 1999 parts of southern Oklahoma City and nearby suburban communities suffered from one of the most powerful tornadoes on record, an F5 on the Fujita scale, with wind speeds estimated by radar at 318 mph (510 km/h). On May 20, 2013, far southwest Oklahoma City, along with Newcastle and Moore, was hit again by a EF5 tornado; it was 0.5 to 1.3 miles (0.80 to 2.09 km) wide and killed 23 people. Less than two weeks later, on May 31, another outbreak affected the Oklahoma City area, including an EF1 and an EF0 within the city and a tornado several miles west of the city that was 2.6 miles (4.2 km) in width, the widest tornado ever recorded.",  answer the following question. Note that the answer is present within the text.  Question: What does the F stand for in EF4?
The answer to this question is:
Fujita