photographed by several eyewitnesses as it passed through residential and commercial areas of Dallas. Visible for much of its path, the tornado, at the time, was the most observed in recorded history: 125 observers produced thousands of photographs and hours of high-quality, 16-mm film measuring in length. The tornado was highly visible due to its slow, forward speed, a lack of precipitation, and its coincidence with ideal, late-afternoon lighting. Occurring shortly before the end of the workday, the tornado passed just west of Downtown Dallas and was seen by many business and factory workers. Many TV studios had time to film the tornado from rooftops. A researcher from the Severe Weather Forecast Unit in Kansas City noticed that several old theories were proven false during the Dallas tornado. One of the theories was that all air and debris flowed inward into the funnel and then upward, but on the outside edges of the funnel debris and people were even lifted. WFAA-TV in Dallas produced a 30-minute documentary about the tornado about one week later. The tornado became the subject of several scientific papers analyzing the life cycle of and wind speed speeds in a tornado. Among the studies was the first-ever photogrammetric analysis of wind speeds in a tornado. The film of the tornado is still regarded as being of exceptionally high quality and sharpness. Additionally, structural surveys following this and the Fargo tornado later in the year provided data that contributed to the development of the Fujita scale. Non-tornadic effects Severe thunderstorm winds reached in Toledo, Lucas County, Ohio, on April 5. hail occurred in Kiowa and Latimer counties in Oklahoma on April 2. Aftermath and recovery See also December 2015 North American storm complex – Yielded an EF4 tornado in Rowlett and Garland that killed nine people, the deadliest in the Dallas–Fort Worth Metroplex since 1957 List of tornadoes and tornado outbreaks List of North American tornadoes and tornado outbreaks 2000 Fort Worth tornado outbreak – Deadly F3 tornado struck Downtown Fort Worth and Arlington on March 28 Notes References Sources F4 tornadoes by date Tornadoes of 1957 Tornadoes in Texas Tornadoes in Oklahoma Tornadoes in Mississippi 1957 natural disasters in the United States Tornado outbreak sequence