Tornado intensity
Doppler radar data, photogrammetry, and ground swirl patterns (cycloidal marks) may also be analyzed to determine the intensity and assign a rating.
The association with track length and duration also varies, although longer-track (and longer-lived) tornadoes tend to be stronger.
[6] For many years, before the advent of Doppler radar, scientists relied on educated guesses for tornado wind speed.
However, one can find accounts (e.g. [1]; be sure to scroll down) of some remarkable work done in this field by a U.S. Army soldier, Sergeant John Park Finley.
[7][8] Some debate exists as to the usefulness of the TORRO scale over the Fujita scale—while it may be helpful for statistical purposes to have more levels of tornado strength, often the damage caused could be created by a large range of winds, rendering it hard to narrow the tornado down to a single TORRO scale category.
Because of this, in 2006, the American Meteorological Society introduced the Enhanced Fujita scale, to help assign realistic wind speeds to tornado damage.
Though the portable radar had the uncertainty of ±5–10 metres per second (11–22 mph; 18–36 km/h), this reading was probably within the F5 range, confirming that tornadoes were capable of violent winds found nowhere else on earth.
Though this reading is just short of the theoretical F6 rating, the measurement was taken more than 100 feet (30 meters) in the air, where winds are typically stronger than at the surface.
While scientists have long theorized that extremely low pressures might occur in the center of tornadoes, no measurements confirm it.
A few home barometers had survived close passes by tornadoes, recording values as low as 24 inches of mercury (810 hectopascals), but these measurements were highly uncertain.
In the United States, they typically only occur once every few years,[14] and account for approximately 0.1 percent of confirmed tornadoes.
[17] Nine EF5 tornadoes have occurred in the United States, in Greensburg, Kansas on May 4, 2007, Parkersburg, Iowa on May 25, 2008, Philadelphia, Mississippi, Hackleburg, Alabama, Smithville, Mississippi, and Rainsville, Alabama (four separate tornadoes) on April 27, 2011, Joplin, Missouri on May 22, 2011, El Reno, Oklahoma on May 24, 2011, and Moore, Oklahoma on May 20, 2013.
Well-built structures are typically unscathed, though sometimes sustaining broken windows, with minor damage to roofs and chimneys.
At this level, damage to mobile homes and other temporary structures becomes significant, and cars and other vehicles can be pushed off the road or flipped.
Vehicles can be lifted off the ground, and lighter objects can become small missiles, causing damage outside of the tornado's main path.
[citation needed] EF3 (T6–T7) damage is a serious risk to life and limb and the point at which a tornado statistically becomes significantly more destructive and deadly.
Few parts of affected buildings are left standing; well-built structures lose all outer and some inner walls.
Large, healthy trees are entirely debarked and snapped off close to the ground or uprooted altogether and turned into flying projectiles.
EF4 damage can be expected to level even the most robustly built homes, making the common practice of sheltering in an interior room on the ground floor of a residence insufficient to ensure survival.
Large, multiple-ton steel frame vehicles and farm equipment are often mangled beyond recognition and tossed miles away or reduced entirely to unrecognizable parts.
The official description of this damage highlights the extreme nature of the destruction, noting that "incredible phenomena will occur"; historically, this has included such displays of power as twisting skyscrapers, ripping roofs off of tornado bunkers, leveling entire communities, and stripping asphalt from roadbeds.
