The intensity of a tornado is usually judge based on an analysis of the damage, which is used to estimate wind speed.
yes
There is no particular instrument used for measuring tornado intensity. Ratings are based primarily on damage assessment. Occasionally doppler radar has measure wind speeds inside a tornado, but such measurements are rare.
The Fujita scale. Sometimes it is called the "F-scale" as well. The original scale rated tornadoes based on damage. The scale ranges from F0 at the weakest to F5 at the strongest. The new scale (as of 2007: Enhanced Fujita, or "EF" scale) rates according to damage. as well, but inspects damage in more detail. The new scale ranges from EF0 at the weakest to EF5 at the strongest.
In most cases the intensity of a tornado is never actually measured. Tornadoes are rated based on the severity of the damage they cause. Mobile Doppler radar can measure wind speeds in a tornado, but since such radar cannot measure ground-level winds the measurements are not used in ratings.
A scale is used to measure things...I see you've tagged "tornadoes" so you may be referring to the Fugita Scale--which is used to measure the damage caused by a tornado.
No, it is possible to measure the strength of a tornado, though direct measurements are rare. Most tornadoes have their strength estimated based on the severity of the damage they cause. Occasionally, though mobile Doppler can obtain wind measurements from a tornado. One tornado on May 24, 2011 was rated EF5 after such a radar indicated winds in excess of 210 mph.
The damage is surveyed and where damage boundaries are is noted. This is the used to show how wide the tornado is. Note that the size is not a factor in how the tornado is rated but how intense the damage is.
The most well, known scale is the Fujita scale, which runs from F0 to F5. In the U.s. it has been replaced by the Enhanced Fujita scale, which, similarly, runs from EF0 to EF5.
The wind of a tornado may be remotely measured using Doppler radar. In rare instances an anemometer makes it inside a tornado and measure wind directly. However, most tornadoes never have their winds actually measured. Instead the wind speed is estimated from the severity of the damage using the parameters of the Enhanced Fujita scale.
Tools such as the Enhanced Fujita (EF) scale, drones, satellite imagery, and ground surveys are commonly used to assess and measure tornado damage. These tools help experts determine the intensity and path of the tornado, which in turn helps with emergency response efforts and rebuilding plans.
There are what can be called Pearson numbers that can be used to rate a tornado's width and the distance it travels, but these are rarely used. In most cases a tornado's width is measured in yards or, if it is a very large tornado, in miles and fractions of a mile (meters and kilometers if you prefer the metric scale).
In some cases, Doppler radar is used to measure winds inside a tornado. However, as these measurements are rare and cannot scan ground-level winds, they are not used in ratings. The intensity rating is determined based on analysis of damage.