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How is a tornado rated?

   Written by: LEE Lap-shun

    A tornado is a violently rotating column of air extending from a thunderstorm to the ground.  It is not easy to measure the maximum winds of a tornado directly and its intensity is usually estimated through examining the damage it caused.  Prior to February 2007, this was commonly done using the Fujita Scale (F Scale) devised by Dr. Fujita Tetsuya.  Since then, the National Weather Service of the United States has adopted an Enhanced Fujita Scale (EF Scale) to rate tornadoes.

    The original F Scale, ranging from 0 to 5, gave only a general description of the damage to a structure that a tornado could cause.  It gave little consideration for the strength of the building hit by a tornado and other factors, such as the impact on a structure by debris that might cause the structure to receive higher damage at lower wind speeds.  As a result, some tornado wind speeds were over-estimated.

    The EF Scale also ranges from 0 to 5 but takes into account the strength of construction.  There are different damage descriptions for estimating the tornado wind speeds for different structures including 23 types of buildings and 5 additional objects like trees and poles.  Details of the damage descriptions in the EF scale can be found at the following website: http://www.spc.noaa.gov/faq/tornado/ef-scale.html .  The EF Scale thus allows more precise assessment of tornado severity.  Furthermore, the EF5 category has no upper limit on the wind speed, unlike the F5 category of the old scale.

    Fujita (F) Scale Wind speed (kilometres per hour) corresponding to F Scale     Enhanced Fujita (EF) Scale Wind speed (kilometres per hour) corresponding to EF
    F0 64-116   EF0 105-137
    F1 117-180   EF1 138-178
    F2 181-253   EF2 179-218
    F3 254-332   EF3 219-266
    F4 333-418   EF4 267-322
    F5 419-512   EF5 over 322
    A comparison of winds speeds
    under the Fujita Scale and the Enhanced Fujita Scale

    In Hong Kong, occurrences of tornado are mostly associated with unstable weather during the rain season from May to September.  Past records showed that there were a total of 9 cases of tornadoes in Hong Kong since 1982.  The last reported tornado in Hong Kong occurred at Chek Lap Kok on 28 October 2005.

    A tornado over waters is called a waterspout.  Since 1959, there were 28 cases of waterspouts sighted within Hong Kong Waters, the last being the one observed at Tai O on 13 August 2005.

    A fast rotating column of air extending from a thunderstorm but not touching the ground or water is called a funnel cloud.  Since 1959, there were 16 cases of funnel clouds sighted within 460 kilometres of Hong Kong.  The last one occurred on 26 June 2007, about 30 kilometres northwest of Chek Lap Kok.

figure1e
Figure 1    Picture of a funnel cloud observed
to the southwest of Chek Lap Kok (taken at 10:29 a.m., 9 July 2003).

figure2e
Figure 2    Picture of a funnel cloud and a waterspout observed
to the southwest of Chek Lap Kok (taken at 8:02 a.m., 31 July 2003).


   Reference:
    "The Enhanced Fujita Scale" webpage of the National Oceanic and Atmospheric Administration (NOAA), USA:  http://www.spc.noaa.gov/efscale

Last revision date: <20 Dec 2012>