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.

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.