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Damage in Hong Kong after Super Typhoon Mangkhut 2018
Damage in Hong Kong after Super Typhoon Mangkhut 2018. (Courtesy Anthony Ivanoff / Wikipedia)

Roughly one week after barreling through the islands of the Philippines, Super Typhoon Mangkhut made landfall on September 16 in China’s Guangdong province.

The typhoon and its resulting landslides have left hundreds dead or missing in the Philippines, but remarkably, Chinese officials have so far only reported four deaths due to falling trees and building materials, zero of which were in the coastal city of Hong Kong. In comparison, Hurricane Florence and the dangerous floods that came with it on the southeastern coast of the United States have left over 40 dead.

In the aftermath of Florence, some U.S. weather experts are pushing to change the nation’s hurricane rating system to reflect what they consider to be a more complete risk profile. Currently, the U.S. categorizes its hurricanes into 5 categories based on the Saffir-Simpson scale, which measures wind speed. Critics say that this nearly 50-year-old system does not always convey the complete strength of the storm because it does not address other impacts, such as rainfall or storm surge, which can result in a misinterpretation of risk. After Hurricane Florence, some locals reported that they did not evacuate after learning the storm had been downgraded to a lower category. However, despite the lessened wind speeds, Florence still brought life-threatening storm surges.

By comparison, Hong Kong has a more complex typhoon rating system than the hurricane categories of the United States, and the system ties together several elements of risk communication that the U.S. system does not.

Firstly, there are 6 classes of tropical cyclones in the Pacific based on wind speed: Tropical Depression, Tropical Storm, Severe Tropical Storm, Typhoon, Severe Typhoon, and Super Typhoon. The Severe and Super Typhoon classes were added in 2009, in a bid to increase the public’s risk perception and encourage extra vigilance upon the approach of more intense storms.

When a tropical cyclone is set to hit Hong Kong, the Hong Kong Observatory will also assign the storm a number. Although the number scale goes up to 10, there are actually only five categories corresponding to the numbers 1, 3, 8, 9, and 10. Categories 1 and 3 serve as general warnings for strong winds. Category 8 signifies strong gale or storm force winds, and is split into 4 subcategories (8NW, 8SW, 8NE, and 8SE) that describe the direction of the storm. From these subcategories, residents automatically know which waterfronts and coastlines to avoid. A category 9 storm means that the gale or storm force winds are increasing or are expected to increase significantly. Finally, a category 10 storm constitutes typhoon force winds of 118 km/hr or above. Each category has its own symbol, including the number and a shape for easy identification.

To account for the amount of rain, Hong Kong has a separate set of rainstorm signals scaling from Amber, to Red, to Black. An Amber rainstorm signal means that heavy rain is falling or expected to fall at a rate of over 30mm/hr and is likely to continue. A Red signal means rainfall over 50mm/hr, and a Black signal over 70mm/hr. These color rainstorm signals should not be confused with the China Meteorological Administration colored hurricane alerts which include blue, yellow, orange, and red, and are also based on wind speed.

Perhaps more important than the ratings themselves, each number and color is associated with a specific action. For example, Category 8 or above means stay indoors, and Category 9 or above means that all transportation is suspended. Similarly, an Amber signal warns farmers and fish pond owners living in low lying areas to take precaution against flooding, while a Red signal precautions travelers to carefully consider road and weather conditions. A Black signal means to shelter indoors unless it is dangerous to do so. These two rating systems are used in conjunction with each other to account for both rain and wind. For example, a storm warning might have both an 8 symbol and a Red rainstorm signal.

Hong Kong’s Labour Department additionally does an excellent job in laying out specific guidelines for when employees are required to report for work depending on the rating of the storm and the timing of the warning. The guidelines strongly emphasize the clear communication of the employer’s dismissal, attendance, and payment protocol before the onset of a storm, such that when there is a typhoon, employees can evacuate or continue to operate smoothly and safely without worrying about job security.

The Hong Kong two-fold system of rating typhoons may be somewhat complicated and confusing to some, but it accounts for both the risks associated with wind and rain. The attachment of specific action and contingency plans for each category also enhances the communication of risk and the smooth recovery of operations after a storm. As U.S. communities work to become more resilient to strong storms such as hurricanes, an important part of the conversation should be this type of clear communication, to better prevent harmful consequences due to misinterpretations of risk.

The different systems used by Hong Kong and the United States in how tropical cyclones and storms are classified.
The different systems used by Hong Kong and the United States in how tropical cyclones and storms are classified.


Sources and Further Readings

Typhoon Mangkhut: Deadly storm makes landfall in south China after leaving at least 64 dead in Philippines – The Independent

Typhoon Mangkhut death toll climbs – SBS News

After the Storm: Photos From Hong Kong, Battered by Typhoon Mangkhut – New York Times

Meteorologist Pushing To Change Hurricane Rating System – CBS Philly

Saffir-Simpson Hurricane Wind Scale – National Hurricane Center

Tropical Cyclone Classification – Hong Kong Observatory

Hong Kong’s Tropical Cyclone Warning Signals – Hong Kong Observatory

Rainstorm Warning System – Hong Kong Observatory

Typhoon alerts – China Meteorological Administration

Best Practices in Disaster Preparedness: Hong Kong – Palafox Architecture

Code of Practice in times of Typhoons and Rainstorms – Hong Kong Labour Department