Debrief-Typhoons

When does a storm become a typhoon?

TYPHOON GONI A house partially submerged in debris yesterday after Typhoon Goni swept through Daraga, Albay province, in the Philippines. TYPHOON MOLAVE A man clearing debris from his house in central Vietnam's Quang Ngai province last Saturday, afte
TYPHOON GONI A house partially submerged in debris yesterday after Typhoon Goni swept through Daraga, Albay province, in the Philippines. PHOTO: REUTERS
TYPHOON GONI A house partially submerged in debris yesterday after Typhoon Goni swept through Daraga, Albay province, in the Philippines. TYPHOON MOLAVE A man clearing debris from his house in central Vietnam's Quang Ngai province last Saturday, afte
TYPHOON MOLAVE A man clearing debris from his house in central Vietnam's Quang Ngai province last Saturday, after Typhoon Molave struck.PHOTO: AGENCE FRANCE-PRESSE
TYPHOON GONI A house partially submerged in debris yesterday after Typhoon Goni swept through Daraga, Albay province, in the Philippines. TYPHOON MOLAVE A man clearing debris from his house in central Vietnam's Quang Ngai province last Saturday, afte
TYPHOON HAISHEN Japanese police at the site of a landslide in Miyazaki prefecture on Sept 8, after Typhoon Haishen hit the island of Kyushu.PHOTO: AGENCE FRANCE-PRESSE

What's the story and why does it matter?

Parts of the Philippines were devastated by Super Typhoon Goni on Sunday, the 18th such storm to hit the country this year. Goni, now downgraded to a tropical storm, is forecast to make landfall in central Vietnam tomorrow night, a region already reeling from several other storms in recent weeks.

Another storm, Atsani, looks set to hit the Philippines later this week. These natural disasters are crimping efforts to bring the Covid-19 pandemic under control in the affected countries.

How are typhoons formed?

Typhoons, hurricanes and cyclones are large rotating storms that form over tropical waters. They can pack wind speeds of at least 120kmh. Storms that form in the Western Pacific Ocean are called typhoons; those that form in the Atlantic or Eastern Pacific are known as hurricanes, while those in the Indian Ocean and South Pacific are termed cyclones.

Warming ocean waters intensify these storms as they provide them with more heat and therefore energy. The stronger the storm when it makes landfall, the more deadly and destructive it is.

Where and when do they strike?

Typhoons usually form more than 10 degrees north and south of the Equator, and move towards the poles and away from the Equator.

The Philippines is the first major landmass facing the Pacific typhoon belt and gets hit by an average of 20 raging storms a year. Vietnam, Cambodia, Thailand and Laos are often in the path of such storms. Beyond South-east Asia, Japan, South Korea, China and India are also prone to severe storms.

Typhoon season usually starts around June and peters out by November. But some of the strongest typhoons in recent years have hit closer to the end of the year.

What do the different category levels mean?

The intensity of a typhoon is measured by its maximum sustained wind speed. But a 15 per cent increase in intensity results in a 50 per cent rise in destructive power.

The most common classification method is the Saffir-Simpson scale, which categorises storms across five levels, with a Category 5 storm (sustained winds of at least 252kmh) being the strongest.

Another classification, from the Joint Typhoon Warning Centre, defines a super typhoon as one with a sustained wind speed of at least 241kmh.

Super Typhoon Goni, a Category 5 storm with gusts of up to 310kmh, is the strongest storm to hit anywhere in the world so far this year.

How are warnings issued, and how do various countries cope?

Tropical storm warnings vary from country to country. In the Philippines, signals are raised at least 36 hours ahead of a storm making landfall, and TV and radio stations play a part in disseminating the information to rural households.

In some parts of rural China, village leaders and volunteers even go door to door to alert villagers.

Generally, residents are evacuated from areas deemed most at risk, with the military on standby.

Boats are brought back ashore, outdoor boarding and hanging signs are taken down, and people and businesses are advised to secure their doors, windows and roofs to prevent damage.

How are typhoons named?

The World Meteorological Organisation maintains lists of names to be used for such storms.

The names have to be short, distinctive, and relevant to their cultural and geographic areas so that they are easy to remember.

For the Western Pacific/South China Sea basin, where many languages are spoken, names are contributed by countries in the region.

If a typhoon is particularly destructive - such as Super Typhoon Haiyan in 2013 - that name will not be used again.

A version of this article appeared in the print edition of The Straits Times on November 03, 2020, with the headline 'When does a storm become a typhoon? '. Print Edition | Subscribe