FAST FACTS: Tropical cyclones, rainfall advisories

Here's what you need to know about tropical cyclones and other terms used in everyday weather forecasts

Rappler.com

9:24:10am September 22, 2017

10:40:52am August 11, 2018

MANILA, Philippines – When does a severe tropical storm become a typhoon? How does signal number 1 differ from signal number 5? Where does a tropical cyclone get its name? What is the meaning of a yellow rainfall advisory?

We've got the answers to those questions on this page, and more, as a handy reference for the rainy season.

The information are based on official data from the Philippines' weather bureau, PAGASA, and from the World Meteorological Organization (WMO).

Kinds of Tropical Cyclones

The Philippines gets around 20 tropical cyclones each year.

PAGASA classifies tropical cyclones into 5 categories, with the severe tropical storm and super typhoon categories officially added only in 2015. The state weather bureau reevaluated the classification, especially taking into account Super Typhoon Yolanda (Haiyan), which killed thousands of people and left a trail of destruction in Eastern Visayas in 2013.

Tropical cyclones are classified based on their maximum sustained winds. 

Tropical depression

maximum winds of up to 61 kilometers per hour (km/h) 

Tropical storm

maximum winds of 62 to 88 km/h

Severe tropical storm

maximum winds of 89 to 117 km/h

Typhoon

maximum winds of 118 to 220 km/h

Super typhoon

maximum winds exceeding 220 km/h

FLOODED. Maring, which hit land as a tropical depression then later became a tropical storm, triggers floods in Manila on September 12, 2017. File photo by Ben Nabong/Rappler

FLOODED. Maring, which hit land as a tropical depression then later became a tropical storm, triggers floods in Manila on September 12, 2017. File photo by Ben Nabong/Rappler

Typhoon vs Hurricane vs Cyclone

Typhoon, hurricane, cyclone – they all essentially mean the same thing. The only difference is where they are located.

The WMO provided this breakdown:

Typhoons – Western North Pacific

Hurricanes – Western North Atlantic, Central North Pacific, Eastern North Pacific, Caribbean Sea, Gulf of Mexico

Cyclones – Bay of Bengal, Arabian Sea

Severe tropical cyclones – Western South Pacific, Southeast Indian Ocean

Tropical cyclones – Southwest Indian Ocean

Image courtesy of NOAA

Image courtesy of NOAA

Naming of Tropical Cyclones

All tropical cyclones are given local names when they form inside or enter the Philippine Area of Responsibility (PAR). PAGASA has 4 sets of tropical cyclone names that are used every 4 years. This means the names for 2018 will also be used in 2022, 2026, and so on.

PAGASA drops the usage of a tropical cyclone name when it has met at least one of these two requirements:

at least 300 deaths

P1-billion worth of damage to agriculture and infrastructure

As for international names, the Regional Specialized Meteorological Center (RSMC) Tokyo-Typhoon Center assigns names to tropical cyclones in the Western North Pacific and South China Sea. The names come from a list contributed to by several countries, including the Philippines.

Only tropical cyclones of tropical storm strength and above – not tropical depressions – get an international name.

International names are also retired if a tropical cyclone is "particularly deadly or costly," noted the WMO.

LOSS OF LIVES. Typhoon Pablo (Bopha), which hit Mindanao in December 2012, left 1,067 people dead and more than 800 others missing. File photo by John Javellana/Rappler

LOSS OF LIVES. Typhoon Pablo (Bopha), which hit Mindanao in December 2012, left 1,067 people dead and more than 800 others missing. File photo by John Javellana/Rappler

Tropical Cyclone Warning Signals

Whenever PAGASA announces a list of areas under tropical cyclone warning signals, there's almost always this comment: "Why is it still sunny where I am, when we're under signal number 1 / 2...?"

It's important to note that the signal numbers are precisely raised as a warning – to give both the government and the public lead time to prepare. When the areas are announced for the very first time, it does not mean that the rains are already pouring or the winds are already howling, which is a good thing as residents can still prepare for the potential effects of a tropical cyclone.

PAGASA uses signal numbers 1 to 5, with 5 just added in 2015. When it announced the additional warning signal back then, the state weather bureau said that "extensive and devastating damage caused by strong typhoons" had already made the 4-level warning system "inadequate." A prime example: the destruction from Yolanda.

TACLOBAN AFTERMATH. Super Typhoon Yolanda (Haiyan) claimed thousands of lives and left a trail of destruction in November 2013. Rappler file photo

TACLOBAN AFTERMATH. Super Typhoon Yolanda (Haiyan) claimed thousands of lives and left a trail of destruction in November 2013. Rappler file photo

Signal number 1

winds of 30 to 60 km/h expected in at least 36 hours 

wave height of 1.25 to 4 meters

no damage to very light damage (makeshift structures may be affected)

some banana plants may be tilted and twigs of small trees may be broken

significant damage to rice crops possible if in flowering stage

Signal number 2

winds of 61-120 km/h expected in at least 24 hours 

wave height of 4.1 to 14 meters

storm surge possible in coastal areas

light to moderate damage to high-risk structures (such as shanties which may lose their roofs), very light to light damage to medium-risk structures, no damage to very light damage to low-risk structures

wooden, old electric posts tilted or downed

poorly constructed signs or billboards damaged

most banana plants, a few mango trees, ipil-ipil, and similar trees downed or broken

some coconut trees tilted, a few broken

rice and corn crops may be damaged

Signal number 3

winds of 121-170 km/h expected in at least 18 hours 

wave height above 14 meters

storm surge possible in coastal areas

heavy damage to high-risk structures (such as shanties); moderate damage to medium-risk structures (old, dilapidated houses or mixed timber structures); light damage to low-risk structures

almost all banana plants downed, some big trees (acacia, mango, etc) broken or uprooted

dwarf-type or hybrid coconut trees tilted or downed

considerable damage to shrubbery

Signal number 4

winds of 171-220 km/h expected in at least 12 hours 

wave height above 14 meters

storm surge 2-3 meters high possible in coastal areas

very heavy damage to high-risk structures (those made of light materials may be totally or partially destroyed), heavy damage to medium-risk structures (roofs blown away, walls collapsed, doors and windows extensively damaged), moderate damage to low-risk structures (made of first-class materials but partially damaged)

all signs and billboards blown down

banana plantations almost totally damaged and most mango trees, ipil-ipil, and similar trees downed or broken

coconut plantations extensively damaged

rice and corn plantations may suffer severe losses

Signal number 5

winds of more than 220 km/h expected in at least 12 hours 

wave height above 14 meters

storm surge more than 3 meters high possible in coastal areas

widespread damage to high-risk structures (homes in coastal areas made of light materials may be totally damaged), very heavy damage to medium-risk structures (roofs blown away, extensive damage to windows and doors), heavy damage to low-risk structures (most residential and commercial buildings of mixed construction may be severely damaged)

all signs and billboards blown down

electricity and communication services severely disrupted

total damage to banana plantations

most trees broken, uprooted (including coconut trees)

WIPED OUT. This aerial photo shows uprooted coconut trees on a hill near the town of Guiuan, Eastern Samar on November 11, 2013, only days after Super Typhoon Yolanda (Haiyan) devastated the town on November 8. File photo by Ted Aljibe/AFP

WIPED OUT. This aerial photo shows uprooted coconut trees on a hill near the town of Guiuan, Eastern Samar on November 11, 2013, only days after Super Typhoon Yolanda (Haiyan) devastated the town on November 8. File photo by Ted Aljibe/AFP

Other Common Weather Terms

Cold front

forms when an advancing cold air mass displaces warmer air in its path, causing the displaced warm air to rise, which then leads to the formation of clouds and precipitation

Intertropical convergence zone (ITCZ)

a belt near the equator where the trade winds of the Northern Hemisphere and Southern Hemisphere meet, usually causing low pressure areas or thunderstorms

Landfall

when the center or eye of a tropical cyclone hits land

Low pressure area (LPA)

"region of the atmosphere in which the pressures are lower than those of the surrounding region at the same level" (WMO)

brings cloudy or rainy weather because air rises and cools near a low pressure area, causing precipitation

Philippine Area of Responsibility (PAR)

the area set by the WMO for PAGASA to monitor

weather disturbances that enter PAR or develop inside PAR should be tracked by PAGASA

Southwest monsoon

also known as hanging habagat 

characterized by warm and humid weather with heavy rainfall, due to southwesterly winds

usually from June to October in the Philippines, but may begin in late May

Thunderstorm

a local-scale weather system that affects only certain areas at a time and usually lasts up to two hours only

brings moderate to heavy rain, thunder, and lightning

SOUTHWEST MONSOON. Monsoon rains cause floods in Quezon City on July 27, 2017. File photo by Darren Langit/Rappler

SOUTHWEST MONSOON. Monsoon rains cause floods in Quezon City on July 27, 2017. File photo by Darren Langit/Rappler

Color-Coded Rainfall Advisories

PAGASA uses color-coded rainfall advisories to warn the public of heavy to torrential rain and possible flooding. These advisories – yellow, orange, and red – are usually issued every 3 hours.

Yellow

heavy rain (7.5-15 millimeters) in next 3 hours

flooding possible in low-lying areas and near rivers

residents should be aware of the weather condition

Orange

intense rain (15-30 mm) in next 3 hours

flooding is threatening in low-lying areas and near rivers

residents should be prepared for possible evacuation

Red

torrential rain (more than 30 mm) in next 3 hours

severe flooding expected

residents should evacuate or take the necessary precautionary measures

PAGASA issues daily weather forecasts every 4 am and 4 pm. When there's a tropical cyclone, bulletins are issued every 5 am, 11 am, 5 pm, and 11 pm. If a tropical cyclone is particularly threatening or many areas are affected, the frequency of bulletins becomes every 3 hours, so there are also bulletins every 8 am, 2 pm, and 8 pm, on top of the 4 original ones. – Rappler.com

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