When does a severe tropical storm become a typhoon? How does Signal No. 1 differ from Signal No. 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, the Philippine Atmospheric, Geophysical, and Astronomical Services Administration (PAGASA), and from the World Meteorological Organization (WMO).
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.
Severe tropical storm
File photo by Ben Nabong/Rappler
Typhoon, hurricane, cyclone – they all essentially mean the same thing. The only difference is where they are located.
The WMO provided this breakdown:
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 2020 will also be used in 2024, 2028, and so on.
PAGASA drops the usage of a tropical cyclone name when it has met at least one of these two requirements:
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.
File photo by John Javellana/Rappler
Whenever PAGASA announces a list of areas under tropical cyclone wind signals, there's almost always this comment: "Why is it still sunny where I am, when we're under Signal No. 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 rain is already pouring or 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 Nos. 1 to 5, with 5 just added in 2015. When it announced the additional 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.
Tropical cyclone wind signals also used to be called tropical cyclone warning signals. But in 2019, PAGASA changed the term to emphasize that such signals are based on wind.
Rappler file photo
Signal No. 1
Signal No. 2
Signal No. 3
Signal No. 4
Signal No. 5
File photo by Ted Aljibe/AFP
Intertropical convergence zone (ITCZ)
Low pressure area (LPA)
Philippine Area of Responsibility (PAR)
File photo by Darren Langit/Rappler
PAGASA uses color-coded rainfall warnings to alert the public that there will be heavy to torrential rain and possible flooding. These warnings – yellow, orange, and red – are usually issued every 3 hours.
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