Heat index in Central Luzon: how often it reaches danger

What PAGASA’s heat index bands mean

The heat index is how hot it feels once humidity is added to the air temperature. PAGASA works it out with the Rothfusz regression, an equation from the United States weather service, and puts each value in one of four bands: caution 27 to 32 °C, extreme caution 33 to 41 °C, danger 42 to 51 °C, and extreme danger 52 °C and above.

PAGASA’s chart warns that heat cramps and heat exhaustion are already possible in the extreme caution band, so a day below danger is not a safe day for hard work outdoors. The scale assumes shade: the US weather service says direct sun can raise the heat index by up to 15 °F, about 8 °C.

Is it getting worse? What the counts show

Average danger days a year, by decade, counting only years with a near-complete record.

“n/a” means fewer than five complete years in that decade. Two things hold up. Danger days are normal on the Zambales coast: Iba has had at least 17 in every complete year since 2002. And afternoons are warmer at most stations: the average 2 PM temperature from March to May rose at Iba and NAIA since 1980 (about 0.4 and 0.2 °C a decade) and slightly at Clark since 1945 (about 0.1 °C a decade), while Subic and Science Garden show no clear change.

Clark’s 2020s are the highest decade in its record as measured, and that is the part we trust least. Around 2016 Clark’s humidity readings jumped: its dew point, compared with NAIA’s, rose by about 1.6 °C, which looks like a sensor change. Take that step out of Clark’s 2017 to 2026 record and its 27.8 danger days a year fall to about 4.5, below its 1980s. Over all its complete years Clark’s count rises by about 2.7 days a decade (95% interval 0.6 to 4.8); leave out 2016, when its humidity sensor probably failed, and it is 2.4 (−0.3 to 5.0), and since 1980 the rise is not clear. Subic shows no clear change, and Science Garden in Quezon City shows fewer danger days than in the 1990s, with its humidity readings falling too.

If someone is overcome by heat

The Department of Health calls heat stroke a medical emergency. Its signs include loss of consciousness, confusion or seizures. DOH says to move the person to a shaded, cool place, remove outer clothing and cool the skin with cold compresses, ice packs or a cold wet cloth, to give sips of cool water only if the person is conscious, and to get them to hospital immediately. The emergency hotlines page lists the numbers.

The World Health Organization’s one-page guide puts calling for emergency help first, before cooling; DOH lists the cooling steps first, then hospital. Our reading: get help on the way and start cooling at once, which follows both.

For school days, DepEd’s October 2026 memo for the El Niño says extreme heat no longer triggers automatic nationwide class suspensions. School heads go by temperature and humidity read inside classrooms at 8 AM, noon and 2 PM: at 27 to 32 °C they may shorten in-person hours, at 33 to 41 °C physical classes stop and learners get lighter home tasks, and at 42 °C and above all classes, in school and at home, stop. Governors and mayors have also suspended classes for heat, as in the cases above.

How we counted

• The reports are PAGASA’s station reports, and before 1992 at Clark and Subic those of the US bases there, from NOAA’s hourly files to 24 August 2025 and from Ogimet’s copies of the same reports to 31 October 2026.
• For each day we took the highest heat index of the 8 AM, 11 AM, 2 PM and 5 PM readings, the hours PAGASA’s own observed table uses, rounded it to a whole degree and put it in PAGASA’s band. A day needs its 2 PM reading.
• A year counts as complete when at least 85% of March to June days and 75% of all days have a value. Readings outside plausible ranges, or far from the rest of that day’s readings, were dropped.
• Cabanatuan and Muñoz are left out: their humidity readings run several degrees above nearby stations, which would inflate their counts.
• Using only the four report hours gives a lower count than using every reading. Counting every hourly reading would have added 72% more danger days at Clark and 60% more at NAIA from 2017 to 2025.
• On the two days where a PAGASA value for these stations was reported in the news (Iba, 28 April 2024, and Clark, 6 May 2024), our values are within a degree of it.