MANILA - Dagupan City in Pangasinan recorded a 51 degrees Celsius heat index on Tuesday, the highest among areas monitored in the country by PAGASA during the day.
It was logged at 2 p.m., according to the state weather bureau.
The heat index in Aparri town, Cagayan and in Sangley Point, Cavite also reached 50 degrees Celsius.
In Metro Manila, a 46 degrees Celsius heat index was measured in NAIA in Pasay City, and 41 degrees at the Port Area in Manila.
According to PAGASA, heat index is the human discomfort index or the "apparent" temperature which humans perceive or feel like the temperature affecting their body."
A 41-54 degrees Celsius heat index is classified as "danger", while 32-41 degrees is "extreme caution."
Under the "danger" range, "heat cramps and heat exhaustion are likely" and "heat stroke is probable with continued activity".
The "extreme caution" range, meanwhile, may possibly cause heat cramps and heat exhaustion, and "continuing activity could result heat stroke."
Last May, Dagupan logged 53 degrees Celsius -- the country's record-high for this year -- and 52 degrees Celsius in the same week.
The high heat index is among factors that contributed to electric supply issues currently being experienced in the country that resulted in power outages, the Department of Energy had said.
The Luzon grid was placed under red alert on Tuesday with potential power outages due to low power supply, the National Grid Corporation of the Philippines said.
The red alert status was in place from 10 a.m. to 5 p.m., the NGCP said in an advisory, as well as from 6 p.m. to 10 p.m.