The heat index: What it is, and how it’s computed

The heat index is the measure of the temperature that a person feels in contrast to the actual air temperature.

Essentially, it measures "human discomfort" and is the "apparent temperature" felt by the human body. It is computed by factoring in the humidity as well as the air temperature.

The apparent temperature may also vary for each person because everyone has a different height and weight: a thin person walking outdoors on a sunny day in light clothing will feel less discomfort than the heavier pedestrian wearing bulky clothing next to him.

According to PAGASA, full exposure to sunshine can raise the heat index by as much as 9°C. Higher levels of humidity also drive up the heat index and make it more difficult for people to cool off.

Last week, it was reported that the heat index (as computed by the formula that the National Oceanic and Atmospheric Administration uses) reached 53°C in Cabanatuan City. According to PAGASA’s scale, heat index temperatures from 41 to 54°C are dangerous and could possibly lead to heat stroke.

But is it possible for the heat index to reach this high? According to Thelma Cinco, head of PAGASA’s Impact Assessment and Applications Section (IAAS) of the Climatology and Agrometeorology Division (CAD), the formula that PAGASA (and NOAA) uses is very sensitive to the measured relative humidity. “Sometimes [heat index values which do not accurately reflect the perceived temperature] happen with high values of relative humidity,” she said.

According to IAAS, there’s no available historical data for heat indexes. Temperature record highs and lows, however, shows that in 1987 the Science Garden in Quezon City reached a record high of 38.5°C.

PAGASA issued some tips on lowering one's heat index: 

• Stay indoors as much as you can, if possible on the lowest floor away from the sunshine (if air conditioning is not available elsewhere). 
• Wear lightweight, light-colored clothing that will reflect the sun's energy rather than absorb it. 
• Stay hydrated: drink plenty of water and avoid liquor as it dehydrates the body. 
• Protein-rich foods can increase metabolic heat. Eat more frequent, smaller meals.

— BM, GMA News