Heat Index or WBGT?

Published in

Our Planet. Our Life.

5 min readAug 8, 2024

My Story of Discovery

A meteorologist measuring ambient conditions. — @RajivChopra. A meteorologist.

Over the last weeks, I have read several articles from Indian publications stating we should measure the heat index rather than the dry bulb temperature. The temperatures recorded and published are dry bulb temperatures, and they tell you what the ambient temperature is at a particular moment. When the government (in India) reports the temperature, it primarily reports the dry bulb temperature.

I live in the National Capital Region of Delhi (Delhi-NCR), and when I returned to this region in 2012, the temperature forecasts for September brought me much happiness. Temperatures in May and June can rise to 42–45 degrees Celsius. So, when I noticed predictions (for September) of temperatures in the 32–25 degrees Celsius range, my happiness quotient rose to the sky. I had just left my corporate career, and even though we spoke much about sustainability, we didn’t differentiate about the ways to measure temperature.

I moved from the comfort of a centrally air-conditioned office to my home office, and to save on electricity bills, I refused to use the air conditioner. The temperature forecasts for September brought no relief, and I wondered why I felt suffocated. When my blood pressure rose, I’d drink copious quantities of water and lie flat on the bed in an air-conditioned room. My wife forced me to use the air conditioner.

This experience prompted me to examine the forecasts with more care, and I noticed, in smaller print, the ‘feels like’ temperature. Further examination and correlation created that ‘Eureka’ moment when I discovered the connection between the dry bulb temperature, humidity, and the ‘real feel temperature.’ A year later, I discovered we must measure the internal humidity of our rooms and homes.

Heat Index or WBGT?

The heat index measures how hot you feel and combines the effects of dry-bulb temperature and humidity. In effect, it tells you how miserable you feel! High humidity levels prevent sweat from evaporating and inhibit the body’s cooling ability. Therefore, the heat index is a good measure but insufficient.

Ever since I discovered the ‘Wet Bulb Globe Temperature,’ I have become convinced this is the best measure because it also gives a simple measure of heat stress and death risk.

I quote from probablefutures.org

Wet-bulb temperature only takes into account heat and humidity and is measured in the shade. Wet-bulb globe temperature combines that heat and humidity measure with sunlight and wind, to create a fuller picture of the conditions a person would experience.

And, I quote again.

Without the body’s self-cooling function working, our body temperature rises, like when we have a fever. If our bodies overheat, our organs become stressed, and ultimately begin to fail. High enough wet-bulb temperatures can completely restrict our ability to be anywhere that isn’t climate-controlled — scientists estimate humans cannot survive in wet-bulb temperatures above 35°C for more than a few hours.

The Complexity of WBGT Calculations

©RajivChopra. Person looking at the temperature on a laptop screen.

Measuring WBGT is complex because it measures heat stress by taking the following factors into account:

· Air temperature

· Wind-speed

· Humidity

· Sun angle

· Cloud cover

The listed factors should give anyone an excellent idea of the complexity of calculating WBGT. Unless you are a climate scientist, a mathematician, or a curious person with excellent mathematical skills, I advise you to avoid the abstruse mathematical calculations required to measure WBGT.

We only need simple standards and warning signs.

Simple measures will suffice: between 30 and 32 Celsius WBGT, the risk of incidents relating to heat stress increases dramatically.

Humans cannot survive over a few hours at a WBGT of 35 degrees Celsius.

My recommendation to the Indian Met (the IMD) is to measure and communicate all three: the temperature, the heat index, and the WBGT. I also believe we must start sensitizing people to the dangers of heat stress and the warning signs by measuring and reporting temperature and understanding the body’s signals.

In my experience, when any discussion becomes technical, people roll their eyes, walk away, and call you a nerd. I believe this is dangerous, but we must acknowledge this behavior and give people information in simple, bite-sized bits that are digestible. Behaving like the proverbial ostrich is not a solution and will lead to catastrophe. Neither do we need to predict doom all the time. I came across an excellent and prescient quote from Carl Sagan.

We’ve arranged a society on science and technology in which nobody understands anything about science and technology, and this combustible mixture of ignorance and power sooner or later is going to blow up in our faces.

People fear science, and it is the government’s job and scientists and concerned citizens' job to educate people without scaring them.

Our Gods will not protect us.

I’ve read far too many articles predicting that North India will be unlivable in the following decades. Hospitalizations because of heat stress have increased, and even the ‘man on the street’ now talks about climate change. People are not fools and are concerned about the impact of climate change and heat.

Most Indians act as though our millions of gods will protect us from all harm, but we place our faith in gods who cannot or will not, pull us out of the impending disaster. If we do not measure and report all three—dry bulbs, heat index, and WBGT—we will be in trouble sooner than we realize.