Get the Heat Index right: an important first step to protect people from heatwaves

India has experienced unprecedented high temperatures this summer. Although this is a worldwide problem tied to climate change, a warm tropical country like India experiences intense heat that readily exceeds human tolerance. Increasingly intense and long heatwaves are expected to occur in India with rising frequency in the coming years, with no end in sight until climate change itself stabilises. Accurately quantifying the effects of heat on people is imperative to reduce morbidity and mortality due to heat stress.

Human beings need to maintain a core temperature within a narrow range centred on 37°C. Metabolic heat generation in the body requires us to expel excess heat to the environment to maintain our core temperature in the range. Heatwaves can cause heat exhaustion, illness or death because people caught in a heatwave are unable to expel excess body heat.

The Heat Index was developed in 1979 by R G Steadman to convey the discomfort experienced by a human body exposed to air temperatures and humidities commonly experienced in the United States. However, the Heat Index was developed under simplified assumptions which do not apply under extreme temperatures and humidities of intense heatwaves. Scientists in the US National Weather Service extrapolated the Heat Index to a range of conditions that Steadman did not explore. These extrapolations have led to erroneously under-reported heat stress under conditions of high air temperature and humidity.

Recently, Yi-Chuan Lu and David M Romps from UC Berkeley published an “extended heat index” (EHI) which accurately quantifies the heat stress even under the extreme conditions of heatwaves that Steadman did not consider. Like the Heat Index, the EHI too quantifies heat stress conditions in terms of an equivalent temperature experienced by a person under certain standard conditions of air movement and humidity. Unlike Steadman’s work, EHI assumes realistic conditions of air movement, radiation input to the skin, and the limitations on the heat transfer from the body’s core to the skin. In addition, the EHI also identifies conditions where a long exposure will cause illness or death from hyperthermia. One limitation of the EHI, similar to that of the Heat Index, is that they both measure heat stress assuming the person is a young adult in optimal health, and undertaking light effort (e.g. walking), with plenty of access to water, and in the shade, (i.e., with no additional thermal load from exposure to sunlight). However, 49 per cent of India’s workers labour outdoors, with no protection from heat.

The EHI gives the same values as the Heat Index for mild conditions. For instance, at 30°C and 40 per cent relative humidity, both the EHI and the Heat Index convey a heat stress of roughly 30°C. However, at 45°C and 40 per cent relative humidity, the EHI conveys a heat stress of 70°C, while the Heat Index conveys a heat stress of 63°C thus significantly underestimating the heat stress. For example, during the severe heatwave of May 2015 in India, the highest Heat Index near Rourkela, Odisha, India [22N 84.75E], was 42°C, 24°C lower than the EHI of 66°C at that same location (see Figures below). Furthermore, the US National Weather Service estimates that exposure to sunlight can raise the Heat Index by about 8°C.  So, at the worst hour in the sun on May 24, 2015 in Rourkela, the EHI had likely reached a deadly 74°C.

Figure 1A: Steadman Heat Index (HI) Map for South Asia for the hottest hour of May 24, 2015

Country borders are as shown in Cartopy Python package software developed by the UK Met Office, and do not claim legal validity.

Consistent with common practice worldwide, HI has been estimated using the US National Weather Service’s approximate formula for Steadman Heat Index. Dash-dot contour lines mark every 2.5°C change in the HI. Weather data from copernicus.eu.

Location of the highest EHI in the area, near Rourkela, Odisha, is denoted by a circle. It does not stand out in the Heat Index map because the HI underestimates the experienced heat stress.  

Figure 1B: Extended Heat Index (EHI) Map for South Asia for the hottest hour of May 24, 2015

Country borders are as shown in Cartopy Python package software developed by the UK Met Office, and do not claim legal validity.

EHI more accurately estimates the heat stress that is underestimated in Figure 1A.

Location of the highest EHI in the area, 66, near Rourkela, Odisha, is denoted with a circle. Dash-dot contour lines mark every 2.5°C change in the extended heat index. Weather data from copernicus.eu.

How can the EHI help protect Indians caught in heatwaves? EHI can identify conditions where heat stress will prove fatal even for a healthy adult performing moderate effort. For healthy adults, EHI will flag conditions where only a short duration of exposure should be allowed. For older people, youngsters, and those with compromised health, these flagged conditions would be dangerous. Publishing the EHI in real time on a website would allow anyone with a smartphone to see whether they need to seek shelter, or intervene to save a friend or family from dangerous heat stress. Government institutions may be able to require intermittent access to cooling facilities if EHI approaches dangerous conditions for outdoor workers. Outdoor work in the sun could be deferred or performed preemptively in the early morning if the EHI in the early afternoon exceeds dangerous conditions. This is not an exhaustive list of interventions.

Unprecedented dangers from unprecedented heatwaves require new ways of protecting our communities. The old ways are no longer adequate because the old temperature records are going to be broken again and again as climate change inexorably presses upon us. In addition to the existing guidelines for Heat Action Plans from the National Disaster Management Authority, it is time to strengthen the way we measure, report and deal with heat emergencies.

Ashok Gadgil is Distinguished Professor Emeritus, and Elif Kilic is Masters Student, in the Civil and Environmental Engineering Department of UC Berkeley.  Both are affiliated with the India Energy and Climate Center of UC Berkeley (http://iecc.gspp.berkeley.edu)

Views expressed are the authors’ own and don’t necessarily reflect those of Down To Earth