Now Reading
As Heat Engulfs North India, Keep an Eye on the Wet-Bulb Temperature

As Heat Engulfs North India, Keep an Eye on the Wet-Bulb Temperature

Women trying to protect themselves from the heat during a protest rally at Jantar Mantar in New Delhi, March 2015. Representative photo: PTI/Subhav Shukla

  • According to the IMD, there is a heatwave on some days when the average maximum temperature on those days is 4.5-6.4º above the long-term average.
  • This is an incomplete definition because one can also feel hotter if the humidity is higher – which is the case in parts of South India and Southeast Asia.
  • The effect of humidity on how we feel the heat is expressed by the wet-bulb temperature: if it exceeds 35º C, outdoor activity can become life-threatening.
  • A combination of high heat and high humidity has created sufficiently higher wet-bulb temperatures in many parts of the world, and sooner than expected.

Bengaluru: Global warming caused by human activities has precipitated a sixth mass extinction event in which birds, animals and insects are being forced to adapt to a changing world too fast for evolution to manage – or risk being wiped out en masse. But right now, North India – including the national capital of New Delhi – is sizzling at a surreal daily high of 45º C, in the grip of a brutal heatwave.

When it is this hot, biological adaptation for humans, a famously hardy species, is almost out of the question.

As climate change continues to distort the world’s weather patterns, India will face more, and more intense, heatwaves. According to the India Meteorological Department (IMD), there is a heatwave on some days when the average maximum temperature on those days is 4.5-6.4º above the long-term average (or above 40º C in the plains or 37º C in coastal areas).

Between 1981 and 2010, the average high in New Delhi was 39.5º C, around May. This means the city began experiencing a heatwave – as the IMD predicted it would – from April 28, when the daily moved towards 44º C, which is 4.5º C plus 39.5º C. The IMD had issued a ‘yellow alert’ for the city – which means you should keep an eye on the mercury – and also said that the heatwave would persist for a few days more.

Evaporative cooling

These are not mere numbers: locked away in their values is one that connects directly to human biology, public health and governance in the era of the climate crisis. This is the wet bulb temperature. It measures the lowest temperature an object can reach when it’s in a hotter environment and is simultaneously cooling down by having water evaporate from its surface.

Consider the skin on your body, for example: the wet-bulb temperature will show the minimum temperature the skin can reach if it’s a hot day outside and there is sweat evaporating from its surface. According to weather-tracker meteologix, South Delhi had a wet-bulb temperature of 19.5º C at 8:30 am on April 26, 2022. This is hospitable. This morning (April 29), it was around 22º C – also hospitable.

But a few hours of outdoor activity with a wet-bulb temperature of 32º C can quickly prove debilitating. One in excess of 35º C will likely lead to death if you spend a few hours outside in the shade, even with unlimited water and no physical activity.

Annual probability of occurrence of heat waves with apparent temperature peaks greater than 40º and 55º C. Warming level specified on the y-axis. Image:

Heat and humidity

If you zoom out on the meteologix map to look at Asia as a whole, you will notice that the wet-bulb temperature increases progressively towards the equator, and is also higher on India’s east coast than on the west. This is because of humidity: the more humid the lower atmosphere, the less sweat will be able to evaporate into it, and the less the body will be able to cool, and the more it will heat up. So simply knowing the ambient temperature in a given area won’t suffice. We also need to check the humidity, and subsequently the wet-bulb reading.

Here is a simple, handy calculator to estimate the wet-bulb temperature.

In 2020, researchers at Columbia University, New York, reported that previous studies that said the planet could reach deadly combinations of heat and humidity later in the century were wrong. Instead, the Columbia team wrote, many parts of Earth’s surface had already started experiencing life-threatening conditions.

Based on weather data collected from 1979 to 2017, they wrote in particular that hotspots in “eastern coastal India” and “northwestern India” featured in the highest 99.9th percentile of the wet-bulb temperature (with a minimum of 31º C).

More broadly, at least one assessment has found that the maximum wet-bulb temperatures in parts of the Central Americas, North Africa, the Middle East, Northwestern and Southeastern India and Southeast Asia have already approached or crossed the 35º C threshold – contrary to our understanding in just 2010 that such values are further in the future.

The authors of the Columbia University study said their work stood out because humidity can vary in the course of a few hours – which in turn can heighten body heat for long enough to prove fatal. To account for this possibility, they used hourly data from the weather stations they surveyed.

RCP 4.5 and 8.5

Global warming is also leading to higher sea levels, and together with heat waves they will render more parts of India uninhabitable. But which parts will become uninhabitable and when depends on representative concentration pathways (RCPs) – how much greenhouse gases human activities will emit through this century.

RCP 4.5 is known as the “intermediate” pathway, in which the world’s greenhouse gas emissions peak in 2040 and then decline. Under RCP 8.5 – the infamous ‘business as usual scenario’ – emissions continue to rise through the 21st century. The UN Intergovernmental Panel on Climate Change (IPCC) considers it to be an unlikely scenario, but it is still used to estimate emissions in the middle of the 21st century, since emissions reduction thus far has been minimal.

According to a 2020 study, the exposure to higher wet-bulb temperatures in the RCP 8.5 scenario could be “100-250-times” higher than in the RCP 4.5 scenario by 2080. Exposure here is measured in person-days, which divided by 24 yields person-hours.

Global mean annual exposure under RCP 4.5 and RCP 8.5 in the 2070-2080 decade to wet bulb temperature of 30-35º C. Exposure to wet bulb temperatures above 30º C is reduced by several orders of magnitude in RCP 4.5 compared to RCP 8.5. Plot: DOI 10.1088/1748-9326/aaa00e

The 2010 study that famously identified the 35º C wet-bulb limit used temperature data for six-hour periods. Assuming that a person could fall significantly ill in six hours with a wet-bulb temperature of 35º C (although it will likely happen much sooner), the difference between RCP 4.5 and RCP 8.5 entails that at least 25-times more people could fall dangerously ill by 2080.

Note that this is a back-of-the-envelope calculation that is unlikely to be accurate – but it serves to highlight the scale of the public health crisis facing the people of north India due to heatwaves alone. What will happen when there is a heatwave together with a drought, or an epidemic, or a major train crash?

What will happen, as is likely today, when people don’t want to wear even a surgical mask, let alone a cloth mask or an N95, because it obstructs breathing on a hot day? The novel coronavirus will spread but how much of a priority would that be in the face of such blistering heat?

Heat action

The 2020 study even singled out “Northeast India and coastal West Africa” for their “currently … scarce cooling infrastructure” and “relatively low adaptive capacity”. Indeed, studies about heatwaves in South Asia have tended to focus on the Indo-Gangetic plain because it is relatively more vulnerable to extreme heat stress in future and because it is home to 400 million people.

The plain also encompasses many of India’s so-called BIMARU states – where public healthcare is underfunded, underequipped, understaffed and undersupplied. On top of these deficiencies, these states are also generally poorer, with many of their residents dependent on daily wages, which typically entails outdoor work with few facilities and little respite. During a heatwave, they will be extremely vulnerable to heat stress – compounded by the shameful irony that the intensity of the heat, so early in the year, is likely rooted in climate climate, to which these people contributed the least.

In 2013, Ahmedabad in Gujarat became India’s first city to adopt a ‘heat action plan’. Thus far, between 30 and 40 cities have adopted similar plans, which include outreach campaigns, installation of early-warning systems and passive cooling solutions (through architecture and building codes, e.g.) and reduction of heat exposure among vulnerable sections. Heatwaves tend to be deadlier in cities thanks to the contribution of the urban heat-island effect.

These steps are necessary but not sufficient. We also need to redefine heatwaves to include humidity, since it increases the wet-bulb temperature, and change the way our cities expand.

For now, according to the IPCC, wet-bulb temperatures in India rarely cross 30º C (like in Delhi in July 2021) – but that could change soon. Residents of all Asian cities are expected to suffer more heat stress in the RCP 4.5 scenario. Under RCP 8.5, the report foretells the possibility of 35+ degrees celsius wet-bulb temperatures everywhere in the country.

Note: This article was first published on April 26 and was republished with some additional details on April 29, 2022.

Scroll To Top