Heatwaves and the role humidity plays in making them deadly

Heatwaves and the role humidity plays in making them deadly

Subject: Geography

Section: Climatology

Context: 

• THE CONSECUTIVE heat waves over South Asia since March 2022 have continued the disturbing tradition of breaking historical temperature records.
• These record temperatures were not accompanied by the high mortality burden observed in previous heatwaves such as the one during 2015.
• The recent IPCC report AR6 has emphasised that humidity is also very important while estimating the physiological stress that extreme heat puts on the human body.
• While the current heatwave is not India’s worst, yet, we endured the hottest March in 122 years.
• This heatwave has also brought the wet bulb temperature concept into mainstream conversation.
• Wet bulb temperature is significant in understanding heat waves in coastal regions of India

What is dry bulb and wet bulb temperature?

• Instead of the “dry bulb” temperature that is usually measured using a regular thermometer, an alternative metric known as the “wet bulb temperature” has been used to measure exposure to extreme heat.
• Sustained exposures to wet bulb temperatures above 35°C are fatal,
• While sustained exposures to wet bulb temperatures above 32°C are dangerous for intense physical activity.
• However, there has been a growing concern about the 35°C threshold and whether parts of South Asia will become“ unsurvivable” in the coming years.
• The wet bulb temperature is usually lower than the dry bulb temperature, and the difference between the two increases dramatically as the air becomes dry.
• Humidity and with high temperature makes uncomfort for human body because humid air hardly absorb body sweat
• In simpler terms, wet bulb temperature tells us at what level our bodies will be unable to cool themselves down by sweating.
• Wet bulb temperature combines heat and humidity to indicate how much evaporation can be absorbed into the air.
• It measures the lowest temperatures that our bodies can reach when we are in hotter environments, by sweating.

How wet bulb temperature is measured:

• Wet bulb temperatures (WBT), measured by putting a wet cloth over a thermometer and passing air through it, or using a wet thermometer in shade and letting water evaporate off it.
• Wet bulb temperatures are always lower than regular or dry bulb temperatures in the same location owing to humidity.

Sustained exposure to wet bulb temperature and Survivability thresholds:

• The research that backs the IPCC AR6 also suggests we are unlikely to experience sustained exposure to wet bulb temperatures beyond the threshold of survivability.
• The hype around survivability thresholds and wet bulb temperatures obscures deeper issues, both physiological and political.
• Firstly, the inability of the body to stabilize its core temperature can have multiple reasons. 
• pre-existing cardiac conditions, pre-existing respiratory problems and diabetes too are potential causes of death.
• Such conditions impair the body’s ability to efficiently transfer heat to the environment.
• The Second obvious issue is that of dehydration.
• Many labourers, especially women, intentionally keep themselves dehydrated due to the lack of toilets in workplaces.
• Dehydration can lead to decreased sweat production and therefore increased vulnerability to heat stroke during heat waves.
• Such public health factors can dramatically reduce the survivability thresholds and underestimate the actual vulnerability of the population.
• Another important issue is political issue here that often goes unnoticed.
• A singular focus on increasing wetbulb temperatures subtly shifts the responsibility of action from the local to the transnational arena.
• Increasing wet bulb temperatures are the by-product of global climate change, and therefore keeping our populations becomes the responsibility of international negotiators at conferences such as COP26.
• Very little can be done at the local level to keep wet bulb temperatures from rising if the factors controlling the mare global
• On the other hand, understanding the factors that increase vulnerability to heatwaves puts the onus on local actors, who have to provide better sanitation facilities, protect the elderly and those with cardiac conditions, and reduce the incidence of diabetes in the population.
• Such a focus will increase the pressure to improve our national health infrastructure, whose fragility has become increasingly clear to everyone over the past two years.
• Refocusing our priorities to highlight critical gaps in our urban and health infrastructure that repeatedly fail to protect the most vulnerable among us will allow us to build a more climate-resilient nation.