What Is Wet Bulb Temperature – Free PDF Download

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The chatter around extreme heat has introduced a new term to our climate change vocabulary — wet bulb temperatures.
As heat wave conditions set in across the subcontinent for the second time this month, concerns around wet bulb temperatures — a measure of both heat and humidity — and their effects on human health have also surfaced.

The consecutive heatwaves over South Asia since March 2022 have continued the disturbing tradition of breaking historical temperature records.
Mercifully, these record temperatures were not accompanied by the high mortality burden observed in previous heatwaves such as the one during 2015.
Why those heatwaves were so deadly is a puzzle that we are yet to resolve….

IPCC report AR6

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.
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.
The report mentions that 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.

Humans lose heat generated within their body by producing sweat that evaporates on the skin. The cooling effect of this evaporation is essential in maintaining a stable body temperature.
As humidity rises, sweat does not evaporate —just like clothes take a long time to dry in humid locations – and makes it difficult to regulate body temperature. This is why we feel more discomfort in humid places.

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.
The wet-bulb temperature is that which would be recorded by a thermometer wrapped in a moist towel. Water evaporating from around the bulb has a cooling effect.

Warm-blooded animals need to keep their internal temperatures reasonably constant. If it is too hot, most use some form of evaporative cooling to do this—functioning, in essence, as living wet-bulbs. Dogs pant, pigs wallow in mud and humans sweat.
Ambient humidity changes how efficient these processes are, which is why muggy Singapore would feel much hotter than dry Sydney at the same ambient temperature.
A wet-bulb temperature of 35°C is regarded as the theoretical limit of what humans can endure. It would be reached at an actual temperature of 45°C if relative humidity were 50%, or at about 39°C if humidity were 75%.
Beyond this point it becomes impossible for sweat to cool the body down, causing people to overheat and in effect cook.

Cells swell, proteins are deformed and organ systems fail, resulting in death. At wet-bulb temperatures above 35°C, it is thought that even young healthy people wearing light clothing—regardless of whether they are parked in front of a fan, in the shade or have unlimited water to drink—will die in about six hours.
Even slightly lower wet-bulb temperatures can be dangerous. At a wet-bulb temperature of 32°C, it is risky to do anything that raises body temperature, such as walking or working outside.

Average global temperatures are currently 1.1-1.3°C warmer than pre-industrial levels.
Even if countries stick to their promises to curb emissions and scrub carbon from the atmosphere, the world is on track for temperatures to rise by the same amount again by the end of the century.

The hype around survivability thresholds and wet bulb temperatures obscures deeper issues, both physiological and political.
Firstly, the inability of the body to stabilise its core temperature can have multiple reasons. For instance, increased strain on the heart during periods of elevated temperature could be fatal for those with pre-existing cardiac conditions and is in fact the leading cause of deaths during heatwaves.
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.
A less obvious issue is that of 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 heatwaves.
Such public health factors can dramatically reduce the survivability thresholds and underestimate the actual vulnerability of the population.

In summary, the recent focus on increasing temperature and humidity is a welcome step in building awareness about the impacts of climate change.
However, it is important to realise that our vulnerability to heatwaves is a systemic problem that has existed for a very long time, and is not dependent only on increasing greenhouse gas emissions.
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.

Question:
What do we call the energy transferred between two systems by virtue of temperature difference?