Rising temperatures, heat, and humidity in urban cities can put millions of people at a dangerous risk of dying from heat stroke, seizure, stomach, and food-related ailments, and other heat-related issues.

The month of March 2022 in India has never been the hottest in the last 122 years. Temperatures crossed the 45 °C mark in March and touched 49 °C in April 2022. This heat crisis has also brought the wet-bulb temperature concept into the scene.

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The unprecedented heat phenomenon during the month of June reached beyond the human tolerance level as the ‘feel’ temperature was around 55 degrees. The high ‘feel’ temperature reaches beyond tolerance level when the heat along with high humidity level makes it difficult for human health and normal activity gets hampered.  Scientists measure this high, unprecedented phenomenon of heat with ‘wet-bulb’ temperature when the heat crosses beyond human tolerance.

What is the ‘wet-bulb’ temperature

Dry temperatures usually encountered by everyday thermostats, don’t give the true picture. Heat laden with humidity is measured with wet-bulb temperature.  According to scientists, “Humidity is measured as wet-bulb temperature. Factoring in the humidity along with the heat, called the heat index, helps us determine what the temperature actually ‘feels like. ”

In usual cases, 35°C is the highest safe wet-bulb temperature. Many Indian coastal states, currently exceeding 32°C, run the risk of reaching wet-bulb temperatures so high that the human body simply won’t be able to keep up and will die of overheating.

As per the finding of the research published in an article by Science.org, ‘The emergence of heat and humidity too severe for human tolerance’, states how humid heat is a dangerous factor for a healthy living environment, “Our findings thus underscore the serious challenge posed by humid heat that is more intense than previously reported and increasingly severe.”

So, How wet-bulb temperature is different from dry-bulb temperature?

Wet-bulb temperature is a new phrase that has entered our lexicon as a result of the conversation surrounding severe heat. Wet-bulb temperatures a measure of both heat and humidity and their implications for human health have come to light as the subcontinent experiences its second heat wave of the month.

A wet-bulb temperature of 32-degree Celsius is usually the maximum that a human body can endure and carry out normal outdoor activities in. This is equivalent to a dry temperature of 55-degree Celsius.

The theoretical maximum wet bulb temperature is 35-degree Celsius – most humans, even with unlimited water supply, are likely to suffer heat strokes at this level, likely leading to death.

How is a Wet Bulb Temperature calculated?

The temperature measured by a thermometer wrapped in water-soaked (water at ambient temperature) fabric (a wet-bulb thermometer) over which air is circulated is known as the wet-bulb temperature (WBT).

Wet-bulb temperature equals air temperature (dry-bulb temperature) at 100% relative humidity; at lower relative humidity, evaporative cooling causes the wet-bulb temperature to be lower than dry-bulb temperature.

The temperature of an air parcel that has been cooled to saturation (100 percent relative humidity) as a result of water evaporating from it, with the latent heat provided by the air parcel, is known as the wet-bulb temperature.

An accurate reading of the wet-bulb temperature is provided by a wet-bulb thermometer. The wet-bulb temperature is the lowest temperature that may be attained solely by the evaporation of water under the current environmental circumstances.

The lowest temperature is attained by adiabatic evaporation of water in the air until the air is saturated without the addition of heat or an equal amount of Enthalpy (absolute humidity rises).

Water must be heated in order to evaporate. If this evaporation is adiabatic, the environment’s heat is removed, resulting in the air cooling and becoming more humid. When we exit the shower or the pool, the same thing takes place. We become cold as a result of the water on our body evaporating and removing our body’s heat.

Even those who are used to extreme heat are unable to engage in routine outdoor activities once the wet-bulb temperature reaches 32 °C (90 °F), which is comparable to a heat index of 55 °C (130 °F). Even with infinite water, a wet-bulb temperature of 35 °C (95 °F), which corresponds to a heat index of 70 °C (160 °F), is the maximum temperature at which humans may theoretically survive for more than a few hours in the shade.

In contrast, the dew point is where condensation (dew) and clouds would develop; it is the temperature at which the ambient air must be chilled to achieve 100% relative humidity, assuming there is no more evaporation into the air.

The wet-bulb temperature is lower than the dry-bulb temperature but higher than the dew point temperature for a parcel of air that is less than saturated, or air with less than 100 percent relative humidity. The gaps between each pair of these three temperatures are larger the drier the air is (the lower the relative humidity). On the other hand, the three numbers line up when the relative humidity reaches 100 percent.

The thermodynamic wet-bulb temperature relates to certain values of the relative humidity and dew point temperature for air at a known pressure and dry-bulb temperature.

Wet-bulb temperature and health

Only within a limited temperature range are living things capable of surviving. Many animals use evaporative cooling to lower their body temperatures when the ambient temperature is too high (humans and horses sweat, dogs and other mammals saliva and water). This helps avoid potentially deadly hyperthermia brought on by heat stress.

Wet-bulb temperature, or more complex calculated quantities like wet-bulb globe temperature (WBGT), which also takes into account solar radiation, give a useful indication of the degree of heat stress and are used by several agencies as the basis for heat stress prevention guidelines. The effectiveness of evaporative cooling depends on humidity.

Even fit and healthy persons sitting in the shade next to a fan at a continuous wet-bulb temperature of above 35 °C (95 °F) are likely to die because at such a degree, the body switches from losing heat to the environment to absorbing it.

On June 9,  the Safdarjung Observatory in Delhi – the base station for the national capital – reported 44 degrees Celsius, a maximum temperature, four scores above normal, while nighttime lows frequently do not drop below 30.

This is the time of the year when Delhiites also bear the wrath of the sun and the heatwaves. Even normal tap water can scorch one’s hand the same way as the water people get directly from geysers.

The soaring heatwaves are not only due to the summer season of the year, however, there are also other factors. Millions of tons of waste are stored in a massive landfill on the outskirts of the capital that was unexpectedly combusted a week ago. The landfill, which is 17 stories high, is still burning, aggravating the already severely filthy air in the city.