Wet bulb temperature is simply the lowest temperature that the human body can cool down to after evaporating all the moisture that its surroundings allow it to. The lower it is, the more easily the human body can cool itself down through perspiration; when high it can lead to various issues including death in extreme cases.
Human beings can’t survive more than 35 degree celsius wet bulb temperature for more than six hours. The people most at risk are elderly, people with heart conditions and people working outside in direct sunlight or without air conditioning. As the wet- bulb temperature approaches the temperature of human body which is 36-37 degrees celsius or 96-97 degree fahrenheit, it becomes more difficult for the body to dissipate heat as the moisture in the air doesn't allow for quick evaporation of sweat; to flush extra heat, blood rushes to the surface layers of body depriving essential organs of blood and oxygen. This causes stress on many organs, especially the heart.
World Weather Attribution’s study says at least 90 people have died in India and Pakistan due to severe heat waves this year. Also worth noting is the fact that the heat wave happened in a relatively dry period of low rainfall, so extreme humidity was not a concern. It’ll be more severe if it happens later in the season with more rain and humidity. Extreme humidity and high temperature is restrictive for industrial and outdoor workers who don’t have air conditioning at their workplace. Central government has had a heat action plan since 2013 that is limited to forecasting spells of extreme heat and warning offices and schools to shut down in some cases. Only 8% of Indians have air conditioning that leaves most of the population vulnerable to heat stress. The human body relies on the evaporation of water from the skin to cool itself down. When it fails, internal organs including the brain get stressed and can lead to coma or even death.
Humidity may look like a simple concept or physical characteristic to measure but there are nuances to it. Climate models predict that the relative humidity will decline as the temperatures rise but the specific humidity will rise with temperature. The difference is in measurement. Specific Humidity measures the absolute vapor concentration in the air– grams of water per meter cube of air; relative humidity measures the ratio of water vapor present in the air and the amount of water vapor the air can hold at that specific temperature (gm/kg). Also the water retention capacity of air increases as it heats up. As an example, 70% humidity at 23°C corresponds to less water vapor in the air than 70% humidity at 38°C, because at 23°C the retention capacity is low. Heat index is measured with the relative humidity at that particular temperature. But if you think, the body’s need to cool itself down at 38°C is more immediate, so the same amount of humidity becomes unbearable at higher temperatures as we perspire more.
Temperature is measured by simply putting a thermometer at the site of measurement, in shade, so the bulb of the thermometer is not heated. This method measures dry bulb temperatures, or simply the temperature of the air. Humidity or the concentration of water vapor in the air is a significant factor, simply measuring temperature fails to measure how certain temperature feels to humans. As global temperature is soaring every year due to climate change, it is becoming increasingly important to monitor the level of heat stress that humans in certain regions of the world are going to face or are already facing.
If you look at your weather app on your phone it will tell you two things– temperature and feels like or Real Feel. This other part is called heat index, it is the subjective measure of what a human will feel in that particular temperature considering the relative humidity of the place. Now, different countries, organizations and mobile applications try to calculate this heat index slightly differently (for example Real Feel is a trademark of AccuWeather). And since extreme heat is becoming a problem, scientists are looking for a more robust and reliable measure of subjective human experience, the concept of Wet-Bulb temperature comes into picture.
Scientists have used GCMs (General Circulation Models) –a kind of climate model that relies on the study of oceanic and atmospheric currents– to predict the change in atmospheric attributes like temperature, pressure and humidity. According to research published in IOPScience, the GCM models that predict the highest increase in temperature also predict the highest decrease in relative humidity. This is expected to have a stabilizing effect on future wet-bulb temperatures. The mean of all GCM predictions show a 2 °C–3 °C rise in wet bulb temperatures in tropical and mid latitude regions of earth by 2070-80. The range of predictions from different models under different RCP (Representative Concentration Pathway) scenarios are 1 °C–2.5 °C for RCP 4.5, and for RCP 8.5 it's 2 –4.5 °C.
As explained earlier, the decrease in relative humidity shows air’s increased capacity for moisture retention and not absolute decrease in humidity in the air. Because of this there are certain regions, which, because of their geographical situation, will see an increase in relative humidity or the increase in wet-bulb temperature. Although concentrated in small spots, these regions will affect millions of people because they are projected to be in India, China and the US. For an RCP 4.5 scenario the number is 250 million and for an RCP 8.5 scenario it is 750 million. This will expose lots of people to heat stress resulting in strokes, organ failures and restriction of movement or work in open daytime situations.
The exploration of the impacts of wet bulb temperatures is very limited. How it correlates with various local factors is unknown and so are its higher order effects. How the alteration of temperature moisture dynamic will influence crop ecosystems and the nature of soil or the forest and wildlife ecosystems. Its effect on individuals with health risks and on epidemiology, how urban island effect comes into play with this phenomenon, how it’ll play out in regions that have historically been arid deserts; in what ways will these phenomena affect the economy of a certain high heat stress hotspots. These are the things that need more research.
The good part is, that even in the worst case scenario (RCP 8.5) the projections tell us that there’s only 15% chance that we will hit 35 °C wet bulb by 2070, and that will be concentrated in small spots of extreme heat stress in India and China. Maybe by working ruthlessly on emission targets and making cities more sustainable we can avoid the worst.
Vivek is a writer who writes to explore. His interests include philosopy, psychology, poetry, cinema, mythology and international relations. Above all he’s interested in making sense of complex systems-how they work and influence each other. An alumnus of Calcutta University, he has a bachelor's degree in Physics.
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