{"id":87149,"date":"2021-04-06T05:31:56","date_gmt":"2021-04-06T05:31:56","guid":{"rendered":"https:\/\/www.alj.com\/?post_type=spotlight-fadyjameel&p=87149"},"modified":"2022-04-19T11:33:05","modified_gmt":"2022-04-19T11:33:05","slug":"deadly-heat-is-cooler-than-you-think-the-chilling-science-of-wet-bulb-temperatures","status":"publish","type":"spotlight-fadyjameel","link":"https:\/\/alj.com\/en\/spotlight-by-fady-jameel\/deadly-heat-is-cooler-than-you-think-the-chilling-science-of-wet-bulb-temperatures\/","title":{"rendered":"Deadly heat is cooler than you think: the chilling science of \u2018wet-bulb\u2019 temperatures"},"content":{"rendered":"

Believe it or not, there is a kind of heat from which you cannot cool down.\u00a0 And one billion people will be forced to endure it for every 1\u00b0C (1.8\u00b0F) rise in global temperatures in the next 50 years.[1]<\/sup><\/a><\/h4>\n

Imagine working in searing heat with no air conditioning.\u00a0 Millions of people don’t have to; they’re doing it right now.\u00a0 Fortunately, they have free access to highly advanced temperature regulation systems:\u00a0 their own bodies.\u00a0 If you were to find yourself under the glaring Indian sun this very moment, working side-by-side with a farm laborer, you would probably be more worried about losing your mind than losing excess heat.\u00a0 But your body would know what to do.\u00a0 Your blood vessels would dilate, allowing blood to transfer its warmth to the surroundings.\u00a0 And, crucially, you would sweat.<\/p>\n

In 1902, Willis Carrier figured out how to keep us cool by extracting moisture from the air, when he designed the first modern air-conditioning system.[2]<\/sup><\/a>\u00a0 \u00a0When it comes to air-conditioning for the human body, though, evolution beat him to, by giving up<\/em> moisture to the air.\u00a0 When our sweat evaporates, it takes heat with it.\u00a0 As long as we drink plenty of water, we can always spare some of it to cool down by sweating.\u00a0 It’s a highly effective method that has served us well for millennia and allowed us to colonize some of the hottest places on the earth.\u00a0 But it’s not foolproof.<\/p>\n

Sometimes sweating doesn\u2019t work.\u00a0 Under certain conditions, it is literally impossible<\/em> for sweat to evaporate, meaning there is no way for our bodies to cool themselves down. \u00a0This life-threatening phenomenon is measured using a system known as the ‘wet-bulb temperature’.\u00a0 Such extreme conditions used to be rare, but thanks to climate change, they are now \u00a0occurring with alarming regularity in some parts of the world.<\/p>\n

What happens when we can\u2019t cool down<\/h2>\n
\"Elfatih
Elfatih Eltahir,
\nProfessor of Hydrology & Climate, MIT<\/figcaption><\/figure>\n

Humans need to keep their core body temperature around 37.5 degrees Celsius (\u00b0C) or 99.5 degrees Fahrenheit (\u00b0F).\u00a0 As it gets hotter, your body works harder to cool down.\u00a0 When your blood vessels dilate, they lower blood pressure, making the heart work harder to pump blood.\u00a0 If the pressure drops too low, your organs won’t get enough blood, and the risk of heart attacks rises.\u00a0 At the same time, you lose fluids and salts through sweat.<\/p>\n

If your temperature keeps rising, you will experience the symptoms of heat exhaustion, including faintness, cramps, and extreme thirst.\u00a0 Long-term exposure to heat stress can lead to kidney stones and heart problems.\u00a0 That’s if you’re lucky. \u00a0If you can’t cool down at all, you enter heat stroke: confusion, loss of consciousness, organ failure, and death.<\/p>\n

If you can\u2019t get out of the heat, sweating is your only lifeline.<\/p>\n

Elfatih Eltahir, Professor of Hydrology and Climate at the Massachusetts Institute of Technology, and a global expert in the wet-bulb phenomenon[3]<\/sup><\/a>, explains it like this:<\/p>\n

\u201cWe humans are equipped to deal with hot and dry environments, if we have enough water.\u00a0 If you keep \u00a0hydrating yourself, you sweat, and the sweat evaporates, and that cools your body. […] If you add humidity to that equation, it basically interferes with the natural process by which humans can cope with heat distress.\u201d<\/em><\/p>\n

How the wet-bulb temperature works<\/h2>\n

The wet-bulb temperature is an indication of how much the human body can cool itself down by sweating.\u00a0 It is measured by wrapping a thermometer’s bulb in a wet cloth and allowing the water to evaporate naturally.\u00a0 The lowest temperature measured is the wet-bulb temperature.<\/p>\n

This temperature is almost always cooler than the temperature recorded by a regular (i.e., dry) thermometer.\u00a0 This is because the bulb is losing heat through the evaporating water: the faster the water evaporates; the lower the thermometer’s temperature will be relative to the air temperature.<\/p>\n

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The evaporation rate is directly influenced by the absolute temperature and how much additional moisture the air can hold, known as the relative humidity.\u00a0 Hotter air can absorb more water vapor at a faster rate than cooler air.\u00a0 However, as the air absorbs more water vapor and becomes increasingly humid, there is less room for additional water vapor, and the rate of evaporation slows.\u00a0 Relative humidity of 0% means the air is completely dry. 100% means it can’t get any wetter.<\/p>\n

That means no matter how much you sweat, it won\u2019t evaporate.<\/p>\n

What\u2019s a dangerous temperature?<\/h2>\n

This isn\u2019t just interesting science.\u00a0 It\u2019s a life-threatening situation.\u00a0 So much so that in the 1950s, the US military used wet-bulb temperatures (WBT) to devise guidelines for keeping soldiers safe.\u00a0 When the WBT reaches 29\u00b0C (84\u00b0F), it’s recommended to stop exercising if you’re not acclimatized. \u00a0Anything above that is classed as an \u2018extreme\u2019 danger, and the risk of heatstroke rises dramatically.[4]<\/sup><\/a>\u00a0 When the wet-bulb temperature is pushing 35\u00b0C (95\u00b0F), a healthy person may not be able to survive outdoors for more than six hours.[5]<\/sup><\/a><\/p>\n

If the humidity keeps rising, eventually, the wet-bulb temperature will equal the actual temperature.\u00a0 In other words, despite being soaked in water, the wet-bulb won\u2019t cool down at all. \u00a0And neither would anyone else unlikely enough to still be around.<\/p>\n

Humidity makes heat kill faster<\/h2>\n

If the relative humidity is 100%, there is no way you can cool yourself down by sweating. \u00a0But it doesn’t have to be that high to spell danger.\u00a0 Similarly, the actual temperature could be over 50\u00b0C (122\u00b0F), well above the human body’s ideal core temperature, but as long as the air is dry enough, your sweat will evaporate and help to cool you down.\u00a0 It all depends on the relationship between temperature and humidity.\u00a0 The higher the humidity, the lower the actual temperature that can cause health problems (and vice versa).<\/p>\n

\"Al
Al Ula, Saudi Arabia<\/figcaption><\/figure>\n

For example, in the context of the Middle East, Riyadh, the capital of Saudi Arabia, is in the middle of the desert.\u00a0 Dubai, in the UAE, is on the Arabian Gulf coast. The temperature in Riyadh is much hotter than the temperature in Dubai, in general.\u00a0 However, the wet-bulb temperature in Dubai is significantly higher than in Riyadh, because Riyadh\u2019s desert air is much drier – and therefore able to absorb more moisture – than coastal Dubai, where the air is already very moisture heavy.\u00a0<\/p>\n

You can sunbathe in higher temperatures than would kill you.\u00a0 For example, if the actual temperature is 32\u00b0C (90\u00b0F) and the relative humidity is 95%, that’s enough to hit a dangerous 85\u00b0F \u00a0wet-bulb temperature.\u00a0 If the humidity is just 45%, the actual temperature would have to be 40\u00b0C (104\u00b0F) or more to trigger the same wet-bulb temperature.[6]<\/sup><\/a><\/p>\n

Terms like “relative humidity” and “wet-bulb temperature” can seem a little, well, dry. \u00a0Plus, there are those who say, we’ve survived heat like this before.\u00a0 Why can’t we do it again?<\/p>\n

Are we as adaptable as we think?<\/h2>\n

The truth is that most humans have always lived in regions with temperatures between 6\u00b0C (43\u00b0F) to 28\u00b0C (82\u00b0F).\u00a0 This is the sweet spot for human health and food production.\u00a0 But as climate change creates greater extremes of weather, the sweet spot is shrinking.\u00a0 As humans, we are particularly vulnerable because the land is warming faster than the oceans, and most population growth is in already hot regions of Africa and Asia.<\/p>\n

A global temperature increase of 3\u00b0C will mean the average human is 7.5\u00b0C warmer.\u00a0 By the end of the century, around 30% of us will live in extreme heat, defined as an average temperature of 29\u00b0C (84\u00b0F).\u00a0 On their own, these temperatures are a significant cause for concern.\u00a0 But the wet-bulb temperature is a different beast.\u00a0 As the humidity rises simultaneously, we’ll see more and more days where the wet-bulb temperature exceeds 32\u00b0C.<\/p>\n

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Who is most at risk?<\/h2>\n

Anyone can be affected by heat, especially as the wet-bulb temperature rises.\u00a0 Some people are more vulnerable than others, though.\u00a0 They include:<\/p>\n