As the U.S. heat wave drags on, experts warn of deadly 'wet-bulb' temperatures. Here’s what those are.

David Knowles
·Senior Editor
Updated ·3-min read
A construction worker drains a bottle of water while working
A construction worker tries to keep hydrated while on the job in South Boston. (David L. Ryan/Boston Globe via Getty Images)

With summer officially underway, millions of Americans are facing another week of heat advisories across the Northeast, Midwest, and South.

As temperatures worldwide continue to rise due to climate change, the limits of the human body’s ability to deal with excess heat are being tested. Last week, more than 1,000 people died on a pilgrimage to Mecca in Saudi Arabia amid temperatures of 125° Fahrenheit.

In the U.S., government agencies have long warned that the combination of higher summer heat and high humidity will become more and more dangerous in places like the Midwest because so-called “wet-bulb” temperatures will prevent the human body from cooling down by sweating.

“The hotter it is, the more strain our bodies feel, and the more we need to sweat to cool down,” NASA states on its website. “But humid air has less capacity to hold additional moisture, so water evaporates more slowly in humid conditions.”

When it gets hot, the human body sweats, and the evaporation of that sweat into the air helps cool the body. But if the humidity in the air is too high, it prevents the evaporation of sweat, depriving the body of its ability to cool itself down.

To approximate that effect, scientists calculate “wet-bulb” temperature by covering a thermometer with a wet cloth and measuring the amount that the evaporation of moisture cools the thermometer, Reuters reported.

“Wet-bulb temperature is the lowest temperature to which an object can cool down when moisture evaporates from it,” NASA says on its website.

The latest long-range weather forecasts are predicting another steamy, record-breaking July. Fortunately, numerous handy websites have popped up to help people calculate wet-bulb temperatures. All you have to do is enter in the current temperature and relative humidity.

Using the Omni Calculator, for instance, an air temperature of 95°F combined with a relative humidity of 80% results in a wet-bulb temperature of 89.47°F, meaning that the evaporation of sweat isn’t doing a great deal to cool the body down.

“Wet-bulb temperatures above 30°C (86°F) pose potential fatal danger to humans outside. It's also very uncomfortable. In these conditions, you should avoid direct sunlight and drink lots of water,” the site adds for that particular calculation.

Effects on the body

Simply put, prolonged exposure to wet-bulb temperatures can kill you.

“This triggers changes in your body. You dehydrate. Your organs become stressed, especially your heart,” NASA says. “Blood rushes to your skin to try to release heat, starving your internal organs. The results can be deadly.”

Studies have shown that wet-bulb temperatures of 95°F or above could render some parts of the globe uninhabitable.

In the meantime, it’s important to take precautions during periods in which high temperatures are coupled with high humidity. As Yahoo News’ Kaitlin Reilly reported, those include the following:

  • Up next
  • Up next
  • Up next
  • Up next

Latest stories

  • Deaths mount as Pakistan swelters in heatwave

    Soaring temperatures in Pakistan have led more people to head to hospitals seeking help.

  • Beat them or eat them: What should we do about invasive species in the ocean?

    Should we beat them, or eat them? In this episode of Ocean Calls, we’re embarking on a journey to unravel the mysteries of invasive species in EU waters. They unsettle the delicate balance of marine ecosystems, so what can we do about them?

  • Climber Rappels Next to 'Raging Waterfall' That Developed During Flash Flood

    A “raging waterfall” cascading down the Morning Glory Arch was not enough to deter an experienced outdoorsman on June 21, with jaw-dropping visuals showing him rappelling down after a flash flood hit the area.Video by 16-year-old Spencer Grover shows Tyler Erickson rappelling from a sandstone cliff outside Moab, Utah, within touching distance of a muddy waterfall that “spontaneously developed”, Spencer’s father, Michael Grover, told Storyful.Speaking to Storyful, Erickson said that although the canyoneering adventure “unexpectedly turned into a test of resilience and preparedness,” at no point were he and his group in serious danger.“We were well prepared, equipped with headlamps, sufficient food, and water,” Erickson said.“Despite the situation, the temperature remained a comfortable 70 degrees, and no one was at risk of hypothermia. The storm was described by search and rescue as a once-in-150-years event, catching even experienced guides off guard.”Erickson said that the unexpected flash flood brought close to four inches of rain in just 20 minutes. He highlighted “the importance of preparedness and staying calm in unexpected situations”, but said the experience, while challenging, was also “an unforgettable and humbling adventure”.“It was a testament to the resilience and teamwork of our group, and a reminder that nature can be unpredictable,” he said.Separately, a group of 20 people had to be rescued during the same extreme conditions, local media said Credit: Spencer Grover via Storyful

  • Bornean elephant assessed as endangered in new threatened species list

    An update to the IUCN Red List of threatened species also warns of a threat to lizards from invasive snakes and impacts of illegal cactus trade.

  • Funnel Cloud Hovers Over Field Near Cedar Rapids

    A funnel cloud was spotted hovering over part of eastern Iowa on June 25, as storms moved through the region.Aaron Horn, who filmed this footage, said he spotted the funnel near Cedar Rapids on Tuesday.The National Weather Service warned of scattered severe thunderstorms on Tuesday evening, with the possibility of strong wind gusts and tornadoes. Credit: Aaron Horn via Storyful

  • World's smallest elephant in danger of dying out

    Only an estimated 1,000 individuals remain in the wild but conservationists remain optimistic.

  • How the Midwest Floods Nearly Took Out a Century-Old Dam

    The warning from the county government was grim. The Rapidan Dam, a feature of the southern Minnesota landscape for more than a century, was “in imminent failure condition.” “We do not know if it will totally fail or if it will remain in place,” officials said Monday as dead trees and other debris piled up at the dam and floods overwhelmed the Upper Midwest. At one point, a support structure alongside the dam partly failed and gushing waters carved out a nearby cliff. Video footage showed a near

  • What happened to Minnesota's Rapidan Dam? Here's what to know about its flooding and partial failure

    Earlier this week, authorities said the Rapidan Dam near Mankato faced an “imminent threat” of collapse but later said an abutment had partially failed. Authorities said the partial failure of the abutment was caused by the recent bout of heavy rain, but a past assessment of the dam revealed it was already at risk. The rush of water destroyed a power station and caused outages for about 600 households, Minnesota Gov. Tim Walz said.

  • 2 giant pandas set off from China to the US - for first time in over 20 years

    For the first time in more than 20 years, giant pandas are travelling from China to the United States. The five-year-old male Yun Chuan and nearly four-year-old female Xin Bao are expected to arrive at the San Diego Zoo in California later this week after setting off from China on Wednesday. The pair will travel 7,000 miles, accompanied by caretakers who will help them adjust to their new environment. Do you have questions about the biggest topics and trends from around the world? Get the answer

  • Tribes honor the birth of a rare white buffalo calf in Yellowstone and reveal its name: Wakan Gli

    In a gathering near a picturesque lake outside Yellowstone National Park, hundreds of people cheered Wednesday as a Native American leader spoke the name revealed on a painted hide for a rare white buffalo that was born in the park earlier this month: Wakan Gli, which means “Return Sacred” in Lakota. Earlier this month, the white buffalo calf was born in Yellowstone National Park's vast and lush Lamar Valley, where huge, lumbering bison graze by the hundreds in scenes reminiscent of the old American West. To the several tribes who revere American bison — they call them “buffalo" — the calf's appearance was both the fulfilment of sacred prophesy and a message to take better care of the Earth.

  • Two people die from floods ravaging US midwest as more storms forecasted

    Days of flooding have submerged homes and farmland across South Dakota, Nebraska, Iowa and Minnesota

  • Factbox-Companies forging a rare earths industry in the EU

    In the rare earths sector, the EU will struggle to meet most of its ambitious goals in new legislation designed to boost domestic output of critical minerals and reduce dependence on China. Below are some key companies working on rare earths, with production figures in metric tons per year. Neodymium and praseodymium (referred to in the industry by their joint elemental abbreviation, NdPr) are key rare earths needed to make permanent magnets.

  • 15 Southeast Cities To Retire on a Budget of $3,000 a Month

    It might not sound like enough money to pay for the necessities each month, but it's surprising how far $3,000 will go in some regions of the country -- especially in the southeastern states. Discover...

  • New drone footage shows 97% of coral is dead in northern Great Barrier Reef

    Bleaching event, which follows a similar one last summer, is fifth in eight years after parts of reef subjected to extreme heat stress

  • Damage to protected chalk stream left unchecked by Natural England for 11 years

    Damage to a protected chalk stream has gone unchecked by Natural England for more than a decade, according to new analysis.

  • In Colombia, a long, perilous romance to save the harpy eagle

    Wounded and wary, the young couple appear to be into each other: their arranged romance, years in the making, is aimed at preserving the rapidly disappearing harpy eagle, one of the world's largest raptors.One of the world's most powerful and largest eagles, able to hunt monkeys and sloths, the harpy eagle has been hunted to near-extinction in some parts of its territory in Central America.

  • 100% of energy from renewables ‘essential’ for the planet, Greens say

    Moving towards renewables would ‘save the public billions’, Scottish Green co-leader Lorna Slater has said.

  • Tesla ordered to stop releasing toxic emissions from San Francisco Bay Area plant

    Tesla must fix air quality problems at its electric vehicle manufacturing facility in the San Francisco Bay Area after racking up more than 100 violations for allegedly releasing toxic emissions into the atmosphere over the last five years, an air quality board said Tuesday. The Bay Area Air Quality Management District planned to issue a written abatement order later this week after Tuesday's announcement. Each of the 112 violations can emit hundreds of pounds of illegal air pollution, the board said.

  • Deadly floods still battering Midwest towns as millions remain under heat warnings

    Flooding killed at least one person in South Dakota and another in Iowa

  • At least 2 are dead from catastrophic Midwest flooding and a Minnesota dam is at risk of failing

    At least two people are dead after severe storms and major flooding hit the Midwest, according to officials, and a dam in southern Minnesota built in the early 1900s is at risk of collapse.