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As climate writers, my colleagues and I spend a lot of time telling readers that places are hot. The Arabian Peninsula? It’s hot. The Atlantic Ocean? It’s hot. The southern U.S. and northern Mexico? Hot and getting hotter.
But here’s a little secret: “Hot” doesn’t really mean … anything. The word is, of course, of critical importance when it comes to communicating that global temperatures are the highest they’ve been in 125,000 years because of greenhouse gases in the atmosphere, or for public health officials to anticipate and prevent deaths when the environment reaches the point where human bodies start malfunctioning. But when you hear it’s “100 degrees out,” what does that really tell you?
Beyond that you’re a fellow member of the Fahrenheit cult, the answer is: not a lot. Humans can “probably avoid overheating” in temperatures of 115 degrees — but only if they’re in a dry room with 10 percent relative humidity, wearing “minimal” clothing, and not moving, The New York Times reports. On the other hand, you have a high chance of life-threatening heat stroke when it’s a mere 90 degrees out … if the humidity is at 95%. Then there are all the variables in between: if there’s a breeze, if you’re pregnant, if you’re standing in the shade or the sun, if you’re a child, if you’re running a 10K or if you’re napping on your couch in front of a swamp cooler.
In order to better specify how hot “hot” is, a number of different equations and techniques have been developed around the world. In general, this math takes into account two main variables: temperature (the one we all use, also known as “dry bulb” or “ambient air temperature,” which is typically measured five feet above the ground in the shade) and relative humidity (the percentage of air saturated with water vapor, also known as the ugly cousin of the trendier dew point; notably Canada’s heat index equivalent, the Humidex, is calculated from the dew point rather than the relative humidity).
In events like the already deadly heat dome over the southern United States and northern Mexico this week, you typically hear oohing and ahhing about the “heat index,” which is sometimes also called the “apparent temperature,” “feels like temperature,” “humiture,” or, in AccuWeather-speak, the “RealFeel® temperature.”
But what does that mean and how is it calculated?
The heat index roughly approximates how hot it “actually feels.”
This is different than the given temperature on the thermometer because the amount of humidity in the air affects how efficiently sweat evaporates from our skin and in turn keeps us cool. The more humidity there is, the less efficiently our bodies can cool themselves, and the hotter we feel; in contrast, when the air is dry, it’s easier for our bodies to keep cool. Regrettably, this indeed means that insufferable Arizonans who say “it’s a dry heat!” have a point.
The heat index, then, tells you an estimate of the temperature it would have to be for your body to be similarly stressed in “normal” humidity conditions of around 20%. In New Orleans this week, for example, the temperature on the thermometer isn’t expected to be above 100°F, but because the humidity is so high, the heat toll on the body will be as if it were actually 115°F out in normal humidity.
Importantly, the heat index number is calculated as if you were standing in the shade. If you’re exposed to the sun at all, the “feels like” is, of course, actually higher — potentially as many as 15 degrees higher. Someone standing in the New Orleans sun this week might more realistically feel like they’re in 130-degree heat.
The heat index graph.NOAA
Here’s the catch, though: The heat index is “purely theoretical since the index can’t be measured and is highly subjective,” as meteorologist Chris Robbins explains. The calculations are all made under the assumption that you are a 5’7”, 147-pound healthy white man wearing short sleeves and pants, and walking in the shade at the speed of 3.1 mph while a 6-mph wind gently ruffles your hair.
Wait, what?
I’m glad you asked.
In 1979, a physicist named R. G. Steadman published a two-part paper delightfully titled “The Assessment of Sultriness.” In it, he observed that though many approaches to measuring “sultriness,” or the combined effects of temperature and humidity, can be taken, “it is best assessed in terms of its physiological effect on humans.” He then set out, with obsessive precision, to do so.
Steadman came up with a list of approximately 19 variables that contribute to the overall “feels like” temperature, including the surface area of an average human (who is assumed to be 1.7 meters tall and weigh 67 kilograms); their clothing cover (84%) and those clothes’ resistance to heat transfer (the shirt and pants are assumed to be 20% fiber and 80% air); the person’s core temperature (a healthy 98.6°F) and sweat rate (normal); the effective wind speed (5 knots); the person’s activity level (typical walking speed); and a whole lot more.
Here’s an example of what just one of those many equations looked like:
One of the many equations in “The Assessment of Sultriness: Part I,”R.G. Steadman
Needless to say, Steadman’s equations and tables weren’t exactly legible for a normal person — and additionally they made a whole lot of assumptions about who a “normal person” was — but Steadman was clearly onto something. Describing how humidity and temperature affected the human body was, at the very least, interesting and useful. How, then, to make it easier?
In 1990, the National Weather Service’s Lans P. Rothfusz used multiple regression analysis to simplify Steadman’s equations into a single handy formula while at the same time acknowledging that to do so required relying on assumptions about the kind of body that was experiencing the heat and the conditions surrounding him. Rothfusz, for example, used Steadman’s now-outdated calculations for the build of an average American man, who as of 2023 is 5’9” and weighs 198 pounds. This is important because, as math educator Stan Brown notes in a blog post, if you’re heavier than the 147 pounds assumed in the traditional heat index equation, then your “personal heat index” will technically be slightly hotter.
Rothfusz’s new equation looked like this:
Heat index = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783x10-3T 2 - 5.481717x10-2R 2 + 1.22874x10-3T 2R + 8.5282x10-4TR2 - 1.99x10-6T 2R 2
So much easier, right?
If your eyes didn’t totally glaze over, it actually sort of is — in the equation, T stands for the dry bulb temperature (in degrees Fahrenheit) and R stands for the relative humidity, and all you have to do is plug those puppies into the formula to get your heat index number. Or not: There are lots of online calculators that make doing this math as straightforward as just typing in the two numbers.
Because Rothfusz used multiple regression analysis, the heat index that is regularly cited by the government and media has a margin of error of +/- 1.3°F relative to a slightly more accurate, albeit hypothetical, heat index. Also of note: There are a bunch of different methods of calculating the heat index, but Rothfusz’s is the one used by the NWS and the basis for its extreme heat alerts. The AccuWeather “RealFeel,” meanwhile, has its own variables that it takes into account and that give it slightly different numbers.
Midday Wednesday in New Orleans, for example, when the ambient air temperature was 98°F, the relative humidity was 47%, and the heat index hovered around 108.9°F, AccuWeather recorded a RealFeel of 111°F and a RealFeel Shade of 104°F.
You might also be wondering at this point, as I did, that if Steadman at one time factored out all these variables individually, wouldn’t it be possible to write a simple computer program that is capable of personalizing the “feel like” temperature so they are closer to your own physical specifications? The answer is yes, although as Randy Au writes in his excellent Substack post on the heat index equation, no one has seemingly actually done this yet. Math nerds, your moment is now.
Because we’re Americans, it is important that we use the weirdest possible measurements at all times. This is probably why the heat index is commonly cited by our government, media, and meteorologists when communicating how hot it is outside.
But it gets weirder. Unlike the heat index, though, the “wet-bulb globe temperature” (sometimes abbreviated “WBGT”) is specifically designed to understand “heat-related stress on the human body at work (or play) in direct sunlight,” NWS explains. In a sense, the wet-bulb globe temperature measures what we experience after we’ve been cooled by sweat.
The Kansas State High School Activities Association thresholds for wet-bulb globe temperature.Weather.gov
The “bulb” we’re referring to here is the end of a mercury thermometer (not to be confused with a lightbulb or juvenile tulip). Natural wet-bulb temperature (which is slightly different from the WBGT, as I’ll explain in a moment) is measured by wrapping the bottom of a thermometer in a wet cloth and passing air over it. When the air is dry, it is by definition less saturated with water and therefore has more capacity for moisture. That means that under dry conditions, more water from the cloth around the bulb evaporates, which pulls more heat away from the bulb, dropping the temperature. This is the same reason why you feel cold when you get out of a shower or swimming pool. The drier the air, the colder the reading on the wet-bulb thermometer will be compared to the actual air temperature.
Wet bulb temperature - why & when is it used?www.youtube.com
If the air is humid, however, less water is able to evaporate from the wet cloth. When the relative humidity is at 100% — that is, the air is fully saturated with water — then the wet-bulb temperature and the normal dry-bulb temperature will be the same.
Because of this, the wet-bulb temperature is usually lower than the relative air temperature, which makes it a bit confusing when presented without context (a comfortable wet-bulb temperature at rest is around 70°F). Wet-bulb temperatures over just 80, though, can be very dangerous, especially for active people.
The WBGT is, like the heat index, an apparent temperature, or “feels like,” calculation; generally when you see wet-bulb temperatures being referred to, it is actually the WBGT that is being discussed. This is also the measurement that is preferred by the military, athletic organizations, road-race organizers, and the Occupational Safety and Health Administration because it helps you understand how, well, survivable the weather is, especially if you are moving.
Our bodies regulate temperature by sweating to shed heat, but sweat stops working “once the wet-bulb temperature passes 95°F,” explains Popular Science. “That’s because, in order to maintain a normal internal temperature, your skin has to stay at 95°F degrees or below.” Exposure to wet-bulb temperatures over 95°F can be fatal within just six hours. On Wednesday, when I was doing my readings of New Orleans, the wet-bulb temperature was around 88.5°F.
The WBGT is helpful because it takes the natural wet-bulb temperature reading a step further by factoring in considerations not only of temperature and humidity, but also wind speed, sun angle, and solar radiation (basically cloud cover). Calculating the WBGT involves taking a weighted average of the ambient, wet-bulb, and globe temperature readings, which together cover all these variables.
That formula looks like:
Wet-bulb globe temperature = 0.7Tw + 0.2Tg + 0.1Td
Tw is the natural wet-bulb temperature, Tg is the globe thermometer temperature (which measures solar radiation), and Td is the dry bulb temperature. By taking into account the sun angle, cloud cover, and wind, the WBGT gives a more nuanced read of how it feels to be a body outside — but without getting into the weeds with 19 different difficult-to-calculate variables like, ahem, someone we won’t further call out here.
Thankfully, there’s a calculator for the WBGT formula, although don’t bother entering all the info if you don’t have to — the NWS reports it nationally, too.
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On Rewiring America layoffs, a FEMA firing, and Vineyard Wind
Current conditions: It’s heating up in the West, where temperatures could hit triple digits in parts of California’s Central Valley today• Despite a soggy start to Friday in the Northeast, conditions will clear up in time for a warm and sunny Mother’s Day• It’s hot and clear in Kerala, India, where forecasters expect a wetter-than-average monsoon season to begin at the end of the month.
Electrification nonprofit Rewiring America announced Thursday that it is laying off 36 employees — about 28% of its workforce — due to the Trump administration’s clawback of Greenhouse Gas Reduction Fund awards, my colleague Katie Brigham reported. CEO Ari Matusiak wrote in a public letter to his employees that “the volatility we face is not something that we created; it is being directed at us.”
Matusiak added on LinkedIn that since February, Rewiring America has been “unable to access our competitively and lawfully awarded grant dollars,” some $2 billion of which were awarded through the GGRF last year to the organization and four other partners to help decarbonize American homes. The Environmental Protection Agency has tried to rescind $20 billion of the GGRF’s $27 billion in total funding, wreaking havoc “on organizations such as Rewiring America, which structured projects and staffing decisions around the grants,” Katie goes on. Read her full report of the layoffs here.
The acting administrator of the Federal Emergency Management Agency, Cameron Hamilton, was fired on Thursday, one day after defending the existence of the department he’d been appointed to oversee, E&E News reports. Testifying before a House Appropriations subcommittee on Wednesday, Hamilton had told lawmakers that “I do not believe it is in the best interests of the American people to eliminate” FEMA — a response to Homeland Security Secretary Kristi Noem’s remarks that “the president has indicated he wants to eliminate FEMA as it exists today.”
Though Noem swiftly appointed Hamilton’s successor — David Richardson, a senior official at DHS with experience in the Marine Corps commanding artillery units — Democratic Senator Patty Murray of Washington slammed the move, telling Noem, “We have seen an upheaval at FEMA that is going to put lives in jeopardy. We are losing indispensable staff just weeks away from fire and hurricane season.” Hurricane season begins in fewer than 23 days, with the possibility of the first named storm of the year forming before then, and forecasters are also anticipating an above-average fire season. “There is a reason the law requires the administrator of FEMA to have state emergency management experience,” Chad Berginnis, the executive director of the Association of State Floodplain Managers, told E&E News.
The Supreme Court declined this week to hear a pair of challenges brought against Vineyard Wind, the offshore wind farm under construction south of Martha’s Vineyard. The petitions were brought by the fishing industry lobbying group Responsible Offshore Development Alliance, which argued the approval of Vineyard Wind violated protections against ocean users and endangered species, as well as the Texas Public Policy Foundation, which represents Rhode Island fishermen and a seafood company. “We will continue to pursue our goal of shutting down the Vineyard Wind project by filing an administrative petition with the Secretary of the Interior,” TPPF Senior Attorney Ted Hadzi-Antich said in a statement, per The New Bedford Light. To date, Vineyard Wind has — haltingly — installed 32 of the planned 62 turbines, with an anticipated eventual capacity of 806 megawatts.
The Japanese-flagged LNG tanker Energy Glory.Dan Kitwood/Getty Images
Energy groups and CEOs are seeking exemptions to the Trump administration’s rule requiring 1% of U.S. liquified natural gas exports to be shipped on American-made, operated, and flagged vessels within four years, with incremental increases up to 15% by 2047. There are 792 LNG carriers worldwide, most of which belong to South Korea and Japan; just five, dating to the 1970s, were made in U.S. shipyards and are not currently in use, Reuters reports.
As a result, energy executives and groups, including the influential American Petroleum Institute, argue that the Trump administration’s rule puts U.S. energy companies at a disadvantage. Exporters “have little control over their ability to comply with [U.S. Trade Representative’s] new requirements but ultimately face the consequences of not doing so,” API CEO Mike Sommers wrote in a letter reviewed by Reuters. Building five LNG tankers in the U.S. by the end of the decade to meet the 1% threshold is not doable, Sommers added, because it takes five years to make such a carrier at one of the two U.S. shipyards capable of such production.
The National Oceanic and Atmospheric Administration announced Thursday that its database of extreme weather events “will be retired” as part of ongoing cost-saving cuts and reorganization at the agency. Though the database doesn’t explicitly focus on climate event attribution, it contains data going back to the 1980s, charting the upward trend of billion-dollar disasters in the U.S., including severe hail, flooding, wildfires, and hurricanes. “This administration thinks that if they stop doing the work to identify climate change that climate change will go away,” Democratic Representative and former meteorologist Eric Sorensen of Illinois told The Washington Post.
Though the Trump administration has made deep and sweeping cuts across the federal government, it has especially singled out climate-related programs and databases. Some grant seekers have been encouraged to reapply with climate-related language removed from their proposals, a rhetorical shift that has also made its way into business branding, as my colleague Katie Brigham and I have covered for Heatmap. In addition to obscuring the picture of how climate change is potentially increasing damage and deaths in the United States, sunsetting the database is also causing headaches for insurance groups and financial risk modeling firms like First Street, whose head of climate implications and co-founder Jeremy Porter told CNN, “without it, replicating or extending damage trend analyses, especially at regional scales or across hazard types, is nearly impossible without significant funding or institutional access to commercial catastrophe models.”
The new pope, Robert Prevost — now known as Leo XIV — is expected to follow in Pope Francis’ footsteps when it comes to calling for urgent action on climate change. Speaking last year, Prevost “reiterated the Holy See’s commitment to protecting the environment, enumerating examples, like the Vatican installing solar panels and shifting to electric vehicles,” Vatican News reports.
The nonprofit laid off 36 employees, or 28% of its headcount.
The Trump administration’s funding freeze has hit the leading electrification nonprofit Rewiring America, which announced Thursday that it will be cutting its workforce by 28%, or 36 employees. In a letter to the team, the organization’s cofounder and CEO Ari Matusiak placed the blame squarely on the Trump administration’s attempts to claw back billions in funding allocated through the Greenhouse Gas Reduction Fund.
“The volatility we face is not something we created: it is being directed at us,” Matusiak wrote in his public letter to employees. Along with a group of four other housing, climate, and community organizations, collectively known as Power Forward Communities, Rewiring America was the recipient of a $2 billion GGRF grant last April to help decarbonize American homes.
Now, the future of that funding is being held up in court. GGRF funds have been frozen since mid-February as Lee Zeldin’s Environmental Protection Agency has tried to rescind $20 billion of the program’s $27 billion total funding, an effort that a federal judge blocked in March. While that judge, Tanya S. Chutkan, called the EPA’s actions “arbitrary and capricious,” for now the money remains locked up in a Citibank account. This has wreaked havoc on organizations such as Rewiring America, which structured projects and staffing decisions around the grants.
“Since February, we have been unable to access our competitively and lawfully awarded grant dollars,” Matusiak wrote in a LinkedIn post on Thursday. “We have been the subject of baseless and defamatory attacks. We are facing purposeful volatility designed to prevent us from fulfilling our obligations and from delivering lower energy costs and cheaper electricity to millions of American households across the country.”
Matusiak wrote that while “Rewiring America is not going anywhere,” the organization is planning to address said volatility by tightening its focus on working with states to lower electricity costs, building a digital marketplace for households to access electric upgrades, and courting investment from third parties such as hyperscale cloud service providers, utilities, and manufacturers. Matusiak also said Rewiring America will be restructured “into a tighter formation,” such that it can continue to operate even if the GGRF funding never comes through.
Power Forward Communities is also continuing to fight for its money in court. Right there with it are the Climate United Fund and the Coalition for Green Capital, which were awarded nearly $7 billion and $5 billion, respectively, through the GGRF.
What specific teams within Rewiring America are being hit by these layoffs isn’t yet clear, though presumably everyone let go has already been notified. As the announcement went live Thursday afternoon, it stated that employees “will receive an email within the next few minutes informing you of whether your role has been impacted.”
“These are volatile and challenging times,” Matusiak wrote on LinkedIn. “It remains on all of us to create a better world we can all share. More so than ever.”
A battle ostensibly over endangered shrimp in Kentucky
A national park is fighting a large-scale solar farm over potential impacts to an endangered shrimp – what appears to be the first real instance of a federal entity fighting a solar project under the Trump administration.
At issue is Geenex Solar’s 100-megawatt Wood Duck solar project in Barren County, Kentucky, which would be sited in the watershed of Mammoth Cave National Park. In a letter sent to Kentucky power regulators in April, park superintendent Barclay Trimble claimed the National Park Service is opposing the project because Geenex did not sufficiently answer questions about “irreversible harm” it could potentially pose to an endangered shrimp that lives in “cave streams fed by surface water from this solar project.”
Trimble wrote these frustrations boiled after “multiple attempts to have a dialogue” with Geenex “over the past several months” about whether battery storage would exist at the site, what sorts of batteries would be used, and to what extent leak prevention would be considered in development of the Wood Duck project.
“The NPS is choosing to speak out in opposition of this project and requesting the board to consider environmental protection of these endangered species when debating the merits of this project,” stated the letter. “We look forward to working with the Board to ensure clean water in our national park for the safety of protection of endangered species.”
On first blush, this letter looks like normal government environmental stewardship. It’s true the cave shrimp’s population decline is likely the result of pollution into these streams, according to NPS data. And it was written by career officials at the National Park Service, not political personnel.
But there’s a few things that are odd about this situation and there’s reason to believe this may be the start of a shift in federal policy direction towards a more critical view of solar energy’s environmental impacts.
First off, Geenex has told local media that batteries are not part of the project and that “several voicemails have been exchanged” between the company and representatives of the national park, a sign that the company and the park have not directly spoken on this matter. That’s nothing like the sort of communication breakdown described in the letter. Then there’s a few things about this letter that ring strange, including the fact Fish and Wildlife Service – not the Park Service – ordinarily weighs in on endangered species impacts, and there’s a contradiction in referencing the Endangered Species Act at a time when the Trump administration is trying to significantly pare back application of the statute in the name of a faster permitting process. All of this reminds me of the Trump administration’s attempts to supposedly protect endangered whales by stopping offshore wind projects.
I don’t know whether this solar farm’s construction will indeed impact wildlife in the surrounding area. Perhaps it may. But the letter strikes me as fascinating regardless, given the myriad other ways federal agencies – including the Park Service – are standing down from stringent environmental protection enforcement under Trump 2.0.
Notably, I reviewed the other public comments filed against the project and they cite a litany of other reasons – but also state that because the county itself has no local zoning ordinance, there’s no way for local residents or municipalities opposed to the project to really stop it. Heatmap Pro predicts that local residents would be particularly sensitive to projects taking up farmland and — you guessed it — harming wildlife.
Barren County is in the process of developing a restrictive ordinance in the wake of this project, but it won’t apply to Wood Duck. So opponents’ best shot at stopping this project – which will otherwise be online as soon as next year – might be relying on the Park Service to intervene.