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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 a late-night House vote, Tesla’s slump, and carbon credits
Current conditions: Tropical storm Chantal has a 40% chance of developing this weekend and may threaten Florida, Georgia, and the Carolinas • French far-right leader Marine Le Pen is campaigning on a “grand plan for air conditioning” amid the ongoing record-breaking heatwave in Europe • Great fireworks-watching weather is in store tomorrow for much of the East and West Coasts.
The House moved closer to a final vote on President Trump’s “big, beautiful bill” after passing a key procedural vote around 3 a.m. ET on Thursday morning. “We have the votes,” House Speaker Mike Johnson told reporters after the rule vote, adding, “We’re still going to meet” Trump’s self-imposed July 4 deadline to pass the megabill. A floor vote on the legislation is expected as soon as Thursday morning.
GOP leadership had worked through the evening to convince holdouts, with my colleagues Katie Brigham and Jael Holzman reporting last night that House Freedom Caucus member Ralph Norman of North Carolina said he planned to advance the legislation after receiving assurances that Trump would “deal” with the Inflation Reduction Act’s clean energy tax credits, particularly for wind and solar energy projects, which the Senate version phases out more slowly than House Republicans wanted. “It’s not entirely clear what the president could do to unilaterally ‘deal with’ tax credits already codified into law,” Brigham and Holzman write, although another Republican holdout, Representative Chip Roy of Texas, made similar allusions to reporters on Wednesday.
Tesla delivered just 384,122 cars in the second quarter of 2025, a 13.5% slump from the 444,000 delivered in the same quarter of 2024, marking the worst quarterly decline in the company’s history, Barron’s reports. The slump follows a similarly disappointing Q1, down 13% year-over-year, after the company’s sales had “flatlined for the first time in over a decade” in 2024, InsideEVs adds.
Despite the drop, Tesla stock rose 5% on Wednesday, with Wedbush analyst Dan Ives calling the Q2 results better than some had expected. “Fireworks came early for Tesla,” he wrote, although Barron’s notes that “estimates for the second quarter of 2025 started at about 500,000 vehicles. They started to drop precipitously after first-quarter deliveries fell 13% year over year, missing Wall Street estimates by some 40,000 vehicles.”
The European Commission proposed its 2040 climate target on Wednesday, which, for the first time, would allow some countries to use carbon credits to meet their emissions goals. EU Commissioner for Climate, Net Zero, and Clean Growth Wopke Hoekstra defended the decision during an appearance on Euronews on Wednesday, saying the plan — which allows developing nations to meet a limited portion of their emissions goals with the credits — was a chance to “build bridges” with countries in Africa and Latin America. “The planet doesn’t care about where we take emissions out of the air,” he separately told The Guardian. “You need to take action everywhere.” Green groups, which are critical of the use of carbon credits, slammed the proposal, which “if agreed [to] by member states and passed by the EU parliament … is then supposed to be translated into an international target,” The Guardian writes.
Around half of oil executives say they expect to drill fewer wells in 2025 than they’d planned for at the start of the year, according to a Federal Reserve Bank of Dallas survey. Of the respondents at firms producing more than 10,000 barrels a day, 42% said they expected a “significant decrease in the number of wells drilled,” Bloomberg adds. The survey further indicates that Republican policy has been at odds with President Trump’s “drill, baby, drill” rhetoric, as tariffs have increased the cost of completing a new well by more than 4%. “It’s hard to imagine how much worse policies and D.C. rhetoric could have been for U.S. E&P companies,” one anonymous executive said in the report. “We were promised by the administration a better environment for producers, but were delivered a world that has benefited OPEC to the detriment of our domestic industry.”
Fine-particulate air pollution is strongly associated with lung cancer-causing DNA mutations that are more traditionally linked to smoking tobacco, a new study by researchers at the University of California, San Diego, and the National Cancer Institute has found. The researchers looked at the genetic code of 871 non-smokers’ lung tumors in 28 regions across Europe, Africa, and Asia and found that higher levels of local air pollution correlated with more cancer-driving mutations in the respective tumors.
Surprisingly, the researchers did not find a similar genetic correlation among non-smokers exposed to secondhand smoke. George Thurston, a professor of medicine and population health at New York University, told Inside Climate News that a potential reason for this result is that fine-particulate air pollution — which is emitted by cars, industrial activities, and wildfires — is more widespread than exposure to secondhand smoke. “We are engulfed in fossil-fuel-burning pollution every single day of our lives, all day long, night and day,” he said, adding, “I feel like I’m in the Matrix, and I’m the only one that took the red pill. I know what’s going on, and everybody else is walking around thinking, ‘This stuff isn’t bad for your health.’” Today, non-smokers account for up to 25% of lung cancer cases globally, with the worst air quality pollution in the United States primarily concentrated in the Southwest.
EPA
National TV news networks aired a combined 4 hours and 20 minutes of coverage about the record-breaking late-June temperatures in the Midwest and East Coast — but only 4% of those segments mentioned the heat dome’s connection to climate change, a new report by Media Matters found.
“We had enough assurance that the president was going to deal with them.”
A member of the House Freedom Caucus said Wednesday that he voted to advance President Trump’s “big, beautiful bill” after receiving assurances that Trump would “deal” with the Inflation Reduction Act’s clean energy tax credits – raising the specter that Trump could try to go further than the megabill to stop usage of the credits.
Representative Ralph Norman, a Republican of North Carolina, said that while IRA tax credits were once a sticking point for him, after meeting with Trump “we had enough assurance that the president was going to deal with them in his own way,” he told Eric Garcia, the Washington bureau chief of The Independent. Norman specifically cited tax credits for wind and solar energy projects, which the Senate version would phase out more slowly than House Republicans had wanted.
It’s not entirely clear what the president could do to unilaterally “deal with” tax credits already codified into law. Norman declined to answer direct questions from reporters about whether GOP holdouts like himself were seeking an executive order on the matter. But another Republican holdout on the bill, Representative Chip Roy of Texas, told reporters Wednesday that his vote was also conditional on blocking IRA “subsidies.”
“If the subsidies will flow, we’re not gonna be able to get there. If the subsidies are not gonna flow, then there might be a path," he said, according to Jake Sherman of Punchbowl News.
As of publication, Roy has still not voted on the rule that would allow the bill to proceed to the floor — one of only eight Republicans yet to formally weigh in. House Speaker Mike Johnson says he’ll, “keep the vote open for as long as it takes,” as President Trump aims to sign the giant tax package by the July 4th holiday. Norman voted to let the bill proceed to debate, and will reportedly now vote yes on it too.
Earlier Wednesday, Norman said he was “getting a handle on” whether his various misgivings could be handled by Trump via executive orders or through promises of future legislation. According to CNN, the congressman later said, “We got clarification on what’s going to be enforced. We got clarification on how the IRAs were going to be dealt with. We got clarification on the tax cuts — and still we’ll be meeting tomorrow on the specifics of it.”
Neither Norman nor Roy’s press offices responded to a request for comment.
The foreign entities of concern rules in the One Big Beautiful Bill would place gigantic new burdens on developers.
Trump campaigned on cutting red tape for energy development. At the start of his second term, he signed an executive order titled, “Unleashing Prosperity Through Deregulation,” promising to kill 10 regulations for each new one he enacted.
The order deems federal regulations an “ever-expanding morass” that “imposes massive costs on the lives of millions of Americans, creates a substantial restraint on our economic growth and ability to build and innovate, and hampers our global competitiveness.” It goes on to say that these regulations “are often difficult for the average person or business to understand,” that they are so complicated that they ultimately increase the cost of compliance, as well as the risks of non-compliance.
Reading this now, the passage echoes the comments I’ve heard from industry groups and tax law experts describing the incredibly complex foreign entities of concern rules that Congress — with the full-throated backing of the Trump administration — is about to impose on clean energy projects and manufacturers. Under the One Big Beautiful Bill Act, wind and solar, as well as utility-scale energy storage, geothermal, nuclear, and all kinds of manufacturing projects will have to abide by restrictions on their Chinese material inputs and contractual or financial ties with Chinese entities in order to qualify for tax credits.
“Foreign entity of concern” is a U.S. government term referring to entities that are “owned by, controlled by, or subject to the jurisdiction or direction of” any of four countries — Russia, Iran, North Korea, and most importantly for clean energy technology, China.
Trump’s tax bill requires companies to meet increasingly strict limits on the amount of material from China they use in their projects and products. A battery factory starting production next year, for example, would have to ensure that 60% of the value of the materials that make up its products have no connection to China. By 2030, the threshold would rise to 85%. The bill lays out similar benchmarks and timelines for clean electricity projects, as well as other kinds of manufacturing.
But how companies should calculate these percentages is not self-evident. The bill also forbids companies from collecting the tax credits if they have business relationships with “specified foreign entities” or “foreign-influenced entities,” terms with complicated definitions that will likely require guidance from the Treasury for companies to be sure they pass the test.
Regulatory uncertainty could stifle development until further guidance is released, but how long that takes will depend on if and when the Trump administration prioritizes getting it done. The One Big Beautiful Bill Act contains a lot of other new tax-related provisions that were central to the Trump campaign, including a tax exemption for tips, which are likely much higher on the department’s to-do list.
Tax credit implementation was a top priority for the Biden administration, and even with much higher staffing levels than the department currently has, it took the Treasury 18 months to publish initial guidance on foreign entities of concern rules for the Inflation Reduction Act’s electric vehicle tax credit. “These things are so unbelievably complicated,” Rachel McCleery, a former senior advisor at the Treasury under Biden, told me.
McCleery questioned whether larger, publicly-owned companies would be able to proceed with major investments in things like battery manufacturing plants until that guidance is out. She gave the example of a company planning to pump out 100,000 batteries per year and claim the per-kilowatt-hour advanced manufacturing tax credit. “That’s going to look like a pretty big number in claims, so you have to be able to confidently and assuredly tell your shareholder, Yep, we’re good, we qualify, and that requires a certification” by a tax counsel, she said. To McCleery, there’s an open question as to whether any tax counsel “would even provide a tax opinion for publicly-traded companies to claim credits of this size without guidance.”
John Cornwell, the director of policy at the Good Energy Collective, which conducts research and advocacy for nuclear power, echoed McCleery’s concerns. “Without very clear guidelines from the Treasury and IRS, until those guidelines are in place, that is going to restrict financing and investment,” Cornwell told me.
Understanding what the law requires will be the first challenge. But following it will involve tracking down supply chain data that may not exist, finding alternative suppliers that may not be able to fill the demand, and establishing extensive documentation of the origins of components sourced through webs of suppliers, sub-suppliers, and materials processors.
The Good Energy Collective put out an issue brief this week describing the myriad hurdles nuclear developers will face in trying to adhere to the tax credit rules. Nuclear plants contain thousands of components, and documenting the origin of everything from “steam generators to smaller items like specialized fasteners, gaskets, and electronic components will introduce substantial and costly administrative burdens,” it says. Additionally the critical minerals used in nuclear projects “often pass through multiple processing stages across different countries before final assembly,” and there are no established industry standards for supply chain documentation.
Beyond the documentation headache, even just finding the materials could be an issue. China dominates the market for specialized nuclear-grade materials manufacturing and precision component fabrication, the report says, and alternative suppliers are likely to charge premiums. Establishing new supply chains will take years, but Trump’s bill will begin enforcing the sourcing rules in 2026. The rules will prove even more difficult for companies trying to build first-of-a-kind advanced nuclear projects, as those rely on more highly specialized supply chains dominated by China.
These challenges may be surmountable, but that will depend, again, on what the Treasury decides, and when. The Department’s guidance could limit the types of components companies have to account for and simplify the documentation process, or it could not. But while companies wait for certainty, they may also be racking up interest. “The longer there are delays, that can have a substantial risk of project success,” Cornwell said.
And companies don’t have forever. Each of the credits comes with a phase-out schedule. Wind manufacturers can only claim the credits until 2028. Other manufacturers have until 2030. Credits for clean power projects will start to phase down in 2034. “Given the fact that a lot of these credits start lapsing in the next few years, there’s a very good chance that, because guidance has not yet come out, you’re actually looking at a much smaller time frame than than what is listed in the bill,” Skip Estes, the government affairs director for Securing America’s Energy Future, or SAFE, told me.
Another issue SAFE has raised is that the way these rules are set up, the foreign sourcing requirements will get more expensive and difficult to comply with as the value of the tax credits goes down. “Our concern is that that’s going to encourage companies to forego the credit altogether and just continue buying from the lowest common denominator, which is typically a Chinese state-owned or -influenced monopoly,” Estes said.
McCleery had another prediction — the regulations will be so burdensome that companies will simply set up shop elsewhere. “I think every industry will certainly be rethinking their future U.S. investments, right? They’ll go overseas, they’ll go to Canada, which dumped a ton of carrots and sticks into industry after we passed the IRA,” she said.
“The irony is that Republicans have historically been the party of deregulation, creating business friendly environments. This is completely opposite, right?”