By Il fait trop chaud. The Paris Olympics officially opened on July 26, just in time for athletes to compete in a hellish heat wave.
Last week, Olympians in the French capital faced daytime temperatures reaching 35° Celsius (95° Fahrenheit), and in southern France temperatures climbed up to 40° C (104° F). Other countries in the Mediterranean region also felt the heat wave. In Spain, the city of Barcelona recorded its highest-ever temperature — 40° C — on July 31. And in Italy, the sweltering weather helped fuel a wildfire in the Monte Mario natural reserve in Rome.
This extreme heat would not have happened without climate change, researchers from the World Weather Attribution Network reported on July 31. “If the atmosphere wasn’t overloaded with emissions from burning fossil fuel, Paris would have been about 3 [degrees] C cooler and much safer for sport,” said climatologist Friederike Otto of the Imperial College London in a statement.
Other parts of the world sweltered, too. In California, Death Valley set the record for the hottest month ever recorded on Earth, averaging 42.5° C (108.5° F) across July. A major heat dome also settled in over the southern United States (SN: 6/21/24). The extreme heat swept across much of the country by the end of the week, with the National Weather Service placing more than 150 million people under extreme heat advisories on August 1. Antarctica’s eastern region also experienced a major heat wave, with ground temperatures rising beyond 28° C.
Amid all this hotness, it’s important to remember that the thermometer doesn’t always indicate how hot our bodies feel. That brings us to this week’s reader question from Robert Ball:
What is the heat index?
The heat index, or apparent temperature, considers humidity along with temperature to indicate how we really feel in the heat. That’s why it’s often called the “feels like” temperature in forecasts.
Humidity is important because the body’s primary mechanism for cooling in hot weather is sweating (SN: 7/27/22). When sweat evaporates from the skin, it carries heat away from the body. But if the air holds a lot of moisture, the evaporation of sweat slows down. In 100 percent humidity, little to no sweat evaporates.
It’s important to note that the heat index has a major limitation: It was devised for shady conditions with little wind. If you’re working or exercising outdoors, a more useful indicator would probably be the wet bulb globe temperature, or WBGT, which provides the apparent temperature in direct sunlight.
Getting the WBGT is a bit more complicated than calculating the heat index. You first measure the air temperature with a thermometer that is shielded from sunlight and moisture. Then, you measure the “wet bulb” temperature with a thermometer that’s wrapped in a wet cloth, mimicking the cooling effect of sweating. Lastly, you need a thermometer inside a copper globe that’s painted black and positioned in direct sunlight, to measure the heat stress from sunlight. Add 10 percent of the air temperature, 70 percent of the wet bulb temperature and 20 percent of the black globe temperature to calculate the WBGT.
The takeaway here is that when compared to temperature alone, the heat index and WBGT can help provide a more complete picture of how we feel the heat, and when we need to take measures to avoid heat stroke and exhaustion. These are vital tools in a world where heat waves are becoming more common and more intense (SN: 4/3/18).
Please keep sending in your questions about Earth’s extreme heat and shifting climate — we’ll look for ones to answer in upcoming Extreme Climate Update columns.
