There is a certain kind of heat that doesn’t make a big impression. It comes on slowly: the morning feels warmer than it should, the afternoon lasts longer than it should, and the night won’t get any better. It looks like that during a heat wave, and you never forget it after going through one.
Most of the time, heat waves are periods of unusually hot weather that last for more than two days. But that medical description doesn’t really show how they feel on the inside. They settle over an area like a heavy lid on top of a pot. High-pressure systems in the upper atmosphere trap heat close to the ground, stopping the flow of air that would normally lower the temperature. The sky stays clear. There is no break in the sun’s beating. And the temperature goes up.
A modern heat wave is not the same as the hot spells of the past because it is not just stronger. It’s sticking with it. In a normal summer, cooler nights let people, buildings, and the land itself get rid of the heat they took in during the day. That daily reset is what helps you handle a string of hot days. That recovery cycle stops working when the nights stop getting cooler, or when the lowest temperature stays high night after night. It gets hotter. The body never fully heals itself. And the health risk to people grows in ways that aren’t always clear until it’s too late.
The numbers that have come out in the last few years are really scary. Between 1992 and 2021, the number of deaths around the world caused by heat rose from about 335,000 per year in the 1990s to about 546,000 per year. That’s an increase of more than 60% in about 20 years. Still, a lot of people don’t report heat deaths, mostly because they look like heart attacks or breathing problems instead of what they are: the body giving up on fighting the heat.
There are people who are really hurt by heat waves. A disproportionate amount of the burden falls on older adults, babies, pregnant women, people with long-term illnesses, people who work outside, and people who don’t have access to air conditioning. People can’t help but notice that places that aren’t set up to handle extreme heat are often the ones that get it the most. The problem that cities have is that buildings, asphalt, and concrete soak up heat during the day and keep it there at night. This is known as the “urban heat island effect.” During a long heat wave, temperatures can be much higher in a dense city than in the surrounding areas. This makes an already dangerous situation even worse.
It’s quieter, but the economic cost is huge. Heat stress caused about $835 billion worth of lost work productivity around the world in 2023 alone. As the temperature rises, people who work outside move more slowly, make more mistakes, and are more likely to get hurt. Crops are hurt. The need for cooling puts a lot of stress on power grids. Infrastructure like roads, train tracks, and even airplanes have limits that can be pushed past by heat in ways that make daily life more difficult in surprisingly normal ways. A rail line that buckles on a Tuesday afternoon doesn’t make the news, but it does affect a lot of people and costs a lot of money to fix.
When you look at how things have changed over several summers, you get the feeling that people are still getting used to how heat waves really are. Hurricanes are given names. If a tornado hits, sirens go off. Heat comes on quietly and kills more people than either, often without a formal announcement or shocking news footage. The early warning systems are getting better. Now, meteorologists can predict heat events days ahead of time, giving communities and the government time to get ready. But getting ready means taking the threat seriously, which isn’t always the case depending on where you live.
The one thing that is clear is that heat waves are no longer rare. They keep showing up on the calendar, and they stay longer and more often than they used to. The trend is what matters, not whether this summer is the hottest on record or just another very hot one. At this point, the pattern is only going one way.
