TEMPO.CO, Jakarta - Another summer, another wave of headlines about record-breaking heat. From Northeast Asia to Europe to the United States, extreme temperatures are contributing to thousands of deaths, fueling wildfires and straining water, food and energy supplies.
But what do the words behind the headlines mean? How do scientists know if temperatures are record-breaking? Why do cities warm up more than rural areas? And does humidity really make everything feel hotter?
DW spoke to climatologists to answer some common questions about how heat is measured, and why understanding heat terminology matters for everyone.
What does 'record-breaking heat' mean anyway?
It depends on what's being compared. Thousands of weather stations around the world track temperatures. A location can set a record if its temperature exceeds the previous high recorded at the station for a specific day, month or season.
Scientists also calculate regional and global temperature records by combining data from weather stations, ships and ocean buoys and determining the average across larger areas.
How far back that comparison goes depends on the dataset. The oldest continuous record of temperatures in the world is the Central England Temperature Data Series, which started in 1659.
But the three most complete datasets — maintained by the United States' Goddard Institute for Space Studies and National Centers for Environmental Information, plus the United Kingdom's Met Office Hadley Centre — began recording comprehensively in 1880.
These are the datasets behind statements such as "the past decade was the warmest on record." When media outlets use the shorthand "hottest day ever," they are talking about recording temperatures.
While Earth has experienced hotter climates and extreme temperatures over its 4.5-billion-year history, today's warming is exceptional in the context of the 12,000 years of human civilization.
But how do scientists know about Earth's climate before humans and thermometers were around?
To understand Earth's earlier climate, scientists rely on natural records known as "climate archives" that help them recreate long-term patterns in temperature. Those include ice cores, tree rings, ocean sediments and fossils.
"These preserve information about past temperatures, rainfall and atmospheric conditions going back thousands to millions of years," Friedrike Otto, a climatologist and founder of research group World Weather Attribution (WWA), told DW in an email.
The most complete continuous ice core record, drilled in Antarctica, stretches back more than a million years, for example. It contains ancient air bubbles that allow researchers to measure past atmospheric carbon dioxide levels. But other "climate archives" stretch back much further.
"As we know the equations that govern the physics of the climate, we can also use climate models to see how changes in different drivers produce different climate and weather conditions," said Otto.
That's helped scientists discern that higher greenhouse gases are closely linked with warmer global temperatures throughout Earth's history. For instance, about 56 million years ago, large amounts of carbon entered the atmosphere, likely from volcanic activity and methane released from ocean sediments, triggering a major period of global heating.
The difference today is climate change is primarily caused by humans burning fossil fuels and is happening at a much faster rate than over the last million years or so, says NASA.
But how do scientists know if heat waves are a trend or an anomaly?
Climate scientists analyze decades of data to distinguish long-term climatic changes from normal weather fluctuations. A single hot year doesn't prove a trend. But as records grow longer, statistical analyses and climate models make it easier to identify persistent patterns.
"You're looking for consistency. You're looking for a convergence of all the evidence," said Gerald Mills, a climatologist at University College Dublin.
Scientific agencies across different continents — including NASA, NOAA and the EU's Copernicus Climate Change Service — track global temperatures, using different methods and data sets. Despite those differences, they've reached the same conclusion: there is a heating trend and that the last decade has produced the hottest years ever recorded.
Researchers also use attribution science to assess the role climate change plays in extreme weather. Using climate models, they compare the likelihood and intensity of an event in today's climate with a simulated world without human-caused warming.
WWA ran exactly that analysis on Europe's June 2026 heat wave. The scientists found it was the most severe on record for the region studied. In 1976, a famously hot year across Europe, those temperatures would have been virtually impossible.
What heat metrics should you pay attention to in a weather forecast?
Air temperature or "actual temperature" don't fully describe how heat affects the human body. That's why weather services also provide "feels like" figures, which account for other factors.
In the US, the National Weather Service's heat index is based on air temperature and humidity. The UK's Met Office's "feels like" temperature also factors in wind chill.
Why does high humidity make it feel hotter and why is a high wet bulb temperature dangerous?
High humidity means more moisture in the air, which makes it harder for sweat evaporate. That reduces the body's ability to cool down, so the air temperature feels hotter in humid conditions.
Scientists also use the wet bulb temperature as a measure for heat stress. It is recorded by wrapping the bulb of a thermometer in a damp cloth. As water evaporates from the cloth, the bulb cools and the temperature reading drops, similar to how evaporation of sweat cools the human body.
As the wet bulb temperature approaches the actual air temperature, the air becomes increasingly saturated, and the body struggles to cool itself, increasing the risk of dangerous heat stress.
At sustained wet bulb temperatures of around 35 degrees Celsius (95 degrees Fahrenheit), the body can overheat even if people are resting in the shade or staying hydrated. But dry heat can be dangerous too, considering the risk of dehydration.
Why are cities hotter than rural areas?
That's because of the urban heat island effect, which makes cities warmer than surrounding areas for a few reasons.
Cities often replace vegetation and trees with roads, buildings and other hard surfaces that absorb and store heat. Greenery, on the other hand, cools an area through shade and evaporation. Dense buildings and narrow streets in urban areas also reduce air ventilation.
"All those things together, depending on where you are in relation to the city, can make it quite uncomfortable," Mills said.
Planting more trees, adding parks and green roofs, unsealing paved areas where possible and using lighter-colored, more reflective building materials can all help reduce the urban heat island effect by increasing shade, evaporation and the reflection of sunlight.
Read: Why Heat Waves Heighten the Risk of Blackouts
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