Have you ever noticed that your weather forecast shows both the actual temperature and a “feels like” temperature?
That’s because 30 degrees Celsius (86 degrees Fahrenheit) when it’s breezy and dry feels very different from 30C (86F) when it’s stagnant and humid.
In this visual explainer, we show the differences between air temperature and “feels like” temperature, and highlight the regions around the world where heat stress is highest.
First, how is temperature measured?
The temperature you see on the news or the weather app on your phone relies on a network of weather stations positioned around the globe.
To ensure accurate readings, weather stations typically use specialist platinum resistance thermometers placed inside shaded instruments known as a Stevenson screen.
Measurements are taken at a standard height of 1.25-2 metres (4-6.5 feet) above the ground. This provides a reading that reflects the air temperature that people actually feel.
There are two well-known scales used to measure temperature: Celsius and Fahrenheit.
Only a few countries, including the United States, use Fahrenheit as their official scale. Most of the world uses the Celsius scale, named after Swedish astronomer Anders Celsius, who invented the 0-100 degree freezing and boiling point scale in 1742.
What does “feels like” temperature measure?
Measuring air temperature alone doesn’t always reflect how hot or cold it actually feels to your body.
That is why weather reports often include the “feels like” temperature in hot conditions.
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This measure adjusts the air temperature to show how your body really experiences it, taking into account factors such as humidity, wind speed, and sun exposure.
Humidity
Humidity measures how much water vapour is in the air.
The human body works best when its temperature is about 37C (98.6F). To stay cool, blood vessels widen to move more blood to the skin, and sweat glands release moisture that carries heat away as it evaporates.
But in humid conditions, evaporation slows, making this cooling system less effective and increasing the risk of overheating even while sweating.
Wind speed
Wind speed affects the “feels like” temperature in two opposite ways, depending on whether it is cold or hot.
In cold weather, wind increases heat loss from your body by blowing away the thin layer of warm air around your skin. The faster the wind, the colder it may feel.
In hot weather, a light breeze can help evaporate sweat, making it feel cooler.
Sun exposure
Sun exposure makes it feel hotter than the shaded air temperature because your body absorbs infrared radiation from the sun. Even if the thermometer reads the same, direct sunlight adds extra warmth, which is why shaded areas feel cooler.
How high can heat stress get?
Heat stress occurs when the body absorbs more heat than it can release. While physical activity can contribute, it can also result from high air temperatures, elevated humidity, and direct sunlight.
To quantify how humans physiologically experience weather, the Universal Thermal Climate Index (UTCI) is used. It is a bioclimatic index with 10 thermal stress categories, ranging from extreme heat (above 46C/115F) to extreme cold (below -40C/-40F).
Under extreme conditions, the “feels like” temperature can differ from the actual air temperature by up to 15C (27F) or even higher.
The graphic below shows the difference between the actual air temperature and the “feels like” temperature across multiple cities. Cities with high humidity or direct sun often show a much higher “feels like” temperature than the actual temperature, while windy or cold locations can feel significantly colder.
Where is heat stress highest?
According to the World Health Organization, heat stress is the leading cause of weather-related deaths and can exacerbate underlying illnesses, including cardiovascular disease, diabetes, mental health and asthma, as well as increase the risk of accidents and the transmission of a number of infectious diseases.
According to data from the Copernicus Climate Change Service (C3S), Baghdad, Iraq has the world’s highest heat stress level at 50.8C (123.4F), followed by Kuwait City, Kuwait at 50.3C (122.5F) and Doha, Qatar at 49.2C (120.6F).
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The regions with the highest heat stress are generally those that combine very high temperatures, high humidity, and intense sun exposure.
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