Measuring temperature in the shade: science or climate conspiracy

Why official thermometers don't measure the heat you feel on the street

Every summer the same complaint comes up: you step outside, the asphalt is boiling, the sun is scorching you, and Aemet (Spain's weather agency) announces 35 degrees when you swear it's 45. It feels like institutional deception. But the technical reality is more nuanced: measuring in the shade is not a trick, but a scientific convention with more than a century of history.

The recurring complaint: "they measure inside a box, it's not real"

The argument is intuitive: if the thermometer hides in a white, ventilated shelter, what it measures is not the temperature a pedestrian endures under direct sunlight. The feeling of heat depends on solar radiation, humidity, wind and the colour of surfaces. A thermometer exposed to the sun does not measure the air temperature, but that of its own casing heated by radiation, which can shoot 20 degrees above the real value. That is why, since the late 19th century, the World Meteorological Organization standardised the use of the weather shelter (the famous white box) to shield the sensor from direct radiation and measure exclusively the air temperature in the shade.

What if we measured it in the sun?

Those who call for "measuring the real temperature in the sun" ignore a basic problem: there is no single temperature in the sun. It depends on whether the sensor is black or white, on whether the wind cools it, on whether it faces south or north. Any standard would be arbitrary. Climate science needs homogeneous, comparable series over time. Changing the method now would invalidate 150 years of records and turn studies on global warming into garbage. Put another way: the alternative is not more accurate, just different.

The heat index: a patch that doesn't convince

Aware of the gap between the official figure and lived experience, weather services have developed indices such as the Heat Index or wind chill. Aemet offers the "apparent temperature" on its website, which combines temperature, humidity and wind. But these indices are not the real temperature, rather a subjective calculation. And the public receives them with scepticism: "first they give us a false figure and then correct it with an invented one".

The business of the 'real temperature'

Outdoor thermometers promising the "real temperature" and showing spectacular figures are proliferating online. Some sensationalist media use them for eye-catching headlines. But those devices, placed in the sun on a façade, do not measure the air but the temperature of the wall. Their value for climate science is nil, though profitable for marketing.

It is likely that, in the near future, weather reports will include a supplement called "effective temperature under the sun" for popular consumption. But until that happens, the annual complaint will live on. And rightly so: it's not that science lies, it's that the citizen wants a figure that describes their suffering, not a global average. Until a universal index is invented that satisfies everyone, the thermometer will stay in its box.