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Comment by glitcher | Hacker News Reader

TIL about heat index being a combination of temperature and humidity to arrive at a perceived temperature (in the shade apparently, why?).

Started plugging in some numbers into a heat index calculator just for kicks (https://www.omnicalculator.com/physics/heat-index).

I live in a very hot and dry area, and noticed that at 15% humidity the heat index matches temperature, below 15% results in a heat index below the temperature, and above 15% gives a heat index above the temperature.

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I believe it's measured in the shade only so it's a true measure of the air temperature and isn't affected by the direct heat transfer from the sunlight. (eg https://www.forbes.com/sites/dennismersereau/2021/06/30/the-... )

That's my understanding too. In fact, the hottest surface temperature recorded on Earth is much higher than the official temperature record because a surface gathers heat not only from air convection/conduction but direct radiation as well.

> Temperatures measured directly on the ground may exceed air temperatures by 30 to 50 °C (54 to 90 °F).[6] The highest natural ground surface temperature ever recorded may have been an alleged reading of 93.9 °C (201.0 °F) at Furnace Creek, California, United States, on 15 July 1972.[7] In 2011, a ground temperature of 84 °C (183.2 °F) was recorded in Port Sudan, Sudan. [1]

[1] https://en.wikipedia.org/wiki/List_of_weather_records#Measur...

Back when I was a kid, the Australian Open in Melbourne, already in summer, was plagued by periods where they stopped play due to heat.

Filling the void and dead air time, commentators resorted to demos like "frying an egg on center court" to show just how hot it was.

> (in the shade apparently, why?)

Because the effect of the sun induces massive, extremely localized variations in apparent temperature. Are you wearing a black shirt or a white shirt? That changes apparent sun temperature. What is the color of the ground you're standing on? What is the material, for that matter?

Note that all of these effects will only increase the temperature (during daytime, at least). The shade temperature is thus a good indicator of what the coolest possible outdoor environment will be.

The heat index measures your body's ability to regulate its temperature by sweating. Higher ambient humidity limits the effectiveness of sweating.

At "wet bulb temperature" your sweat doesn't cool you down anymore.

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