Wikipedia:Reference desk/Archives/Science/2020 January 20

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January 20

Why do heat index values go below the actual temperature?

Comparison of NWS heat index values (circles) with the formula approximation (curves). In the SVG file, hover over a graph to highlight it.

In extended heat index tables, the feels-like temperature is below the actual temperature for low humidities, as shown by the dotted line in the graph. Would anyone know why this so?

What is then the base humidity where the feels-like and actual temperatures are equal? Why was this value chosen?

Finally, I can't find a table for zero relative humidity. Is it defined?

Thanks,
cmɢʟee⎆τaʟκ 00:24, 20 January 2020 (UTC)

Humans perceive high relative humidity to be uncomfortable. That is why high temps and high humidities are associated with a higher-than-actual temp. High temps are mitigated by the cooling effect of evaporation of perspiration. At low relative humidities perspiration evaporates rapidly, leading to increased cooling and the perception of a lower-than-actual temp. Dolphin (t) 00:40, 20 January 2020 (UTC)

• Dew point comfort varies by what you are used to: [1]. So, any type of heat index based on what it feels like is subjective. There are objective measures, like wet bulb temperature, but there's no guarantee that those will match human perception.

• Zero humidity shouldn't be possible on the surface of the Earth, although it has gotten close: [2]. You could create a zero humidity environment in a lab, but once you put a human in there to determine how uncomfortable it would be, the water vapor sucked out of them would make it a non-zero humidity again. NonmalignedNations (talk) 04:44, 20 January 2020 (UTC)

• I suppose since it's called feels-"like" temperature, it should compare to a realistic and imaginable dry conditions that people can relate to. Zero humidity doesn't exist, so obviously you can't relate to it. I'd also question how comfortable a totally dry environment would be, especially below 30°C. I really doubt I'd enjoy 25°C at a parching 1% humidity more than 25°C at say 70%. 89.172.58.25 (talk) 06:37, 20 January 2020 (UTC)

The articles Heat index and Wind chill should be read together. DroneB (talk) 13:53, 20 January 2020 (UTC)