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Comment by quantadev | Hacker News Reader

Thermodynamics dude here. :)

All moving air does is increase the rate of the heat transfer from the higher temp substance to the lower temp one. If the air outside is hotter than your body temperature then a fan will be heating you UP not cooling you DOWN.

Without electricity or chemical reactions the only way to stay cool in high heats is to use evaporative effects. Evaporation is endothermic (takes in heat) and actually cools down the thing being evaporated off of. So you can basically soak your shirt and as it evaporates it cools you down. There are things called 'evaporative vests' which can cool you off for hours.

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Those two paragraphs are at odds with one another, no? Moving air also increases the evaporation rate.

This is why fans are found to be generally effective to 104 degrees. https://time.com/5644737/fans-can-make-you-hotter/

It’s poorly worded, not incorrect.

I’d replace top with something like: A fan isn’t directly helpful, unlike an AC it’s not going to cool an empty closed room. However, it can make a wider range of temperatures and humidity levels comfortable through aiding evaporation or bringing in cooler air when possible.

Continuing…

This is why as the article mentions a fan alone can be worse than doing nothing in sufficiently hot and dry air. People only sweat so fast so at some point without supplemental moisture the fan acts like a convection oven simply warming people faster. This can be offset by adding supplemental moisture via a 'evaporative vests' or mister the maximum temperature increases.

This and other reasons mean health organizations are concerned with finding and communicating the maximum temperature fans are still useful.

If you think about it, fan use during super high heats could lead to fatalities, because people using them could possibly be so hot, they're not even able to realize the moving of hot air over them is actually heating them up more. Anyway, like I said the only REAL solution is evaporative effects, if there's no AC to be had.

> fans are found to be generally effective to 104 degrees

The OP goes through a lot of evidence saying the threshold is lower.

I wouldn't dispute that there's a very narrow band of temperature differential and air flow rates where the speeded up rate of airflow increases evaporation cooling effects enough to compensate for the faster rate of heat delivery from the air itself. The hotter the air tho, the slower you'd want your fan to be, in this narrow range of conditions.

> So you can basically soak your shirt and as it evaporates it cools you down. There are things called 'evaporative vests' which can cool you off for hours.

Or like, sweat? Which is something people who aren't dehydrated tend to do at or above 95F (35C).

Have you ever spent hours sweating (working) in humid 35 degrees? You wish you'd rather die. Any help is appreciated.

While this is an exaggeration, it really is very very bad. I have stayed for short amount in very hot and humid areas where sadly electricity was rare and yes you just want to get out of there as soon as you can.

It makes a huge difference if you get introduced to high temperatures and humidities over a period of weeks or months rather than all of a sudden. People live in hot climates year round without feeling miserable all the time. Most bodies are able to do a surprisingly good job of adapting to high temperatures if the transition is done very gradually.

Living in hot and humid climate is miserable even for people living there all the time. Yes humans persist and carry on but it doesn't make it less miserable, and any lucky person who was able to escape will never ever want to go back to that lifestyle.

30y Floridian here.

> People live in hot climates year round without feeling miserable all the time.

Not this person, nor my sons (born here) or siblings. Heat makes us all sick.

Florida has 6 or 8 actual seasons and none of them are Fall, Spring or Winter.

Note I didn’t say all people, but out of curiosity how much of your time do you spend in air conditioned environments?

> but out of curiosity how much of your time do you spend in air conditioned environments?

Over 30y, I can give all possible answers to that question. I was 15y in scouting leadership. During June-Aug, I would be outdoors for weeks at a time. What I got used to was feeling sick every day.

That said, early heat stroke symptoms get old after a decade or three. Now I go outside after sunset.

I wouldn't want to live in an environment that's basically too hot for human life to thrive. There are people all over the world right now living in areas that are genuinely deadly if the AC goes out. People say "Oh, we do just fine in Death Valley, etc" (the American West in general) and that's just a person who is alive only because of AC. lol.

Tell that to my usual Spring. Here come the 80s/90s and humidity until November.

I've never even thought about whether evaporative cooling CAN happen in like 100% humidity? Whether from sweat or from wet clothing. It seems like if there's so much air moisture evaporation can't happen, and the additional kinetic energy of hot water molecules just seems like energy that simply cannot be gotten rid of. Maybe one of my personal troll followers can look this up on ChatGPT and tell me I'm wrong? :)

I spend a lot of time riding my bike in the summer here in Massachusetts. Temperatures regularly get up to around 35 °C (95 °F), but fortunately the humidity isn't usually too bad. These rides are usually at least two hours long, and sometimes up to six. I'm always surprised at how tolerable those temperatures are when air is moving by you at a decent speed. It definitely requires a lot of water and mineral replacement to sustain. Aside from having to make more stops to refill and pay closer attention to hydration, I surprisingly find it more pleasant than riding at 30 °C (86 °F).

> but fortunately the humidity isn't usually too bad

...and this is what makes the actual difference and matters much more than the temperature itself.

35 degrees in dry air is a nice summer vacation.

Weird! I replied to that comment because I didn’t think they mentioned humidity, though I see they did. I appear to have just completely overlooked it. Sorry about that!

Yes, I have. Evaporative cooling with a fan in those conditions is still better than no fan.

The article mentions but doesn’t do a good enough job explaining at high temperatures and low humidity a fan can remove sweat faster than your body can supply it. Making excessive fan speeds counter productive and actually increasing body temperatures, ie there’s humidity + temperatures where low works better than high and edge cases where even low is counterproductive.

This flips with supplemental water which once again makes faster fan speeds useful up to significantly higher temperatures. Obviously there’s another limit as a mister alone can’t lower a rocket engine exhaust to be comfortable.

Net result a lot of seemingly contradictory recommendations shown in the article.

Yes, and evaporation happens faster with a fan.

And you don't need to soak your shirt, you just need to sweat which happens automatically in most people.

So even with hotter air, the fan can be cooling you DOWN.

Yeah as a BSME I'm well aware of what sweat does. That's your body's way of doing evaporation cooling. But if you want serious cooling effects then wet clothing can basically make the difference between life and death in extreme conditions. It's a valuable skill to know. Saying "Just sweat" is not a skill to know.

Doesn't moving air help evaporation anyhow?

If evaporation is possible, yes.

Thanks. I get the idea, but the OP says,

"... results in dry conditions starkly illustrated the dangers of misusing fans. The scientists stopped the experiments after testing just 14 people because their cardiac stress went through the roof as the fan blew hot air on them. Nearly half the participants couldn’t stay in the test room for 3 hours. The reason for that dramatic response: The air was so dry that any sweat quickly evaporated, resulting in little additional cooling as the fan bathed participants in extremely hot air, Jay says."

Why would faster evaporation result in less cooling? I could make some guesses but I wonder what the reality is.

Possibly, they mean that the evaporation was exhausted quickly and after that, it was just a convection oven.

evaporation done with hot wind has two effects working in opposite directions. The evaporation is a cooling effect, but the hot wind is a heating effect. So if the hot wind is moving too fast or is too hot, it's effects will be larger than the effects of any evaporation, resulting in heating rather than cooling, due to increased airflow, or too high a temp.

They connected the insufficient cooling to the rate of evaporation being too rapid:

"The air was so dry that any sweat quickly evaporated, resulting in little additional cooling ..."

I wasn't correcting or disagreeing, just providing more info that there's two effects going on. Hot airflow always adds heat, and evaporation always reduces heat, so you only get cooling if the evap energy loss exceeds the heat transfer rate.

I can't get my wife to understand this with respect to our pets. It's infuriating, because she's not retarded, just stubborn as hell.

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