21 ms·

Wet-Bulb Temperature

112 points by hkc 5y ago

bpoyner 5y ago
This reminds me of swamp cooling. When I describe swamp cooling (evaporative cooling) to somebody who didn't grow up with it, it blows their mind.
- frankthedog 5y ago
  Please describe it
  - wing-_-nuts 5y ago
    You basically have a fan that sprays a very fine mist. The water evaporates, and in doing so the temperature of the air drops. Note that this only works when humidity is low enough for evaporation to actually happen. If the humidity is too high this doesn't work. Edit: wiki goes in to way more detail about other methods I was not familiar with: https://en.wikipedia.org/wiki/Evaporative_cooler https://en.wikipedia.org/wiki/Evaporative_cooler
    - deleted 5y ago
      [deleted]
    - joak 5y ago
      I dunno, I think that evaporation happens even if there is a lot of humidity in the air. The fastest (hottest) water molecules are going to escape the liquid water. The wind will carry them away. I think this phenomenon does not take in account the wind. That actually if you have water to wet yourself and a fan you can cool you down. Not sure though since I seem to be alone to think so.
      - oh_sigh 5y ago
        You've repeated this take twice, but it is still wrong. Yes, the fastest water molecules on your skin escape. But you also have water from the air also latching onto your skin which you aren't accounting for. You're free to see how well evaporative cooling works when there is a lot of water in the air. You can just take a fan into a bathroom and run the hottest shower possible for a few minutes.
  - phdelightful 5y ago
    I live in Southwest US where it’s common to use a swamp cooler (aka evaporative cooler) for your house. Instead of the normal refrigerated air unit that sits outside your house, there is basically a box with a big fan that pulls air across a big wooden or synthetic “sponge” and pushes it into your house ducts. A small pump like in a fish tank continuously pulls water across the sponge to keep it wet. You leave a couple windows in your house cracked open to leave an exit as new cold air is pushed in. Has a nice humidifying effect which is good in the desert too!
- swimfar 5y ago
  They used to make swamp coolers that you would put in your car window and it would blow cool air into the car. I always thought those were really cool. Completely unpowered, no electricity. It's such a simple design compared to AC units.
  - SilasX 5y ago
    Hah, I had the same reaction to setting up a siphon. That's for when you need to get a liquid up over a hump and down to a lower point. You draw the liquid into a tube from the source [1] and drag the outlet to a lower point. Then, as if by magic, the higher pool gets sucked through the outlet until it's drained, without any mechanical pump. https://en.wikipedia.org/wiki/Siphon https://en.wikipedia.org/wiki/Siphon [1] you can do it by sucking it in like a straw, but there are less gross ways like using a flexible bottle to provide pumping action
deleted 5y ago
[deleted]
jonathanrbuzan 5y ago
Hi Everyone, I am a researcher that studies the interface between human heat stress and climate change. I list some news articles I was involved in and some peer reviewed manuscripts I coauthored below. Tw at ~35°C (95°F), what does this mean? Putting it terms that may be more relatable, like the NWS Heat Index: whether at low or high relative humidity, a 95°F Tw is ~160°F Heat Index. A Finnish sauna (dry) temperature is ~160-190°F. However, a steam sauna is usually between 90-120°F, aka Tw of 90-120°F. Generally, you do not want to be in a steam sauna for more than 20 minutes, because it will very likely kill you. But, you do sweat a lot, and that can be quite relaxing, for short periods of time. Tw 35°C is extremely rare in the outdoor environment. Persian Gulf/Pakistan area, if they occur (the veracity of those measurements are difficult to confirm). What happens more often (although, still quite rare) is Tw of 32°C (89.6°F). Heat waves, such as the Chicago 1995 (>700 dead) or the Indian-Paksitan Heatwave of 2015 (>2500 dead) peaked at those temperatures. Humans cease being able to function/perform labor at 32°C (89.6°F) Tw, regardless of the air temperature or humidity. And this is the real concern we have a climate scientists. Wet bulb temperatures peaking for extended periods of time (>6 hours) rarely exceed 28°C (82.4°F) today. But, the major work that’s come out recently (some of it mine, but is also verified by independent statisticians), is that global maximum Tw is tied to global mean surface temperatures, and scales readily with global mean changes. In the tropics, this change is reliably projected to be ~0.9°C (1.6°F) Tw per 1°C (1.8°F) of global warming. So, you can do thought experiments. At what point of global warming will there be sustained (>6 hours) of Tw at 32°C (89.6°F)? Current max Tw at >6 hours is 28°C in the tropics: Y = mx +b b = 28°C (today) Y = 32°C (future) m = 0.9°CTw / 1°C global T change x = (Y - b) / m X = ~4.4°C global T change. 32°C (future) = (0.9°CTw / 1°C global T change) 4.4°C global T change + 28°C (today) 4.4°C change in global temperature is equivalent to ~8°F of global change. This is a lot of global warming. But, not unlikely, given our current trajectories. Our estimate of global change by 2100 is about +4°C (7.2°F). Cheers, -Jonathan R. Buzan PhD Climate and Environmental Physics and Oeschger Centre for Climate Change Research University of Bern, Switzerland Some News Articles: https://www.bbc.com/sport/olympics/57904094 https://www.bbc.com/sport/olympics/57904094 https://www.theverge.com/2017/9/14/16290934/india-air-conditioner-cooler-design-climate-change-cept-symphony https://www.theverge.com/2017/9/14/16290934/india-air-condit... https://www.nytimes.com/2015/06/07/opinion/sunday/the-deadly-combination-of-heat-and-humidity.html https://www.nytimes.com/2015/06/07/opinion/sunday/the-deadly... Some Peer review manuscripts: https://iopscience.iop.org/article/10.1088/1748-9326/abeb9f https://iopscience.iop.org/article/10.1088/1748-9326/abeb9f https://www.annualreviews.org/doi/abs/10.1146/annurev-earth-053018-060100 https://www.annualreviews.org/doi/abs/10.1146/annurev-earth-... https://gmd.copernicus.org/articles/8/151/2015/ https://gmd.copernicus.org/articles/8/151/2015/
xyzzy21 5y ago
I think the claim "Even heat-adapted people cannot carry out normal outdoor activities past a wet-bulb temperature of 32 °C (90 °F)" is dubious. When I lived in Taipei, it would common to have 100F @ 100% RH in July/August and life/activity didn't suddenly stop because of it including people working outdoors. Now they may have modulated or changed how they worked outdoors but it's not absolutely so dire as this line claims. The reality of seeing that is self-evident proof is wrong. I'm wondering where this actually cites from...
avnigo 5y ago
Obligatory psychrometric chart: [0]. I find it easier to understand how it's all interrelated using a chart. [0]: https://en.wikipedia.org/wiki/Psychrometrics#Psychrometric_charts https://en.wikipedia.org/wiki/Psychrometrics#Psychrometric_c...
- hkc 5y ago
  Thank you for sharing the chart.
- samatman 5y ago
  typo: psychometric in the text, psychrometric in the link (correct).
  - avnigo 5y ago
    Yes, thanks for catching that. 'Psychro', Greek for cold, i.e. psychrometric, the measurement of cold. I think the chart does a lot more than just that, though!
ordinaryradical 5y ago
For a harrowing depiction of what sustained wet-bulb temperatures could look like, check out the opening of Kim Stanley Robinson’s Ministry for the Future.
- jwhiles 5y ago
  presumably you mean sustained high wet-bulb temperatures? If I understand the concept correctly you can perform a wet-bulb measurement at any temperature, and a wet-bulb temperature of 10 Celsius is presumably not an issue.
- wing-_-nuts 5y ago
  It is kind of terrifying to realize that large portions of the third world are soon going to become so hot that it will be impossible to do any sort of physical exertion outside for large parts of the day without putting yourself at risk for heat stroke. That's at lower warming scenarios. At even higher temps it becomes damned near impossible to live without AC (which many in the third world don't have)
- twright 5y ago
  Found an excerpt of the first chapter for anyone interested: https://www.orbitbooks.net/orbit-excerpts/the-ministry-for-the-future/ https://www.orbitbooks.net/orbit-excerpts/the-ministry-for-t...
- pjerem 5y ago
  This opening is truly frightening.
- asterizer 5y ago
  The entire book is super important. Plausible and scary predictions for the next 40 years with many positive solutions.(Pumping the water from underneath the major glaciers to keep them from sliding into the ocean.) It's heartening to see the world come together over Ukraine, a template for how we could come together over climate change.
  - time_to_smile 5y ago
tomhoward 5y ago
This doesn't seem to be mentioned in the article, but an important related concept is delta-t, which is the dry-bulb temperature minus the wet-bulb temperature. It varies roughly inversely with relative humidity. It's a particularly important metric for farmers/growers who use pesticide spray, as it influences the way the spray behaves when released from the applicator. If the delta-t is too high, the spray droplets will evaporate too fast, before they land on the plant. Whereas if it's too low, it can remain in droplet form for too long, which can reduce plant uptake, and in combination with other factors (particularly thermal inversions and wind) can lead to spray drift onto other crops, which can be hugely damaging. https://www.cropsmart.com.au/using-delta-t-for-assessing-spray-conditions/ https://www.cropsmart.com.au/using-delta-t-for-assessing-spr...
el_nahual 5y ago
This is the key part: > A sustained wet-bulb temperature exceeding 35 °C (95 °F) is likely to be fatal even to fit and healthy people, unclothed in the shade next to a fan; at this temperature human bodies switch from shedding heat to the environment, to gaining heat from it. The important thing is that passive/evaporative cooling simply stops working. In fact, sitting in front of a fan would actually speed up your death. You'll also die shockingly quickly (20 or so minutes, even for healthy, fit adults). The only thing that can save you is: - air conditioning - getting in a cold tub - going somewhere cooler (obviously) like a basement or parking structure There are two thing that make this frightening: 1. We have already, for the first time in history, started seeing spikes of wet-bulb temperatures that exceed these values. Fortunately so far they have been in uninhabited areas. 2. If we ever see these temperatures in a city, odds are that the electrical grid would fail on account of the strain by A/C units. So picture this: a heat wave of this magnitude hits a large city. Bangkok, Phoenix, Melbourne. It lasts for 48+ hours. The grid fails on account of the strain. We're looking at a mass casualty event with millions of dead in a matter of hours/days.
- causi 5y ago
  That prompts a lot of questions I'd never thought about before. Assuming a failed grid, are there enough off-grid cooling units to keep the population alive? If such a thing happened in my city people would pile into vehicles and use that AC, but my city has an average car ownership of two per household. The next century will definitely create a market for refrigerated trucks full of bunkbeds. I wonder how long a fridge-sized freezer filled with water and then frozen could keep you alive in such a heatwave.
  - jazzyjackson 5y ago
    I don't think you need active phase-change cooling to survive a heat wave, there are architectural solutions found in deserts the world over (India and Iran have great examples). Namely, thick stone walls as heat mass so interior stays cool where nights are cool, and encouraging airflow through great public halls (wind will form where-ever there's a temperature gradient, breezeblocks or jaali can aid in increasing airflow while blocking sunlight) Courtyards with a pool of water will have noticeably cooler ambient temps, these are found all over the middle east. At the extreme, you get underground cities like Nūshābād.
joak 5y ago
I'm not sure of that. If you wet your body (or clothes) and sit in front if a fan (or in the wind of there is some) you will cool down. Why? The fastest molecules tend to jump out of the surface of the liquid water. The wind push them away. So the wind removes the hottest molecules. That's why you are cooler when it's windy. This mechanism works at any temperature. Am I wrong?
- snowwrestler 5y ago
  At equilibrium, water condenses as fast as it evaporates. So while the fan is blowing water molecules away from you, it’s also blowing water molecules toward you, which condense on your skin. Net heat transport would be zero.
- bernulli 5y ago
  You can only have a net outflow of water if the air is not saturated with water, which is a function of the air temperature. If it is saturated, a dynamic equilibrium will establish, where water molecules from the air jump back into the surface of the liquid film. That's why a 100F day in Arizona (dry!) will be much more pleasant than a 100F day in Florida (humid!) - if it's dry, your body can still get rid of the heat. If it's humid, it will just 'stick to your body'.
- aidenn0 5y ago
  The wet bulb temperature already takes this into account. If you wet your body and sit in front of a fan, your surface skin temperature will be very similar to the wet bulb temperature. Once you know the skin temperature, everything exterior to your body has already been taken into account and it's a question of how many watts of thermal energy can be transferred from your core to your skin. When your body is warm (i.e. not trying to conserve heat by reducing circulation) then this is essentially linearly related to the difference between core temperature and skin temperature. When the wet bulb temperature is above 95F, then you create more thermal energy than can leave your body when your core is at 99F, so your core body temperature rises and all the bad things that go along with it progress.
- fhars 5y ago
  Yes, you are, sort of. The wind will still cool you, but only down to the wet bulb temperature (that's sort of the definition of the wet bulb temperature). So you will no longer be able to keep your core temperature constant, it will rise to lethal levels.
aidenn0 5y ago
Dew point is another measure I like. I find the commonly reported form of humidity (relative humidity) to be the least useful. Growing up, when the dew point hit 70, I knew it was going to be a miserable day, even if the relative humidity wasn't as high as other days (where the relative humidity was quite high because of a storm moving through)
aaron695 5y ago
phreenet 5y ago
Reminds me of my Army days using the Wet Bulb device to obtain the "Flag" status. Used a dry bulb, wet bulb, and a black bulb fed into a slide rule calculator to determine the heat index and thus working conditions. http://www.wetglobe.com/ http://www.wetglobe.com/
- jonathanrbuzan 5y ago

112 points by hkc 5y ago

This reminds me of swamp cooling. When I describe swamp cooling (evaporative cooling) to somebody who didn't grow up with it, it blows their mind.

frankthedog 5y ago
Please describe it
- wing-_-nuts 5y ago
  You basically have a fan that sprays a very fine mist. The water evaporates, and in doing so the temperature of the air drops. Note that this only works when humidity is low enough for evaporation to actually happen. If the humidity is too high this doesn't work. Edit: wiki goes in to way more detail about other methods I was not familiar with: https://en.wikipedia.org/wiki/Evaporative_cooler https://en.wikipedia.org/wiki/Evaporative_cooler
  - deleted 5y ago
    [deleted]
  - joak 5y ago
    I dunno, I think that evaporation happens even if there is a lot of humidity in the air. The fastest (hottest) water molecules are going to escape the liquid water. The wind will carry them away. I think this phenomenon does not take in account the wind. That actually if you have water to wet yourself and a fan you can cool you down. Not sure though since I seem to be alone to think so.
    - oh_sigh 5y ago
      You've repeated this take twice, but it is still wrong. Yes, the fastest water molecules on your skin escape. But you also have water from the air also latching onto your skin which you aren't accounting for. You're free to see how well evaporative cooling works when there is a lot of water in the air. You can just take a fan into a bathroom and run the hottest shower possible for a few minutes.
- phdelightful 5y ago
  I live in Southwest US where it’s common to use a swamp cooler (aka evaporative cooler) for your house. Instead of the normal refrigerated air unit that sits outside your house, there is basically a box with a big fan that pulls air across a big wooden or synthetic “sponge” and pushes it into your house ducts. A small pump like in a fish tank continuously pulls water across the sponge to keep it wet. You leave a couple windows in your house cracked open to leave an exit as new cold air is pushed in. Has a nice humidifying effect which is good in the desert too!
swimfar 5y ago
They used to make swamp coolers that you would put in your car window and it would blow cool air into the car. I always thought those were really cool. Completely unpowered, no electricity. It's such a simple design compared to AC units.
- SilasX 5y ago
  Hah, I had the same reaction to setting up a siphon. That's for when you need to get a liquid up over a hump and down to a lower point. You draw the liquid into a tube from the source [1] and drag the outlet to a lower point. Then, as if by magic, the higher pool gets sucked through the outlet until it's drained, without any mechanical pump. https://en.wikipedia.org/wiki/Siphon https://en.wikipedia.org/wiki/Siphon [1] you can do it by sucking it in like a straw, but there are less gross ways like using a flexible bottle to provide pumping action

deleted 5y ago

[deleted]

jonathanrbuzan 5y ago

Hi Everyone, I am a researcher that studies the interface between human heat stress and climate change. I list some news articles I was involved in and some peer reviewed manuscripts I coauthored below. Tw at ~35°C (95°F), what does this mean? Putting it terms that may be more relatable, like the NWS Heat Index: whether at low or high relative humidity, a 95°F Tw is ~160°F Heat Index. A Finnish sauna (dry) temperature is ~160-190°F. However, a steam sauna is usually between 90-120°F, aka Tw of 90-120°F. Generally, you do not want to be in a steam sauna for more than 20 minutes, because it will very likely kill you. But, you do sweat a lot, and that can be quite relaxing, for short periods of time. Tw 35°C is extremely rare in the outdoor environment. Persian Gulf/Pakistan area, if they occur (the veracity of those measurements are difficult to confirm). What happens more often (although, still quite rare) is Tw of 32°C (89.6°F). Heat waves, such as the Chicago 1995 (>700 dead) or the Indian-Paksitan Heatwave of 2015 (>2500 dead) peaked at those temperatures. Humans cease being able to function/perform labor at 32°C (89.6°F) Tw, regardless of the air temperature or humidity. And this is the real concern we have a climate scientists. Wet bulb temperatures peaking for extended periods of time (>6 hours) rarely exceed 28°C (82.4°F) today. But, the major work that’s come out recently (some of it mine, but is also verified by independent statisticians), is that global maximum Tw is tied to global mean surface temperatures, and scales readily with global mean changes. In the tropics, this change is reliably projected to be ~0.9°C (1.6°F) Tw per 1°C (1.8°F) of global warming. So, you can do thought experiments. At what point of global warming will there be sustained (>6 hours) of Tw at 32°C (89.6°F)? Current max Tw at >6 hours is 28°C in the tropics: Y = mx +b b = 28°C (today) Y = 32°C (future) m = 0.9°CTw / 1°C global T change x = (Y - b) / m X = ~4.4°C global T change. 32°C (future) = (0.9°CTw / 1°C global T change) 4.4°C global T change + 28°C (today) 4.4°C change in global temperature is equivalent to ~8°F of global change. This is a lot of global warming. But, not unlikely, given our current trajectories. Our estimate of global change by 2100 is about +4°C (7.2°F). Cheers, -Jonathan R. Buzan PhD Climate and Environmental Physics and Oeschger Centre for Climate Change Research University of Bern, Switzerland Some News Articles: https://www.bbc.com/sport/olympics/57904094 https://www.bbc.com/sport/olympics/57904094 https://www.theverge.com/2017/9/14/16290934/india-air-conditioner-cooler-design-climate-change-cept-symphony https://www.theverge.com/2017/9/14/16290934/india-air-condit... https://www.nytimes.com/2015/06/07/opinion/sunday/the-deadly-combination-of-heat-and-humidity.html https://www.nytimes.com/2015/06/07/opinion/sunday/the-deadly... Some Peer review manuscripts: https://iopscience.iop.org/article/10.1088/1748-9326/abeb9f https://iopscience.iop.org/article/10.1088/1748-9326/abeb9f https://www.annualreviews.org/doi/abs/10.1146/annurev-earth-053018-060100 https://www.annualreviews.org/doi/abs/10.1146/annurev-earth-... https://gmd.copernicus.org/articles/8/151/2015/ https://gmd.copernicus.org/articles/8/151/2015/

xyzzy21 5y ago

I think the claim "Even heat-adapted people cannot carry out normal outdoor activities past a wet-bulb temperature of 32 °C (90 °F)" is dubious. When I lived in Taipei, it would common to have 100F @ 100% RH in July/August and life/activity didn't suddenly stop because of it including people working outdoors. Now they may have modulated or changed how they worked outdoors but it's not absolutely so dire as this line claims. The reality of seeing that is self-evident proof is wrong. I'm wondering where this actually cites from...

avnigo 5y ago

Obligatory psychrometric chart: [0]. I find it easier to understand how it's all interrelated using a chart. [0]: https://en.wikipedia.org/wiki/Psychrometrics#Psychrometric_charts https://en.wikipedia.org/wiki/Psychrometrics#Psychrometric_c...

hkc 5y ago
Thank you for sharing the chart.
samatman 5y ago
typo: psychometric in the text, psychrometric in the link (correct).
- avnigo 5y ago
  Yes, thanks for catching that. 'Psychro', Greek for cold, i.e. psychrometric, the measurement of cold. I think the chart does a lot more than just that, though!

ordinaryradical 5y ago

For a harrowing depiction of what sustained wet-bulb temperatures could look like, check out the opening of Kim Stanley Robinson’s Ministry for the Future.

jwhiles 5y ago
presumably you mean sustained high wet-bulb temperatures? If I understand the concept correctly you can perform a wet-bulb measurement at any temperature, and a wet-bulb temperature of 10 Celsius is presumably not an issue.
wing-_-nuts 5y ago
It is kind of terrifying to realize that large portions of the third world are soon going to become so hot that it will be impossible to do any sort of physical exertion outside for large parts of the day without putting yourself at risk for heat stroke. That's at lower warming scenarios. At even higher temps it becomes damned near impossible to live without AC (which many in the third world don't have)
twright 5y ago
Found an excerpt of the first chapter for anyone interested: https://www.orbitbooks.net/orbit-excerpts/the-ministry-for-the-future/ https://www.orbitbooks.net/orbit-excerpts/the-ministry-for-t...
pjerem 5y ago
This opening is truly frightening.
asterizer 5y ago
The entire book is super important. Plausible and scary predictions for the next 40 years with many positive solutions.(Pumping the water from underneath the major glaciers to keep them from sliding into the ocean.) It's heartening to see the world come together over Ukraine, a template for how we could come together over climate change.
- time_to_smile 5y ago

tomhoward 5y ago

This doesn't seem to be mentioned in the article, but an important related concept is delta-t, which is the dry-bulb temperature minus the wet-bulb temperature. It varies roughly inversely with relative humidity. It's a particularly important metric for farmers/growers who use pesticide spray, as it influences the way the spray behaves when released from the applicator. If the delta-t is too high, the spray droplets will evaporate too fast, before they land on the plant. Whereas if it's too low, it can remain in droplet form for too long, which can reduce plant uptake, and in combination with other factors (particularly thermal inversions and wind) can lead to spray drift onto other crops, which can be hugely damaging. https://www.cropsmart.com.au/using-delta-t-for-assessing-spray-conditions/ https://www.cropsmart.com.au/using-delta-t-for-assessing-spr...

el_nahual 5y ago

This is the key part: > A sustained wet-bulb temperature exceeding 35 °C (95 °F) is likely to be fatal even to fit and healthy people, unclothed in the shade next to a fan; at this temperature human bodies switch from shedding heat to the environment, to gaining heat from it. The important thing is that passive/evaporative cooling simply stops working. In fact, sitting in front of a fan would actually speed up your death. You'll also die shockingly quickly (20 or so minutes, even for healthy, fit adults). The only thing that can save you is: - air conditioning - getting in a cold tub - going somewhere cooler (obviously) like a basement or parking structure There are two thing that make this frightening: 1. We have already, for the first time in history, started seeing spikes of wet-bulb temperatures that exceed these values. Fortunately so far they have been in uninhabited areas. 2. If we ever see these temperatures in a city, odds are that the electrical grid would fail on account of the strain by A/C units. So picture this: a heat wave of this magnitude hits a large city. Bangkok, Phoenix, Melbourne. It lasts for 48+ hours. The grid fails on account of the strain. We're looking at a mass casualty event with millions of dead in a matter of hours/days.

causi 5y ago
That prompts a lot of questions I'd never thought about before. Assuming a failed grid, are there enough off-grid cooling units to keep the population alive? If such a thing happened in my city people would pile into vehicles and use that AC, but my city has an average car ownership of two per household. The next century will definitely create a market for refrigerated trucks full of bunkbeds. I wonder how long a fridge-sized freezer filled with water and then frozen could keep you alive in such a heatwave.
- jazzyjackson 5y ago
  I don't think you need active phase-change cooling to survive a heat wave, there are architectural solutions found in deserts the world over (India and Iran have great examples). Namely, thick stone walls as heat mass so interior stays cool where nights are cool, and encouraging airflow through great public halls (wind will form where-ever there's a temperature gradient, breezeblocks or jaali can aid in increasing airflow while blocking sunlight) Courtyards with a pool of water will have noticeably cooler ambient temps, these are found all over the middle east. At the extreme, you get underground cities like Nūshābād.

joak 5y ago

I'm not sure of that. If you wet your body (or clothes) and sit in front if a fan (or in the wind of there is some) you will cool down. Why? The fastest molecules tend to jump out of the surface of the liquid water. The wind push them away. So the wind removes the hottest molecules. That's why you are cooler when it's windy. This mechanism works at any temperature. Am I wrong?

snowwrestler 5y ago
At equilibrium, water condenses as fast as it evaporates. So while the fan is blowing water molecules away from you, it’s also blowing water molecules toward you, which condense on your skin. Net heat transport would be zero.
bernulli 5y ago
You can only have a net outflow of water if the air is not saturated with water, which is a function of the air temperature. If it is saturated, a dynamic equilibrium will establish, where water molecules from the air jump back into the surface of the liquid film. That's why a 100F day in Arizona (dry!) will be much more pleasant than a 100F day in Florida (humid!) - if it's dry, your body can still get rid of the heat. If it's humid, it will just 'stick to your body'.
aidenn0 5y ago
The wet bulb temperature already takes this into account. If you wet your body and sit in front of a fan, your surface skin temperature will be very similar to the wet bulb temperature. Once you know the skin temperature, everything exterior to your body has already been taken into account and it's a question of how many watts of thermal energy can be transferred from your core to your skin. When your body is warm (i.e. not trying to conserve heat by reducing circulation) then this is essentially linearly related to the difference between core temperature and skin temperature. When the wet bulb temperature is above 95F, then you create more thermal energy than can leave your body when your core is at 99F, so your core body temperature rises and all the bad things that go along with it progress.
fhars 5y ago
Yes, you are, sort of. The wind will still cool you, but only down to the wet bulb temperature (that's sort of the definition of the wet bulb temperature). So you will no longer be able to keep your core temperature constant, it will rise to lethal levels.

aidenn0 5y ago

Dew point is another measure I like. I find the commonly reported form of humidity (relative humidity) to be the least useful. Growing up, when the dew point hit 70, I knew it was going to be a miserable day, even if the relative humidity wasn't as high as other days (where the relative humidity was quite high because of a storm moving through)

aaron695 5y ago

phreenet 5y ago

Reminds me of my Army days using the Wet Bulb device to obtain the "Flag" status. Used a dry bulb, wet bulb, and a black bulb fed into a slide rule calculator to determine the heat index and thus working conditions. http://www.wetglobe.com/ http://www.wetglobe.com/

jonathanrbuzan 5y ago