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Why 80 degrees "feels like" 95 degrees (and why this is stupid).

 
 
maxdancona
 
  1  
Reply Fri 30 Jul, 2021 12:24 pm
Consider a spherical cow of uniform density...

This is a punchline to a Physics joke, the point being that Physicists are very good at finding perfect solutions to certain unrealistic problems. Of course cows aren't spheres and their insides aren't uniform.

Science would have a very difficult time creating a general equation for the loss of heat from a human body. Humans come in different shapes, sizes and consistancies. The amount of heat you lose will depend on your weight and your size and the make up of your body. Humans are made up of fat, and muscle and bones and skin and dangly bits all in different configurations.

Anyone who has studied differential equations in college knows exactly what I am talking about. The flow of heat from an warm object to a cooler substance is complicated even for a pure metal cube.

It would be impossible for someone to construct an accurate mathmatical model of heat loss from a human body without specifying the human body in question. The heat loss from a 110lb woman would be quite different than that from a 220lb man... and there would probably be differences between 220 lb men based on the ratio of muscle to fat.

Before looking into this, I thought the "feels like" temperature referred to a scientific value called the "wet bulb temperature". The Wet bulb temperature has a very well-defined and precise meaning... two scientists would calculate the same value given set conditions.

But it seems like this isn't the case, and that there are many different arbitrary ways to calculate the "feels like" temperature. Some include wind speed, some don't. Worse, no one seems to tell you how it is calculated.

You are given a number out of context with no indication on how it is measured. As a former science teacher, this is a bad thing.
hightor
 
  1  
Reply Fri 30 Jul, 2021 01:17 pm
@maxdancona,
I really think you're over-complicating what is really a non-issue.
Quote:
Science would have a very difficult time creating a general equation for the loss of heat from a human body.

But that's not what the THI is supposed to do. The "feels like" association is a simplification that got boiled down into a cute catchy phrase, much like "defund the police" and other crude slogans meant to convey an idea which cries out for more explanation.

For every cubic centimeter of water evaporated the surface temperature is cooled one degree centigrade*. The less water evaporated the less cooling. High temperatures cue the body to begin cooling itself through perspiration. Air movement aids in evaporation. But the higher the humidity, the less moisture is absorbed by the atmosphere. At a certain level of humidity, even a stiff wind will not evaporate moisture from a surface. The THI simply (simplistically) gives the average person listening to a short weather report a quick way to gauge how comfortable it will be doing activities outdoors.

I can't believe I'm actually defending a dumbed-down "measurement" used by weather forecasters, whose reports are already significantly dumbed-down for popular consumption. You raise some valid points, and thank you for bringing this to the attention of this forum, but eventually it's like using a baseball bat to dispatch a cockroach. It's overkill. It's just a guide for people who might not otherwise grasp the relation between temperature, humidity, and personal comfort.

Quote:
As a former science teacher, this is a bad thing.

No, it's not "bad". Nor is it "good". It's not a something to which such values can be applied. Inaccurate, arbitrary, unscientific it may be but it isn't bad. People aren't harmed by it and it's intended to be helpful.

EDIT: *This is the definition of one degree centigrade which I learned in the 8th grade. If it's incorrect, there is still a way to measure cooling by surface evaporation as that is a measurable phenomenon.
maxdancona
 
  0  
Reply Fri 30 Jul, 2021 02:19 pm
@hightor,
Your science explanation is completely wrong. What you describe sounds an awful lot like the definition of a joule. This is only indirectly related to the concept of evaporation.

To calculate the change to "surface temperature" you would start with the heat of evaporation for water. But surface temperature is complex and involves a constant flow of energy in a complex system. You would need partial differential equations to solve this system, and the temperature and quantity of water would impact the result.

In short, I am not over complicating this. You need four semesters of calculus to start to make these calculations.
coluber2001
 
  1  
Reply Fri 30 Jul, 2021 04:10 pm
I take a hike every day in the wild even during the summer in Dallas. I hike in the early evening when the temperature cools down to around 9o degrees. The humidity varies, but it's fairly dry in the summer. Notably, if there is no breeze-- and there usually isn't--it feels hot. However, to stop and rest is oppressive. Just the mild breeze created by walking, two to three miles per hour, is enough to noticeably cool you off.

That's why I think it's important to take the air movement in account when figuring the heat index. Eighty degrees in a closed room with no air movement is oppressive, but with a fan it's comfortable.

hightor
 
  1  
Reply Sat 31 Jul, 2021 03:48 am
@coluber2001,
Quote:
Eighty degrees in a closed room with no air movement is oppressive, but with a fan it's comfortable.

The comfort is derived from the cooling effect that the evaporation of moisture from your skin provides. If the air is saturated with moisture the evaporation won't occur and neither will the feeling of comfort, although we may prefer it to the room with no air movement.
0 Replies
 
hightor
 
  1  
Reply Sat 31 Jul, 2021 04:09 am
@maxdancona,
Quote:
Your science explanation is completely wrong.

Not "completely". In my edit (not trusting things I was taught in junior high school) I said, "If it's incorrect, there is still a way to measure cooling by surface evaporation as that is a measurable phenomenon." You suggested a method by which it could be scientifically measured, but the principle of evaporative cooling is sound.

Quote:
To calculate the change to "surface temperature" you would start with the heat of evaporation for water. But surface temperature is complex and involves a constant flow of energy in a complex system. You would need partial differential equations to solve this system, and the temperature and quantity of water would impact the result.


But, for the purposes of the THI, you don't need these calculations; you're over complicating it, assuming it's some sort of precise scientific measurement when it isn't and doesn't purport to be. The evaporation of moisture on your skin cools you; that's why we sweat.
Quote:
Water evaporating takes quite a lot of heat away -- 540 calories per gram -- when it evaporates.

The THI merely indicates that the effectiveness of this evaporative cooling decreases as the humidity increases.

Quote:
You need four semesters of calculus to start to make these calculations.

But you don't need to do those calculations when you're employing something as dumbed down as the THI.
maxdancona
 
  0  
Reply Sat 31 Jul, 2021 09:10 am
@hightor,
You are being silly Hightor. You are making an argument based on a vague misremembered lesson from junior high. You clearly don't know what you are talking about (and you seem to admit this).

Let's be clear about the facts.

1. Yes, humans use evaporative cooling. Under normal conditions this means that humans are more effective at cooling themselves at lower humidity. In short the principle "less humidity, more comfortable human" is true in most circumstances where humans might be too hot.

2. The precise amount that humans cool down from evaporative cooling varies from humman to human and is scientifically quite complex.

3. Predicting the effects of evaporative cooling based on on a wide range of humans is scientifically impossible without taking into account the body weight, height, fat content and distribution and probably metabolism of the individual human... and even then it would be a probably impossibly difficult calculation.

I would like to know if you accept these facts?
maxdancona
 
  -1  
Reply Sat 31 Jul, 2021 09:19 am
@maxdancona,
My point is this.

1. The "Feels Like" temperature (or RealFeel or however else it is being marketed) is not a scientifically accurate measurement about how the wide range of human beings experience the effects of heat in different conditions.

2. The news outlets don't actually tell us how they are calculating these numbers (googling seems to show that these calculations are different from place to place). However, these caculated values do appear go up as the humdity goes up... which is good.

If people believe that these "Fells like" temperatures are scientifically valid indications of how they are feel they are wrong. My fear is that some proportion of viewers are being deceived. These "feels like temperatures" are presented as authoritative numeric values... even though no one knows what they actually mean.

0 Replies
 
hightor
 
  2  
Reply Sat 31 Jul, 2021 10:34 am
@maxdancona,
Quote:
You are being silly Hightor.

You are being characteristically obtuse, maxdancona.

Quote:
I would like to know if you accept these facts?


Sure. I've said so repeatedly. And I'm not basing my argument on misinformation I received in junior high school. The principle that evaporation results in cooling is accepted by you.

What I'm saying is that you're mistakenly treating the THI as a precise, scientific measurement and it isn't. You're trying to make it do more than it's meant to do.
maxdancona
 
  0  
Reply Sat 31 Jul, 2021 10:39 am
@hightor,
Actually it sounds like we are in agreement on both counts.

I suppose we can stop arguing now.
0 Replies
 
edgarblythe
 
  1  
Reply Sat 31 Jul, 2021 10:53 am
I've always thought it was the propensity to sensationalism inspiring the TV weather forecasters to bear down on the "feels like" instead of the actual.
hightor
 
  1  
Reply Sat 31 Jul, 2021 11:11 am
@edgarblythe,
TV meteorology can be pretty awful sometimes. There's just enough analysis of graphs and historical data to make these reporters appear "scientific" when, in reality, they're more interested in entertainment. Do you remember when weather reporters were regularly brought out, as "experts", to denounce the whole idea of climate change?
maxdancona
 
  0  
Reply Sat 31 Jul, 2021 11:16 am
@hightor,
I have to disagree with you about TV meterology

The TV will tell me with remarkable accuracy what the weather will be like for me several days in advance. This is pretty amazing and shows the advancement of meterological science.

When the TV tells me to expect a foot of snow in threr days... that is useful information that woukd have been impossible to give me 50 years ago.

The entertainment is extra.
edgarblythe
 
  1  
Reply Sat 31 Jul, 2021 11:17 am
@hightor,
I always sort of went deaf when they did that. I mostly tune in to see the radar and find out if fronts and pressure systems are moving.
0 Replies
 
hightor
 
  1  
Reply Sat 31 Jul, 2021 11:25 am
@maxdancona,
Quote:
I have to disagree with you about TV meterology

Oh, good! I was starting to worry. Smile
I have to admit, I no longer even watch TV. But those forecasts could simply be given in a straight forward format without all the show biz.
0 Replies
 
edgarblythe
 
  1  
Reply Sat 31 Jul, 2021 12:15 pm
Forecasting Houston weather is generally accurate on TV, except our position relative to the gulf can throw in a monkey wrench. A front with lots of rain will be coming and they call for an 80 or 100 percent chance of rain, but it often sweeps off the side just before reaching Houston. Or the front will slow down and just stop before reaching here.
0 Replies
 
 

 
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