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Coloured Clothes

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eggshaped
167143.  Wed Apr 18, 2007 3:43 am Reply with quote

What colour clothes should you wear in hot weather?

Forfeit: Light coloured clothes

Answer: It doesnít matter, just wear something light and loose if you want to stay cool.

It is strange that Bedouins, who live in hot deserts, wear black robes, but a study done by Tel Aviv University and Harvard has shown that the amount of heat gained by a person in the desert is the same whether he wears a black or a white robe. The additional heat absorbed by the black robe was lost before it reached the skin.

A man stood in the desert for 30 minutes, facing the sun four times, once in a black robe, once in a white one, once in an Army uniform and once half-naked. The results were surprising, they found that the man was equally cool irrespective of the colour of his robe. It worked like a chimney, because the air is hotter underneath the black robe, it rises more quickly up and out of the loose robe and, by convection, is replaced by the cooler air beneath it.

link

 
MatC
167170.  Wed Apr 18, 2007 4:54 am Reply with quote

Well, well, well ... so thatís half the Mystery of the Fez (post 17260) solved, then ...

 
dr.bob
167600.  Thu Apr 19, 2007 3:41 am Reply with quote

eggshaped wrote:
It worked like a chimney, because the air is hotter underneath the black robe, it rises more quickly up and out of the loose robe and, by convection, is replaced by the cooler air beneath it.


That's a pretty screwed up paper. They start off by saying that black hair on cattle and black feathers on pigeons transmit less radiation than white ones (fair enough. Just 'cos it's black in the visible doesn't mean it's black in the infra-red, where it counts). However, they then go on to say:

Quote:
The additional heat absorbed by the black robe was lost before it reached the skin.


What additional heat? You've just finished telling us that black things absorb less heat. Make up your bloody mind!

 
eggshaped
167603.  Thu Apr 19, 2007 3:55 am Reply with quote

They say (albeit not very clearly) that black plumage absorbs less heat at high wind speeds (this is because the heating of white feathers is affected less by convective cooling than black ones (Walsberg et al)).

This does not contradict that fact that black robes conduct more heat, unless the bedouins are swooping through the air in robes made from black feathers.

 
dr.bob
167611.  Thu Apr 19, 2007 4:18 am Reply with quote

OK, Walsberg et al say that heating of the skin of birds is higher if they've got white feathers when the wind is above 3 m/s. They claim this is because the black feathers absorb the solar radiation more effectively at the surface, preventing more heat from radiating nearer the skin. The heat at the surface is then much more easily convected away by the wind.

Unfortunately, their language is rather confused. They say that, at higher wind speeds:

Quote:
black plumages acquire lower radiative heat loads than do white plumages.


I'm sorry, but this is plain wrong. The black plumages will acquire higher radiative heat loads because they absorb more, but the total heat load is lower because the convective removal of heat is greater for the black plumages.

Mind you, both these papers were written by zoologists and not physicists, so I reserve the right to look down my nose haughtily at them both.

Neither of the abstracts makes any mention of the absorptivity of the different coloured fur/feathers/robes in the infra-red, which is going to be by far the most important point. They may deal with it in more detail in the rest of the article, but I don't have access to that.

 
eggshaped
167616.  Thu Apr 19, 2007 4:35 am Reply with quote

Ok, point taken. I suppose the main question is "do you think the question stacks up?". IMO it does, despite the ambiguous nature of the two papers. What do you think?

 
dr.bob
167649.  Thu Apr 19, 2007 5:43 am Reply with quote

Personally I'm not convinced the question stacks up for a variety of reasons.

1) The paper talks about bedouin tribes who are accustomed to the hot conditions. The fact that they are able to wear dark colours in hot weather may be unremarkable since they're used to dealing with the hot weather. So the question "What colour clothes should you wear in hot weather" may have a different answer to "What colour clothes should a bedouin tribesperson wear in hot weather"

2) The paper claims that the convective cooling is caused by the loose fitting robes. So a general question such as "What colour clothes should you wear in hot weather" may not work since, more tightly fitting clothes may not benefit from this supposed convective cooling.

3) At least one of the references mentions convective cooling at a certain windspeed, so the question may have to be "What colour clothes should you wear in hot weather if the windspeed is high enough"

I think there are too many grey areas for a simple question, and a complicated question with all the caveats would be less interesting.

At least, that's my 2p worth. Sorry to be the doom-monger on this one.

 
eggshaped
167659.  Thu Apr 19, 2007 5:53 am Reply with quote

For the record, I disagree almost 100% with the above post.

1. The study unambiguously shows that in this instance white and black clothes have no effect on body temperature - Bedouin or non-Bedouin.

2. The loose fitting caveat was included in the original answer.

3. The convective wind cooling was in reference to birds, not bedouins.

But it's up to the question wranglers.

 
dr.bob
167667.  Thu Apr 19, 2007 5:58 am Reply with quote

eggshaped wrote:
For the record, I disagree almost 100% with the above post.


Fair enough :)

eggshaped wrote:
1. The study unambiguously shows that in this instance white and black clothes have no effect on body temperature - Bedouin or non-Bedouin.


I was drawing my conclusions on the basis of the abstract which only seems to talk about Bedouins. Of course, if the full text says something different....

eggshaped wrote:
2. The loose fitting caveat was included in the original answer.


True enough.

eggshaped wrote:
3. The convective wind cooling was in reference to birds, not bedouins.


But I thought you said:

Quote:
It worked like a chimney, because the air is hotter underneath the black robe, it rises more quickly up and out of the loose robe and, by convection, is replaced by the cooler air beneath it.


That would imply to me that, if you were wearing something more closely fitting that didn't permit the movement of air in this chimney effect, the convective cooling wouldn't be anything like as good.

 
eggshaped
167672.  Thu Apr 19, 2007 6:00 am Reply with quote

Yes, maybe I'm getting confused.

I took it that windspeed has an effect on birds in a similar way to the wind-chill factor, while a convection current does not rely on wind, but rather on the dynamic nature of hot air.

 
dr.bob
167688.  Thu Apr 19, 2007 6:11 am Reply with quote

eggshaped wrote:
Yes, maybe I'm getting confused.


You and me both :)

I like the idea of debunking the "fact" that dark clothes make you hotter, but I'm not entirely convinced by the science involved. For me, basing a debunking question on dodgy science is a dangerous idea.

Mind you, as I've said, I've only got the "executive summary" of the abstract to go on, so there may be something much more meaningful in the actual text of the paper. So I'm entirely prepared to be told I've missed the point entirely, but I'd just like to raise a wee note of caution to make sure this is done right if it's used as a question.

 
Jenny
167967.  Thu Apr 19, 2007 6:19 pm Reply with quote

Couldn't the dark clothes/Bedouin/desert connection be at least partly to do with the absence of water to wash them in? Or is that just me expressing my inner housewife?

 
Gray
168034.  Fri Apr 20, 2007 3:59 am Reply with quote

The problem is that there's a windspeed threshold above which the colour of clothes makes no difference (witness the radiative and convective properties of black vs. white). Below that windspeed, the major factor is the looseness of the clothing, allowing just the convection factor to operate.

Of course, convective chimneys work best when there's a wind blowing across the top, as in the Bernoulli Effect, so the windspeed comes into effect there too.

It's a tricky one, I agree, but I think if we can get the explanation into a concise paragraph, this should work.

 
Flash
168039.  Fri Apr 20, 2007 4:03 am Reply with quote

If you're sitting or lying down (which you would be - it's hot, dammit) then I suppose you wouldn't get the chimney effect in any case?

 
Gray
168055.  Fri Apr 20, 2007 4:23 am Reply with quote

That's true.

I expect the ambient temperature of the air is going to be a large factor as well, which would definitely tip it in black's favour, as it can radiate more efficiently. In the desert the air is extremely hot, whereas in Devon, say, it's not.

 

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