Protction from Sun rays

[kmpillai]

Hi,
which colour umbrella will give best protection from Sun rays during very hot sunny day?

Thanks,
KMPillai

 

[martindup]

Not so much the colour of the umbrella but the material should not let UV rays through. Different materials let UV rays through at diffrent quantities. If you were to go to colours you want something that will reflect the bulk of the harful rays like the UV's, my guess would be that a reflective surface (like a silver, gold, or most other metalic) would be the best. Except if you could get an Ultra-Violet coloured umbrella but I don't think you get UV pigments.
Martin

 

[kmpillai]

Dear Martin,
Thank you for your response. I agree with your reply. Let us say the material is cloth.In some regions in south Asia most of the people are using black colour umbrella during summer also. Is there any reason behind that. From the heat transer point of view, I think white colour will give best heat protection.

KMPillai

 

[BlueBeam]

Hi!
Funny question here, because it reminds me to a quite long discussion i had during my study. Well, the conclusion was that the color you SEE is not relevant. Since you ask for the thermal properties you must consider the IR-range of the sunlight. This is in fact the reason for the heat you feel, but the visible color of e.g. a fabric does not necessarily represent the properties for infrared. You can have black surfaces which reflect more IR then a white one. It depends on the material itself and not on its color. Though you find that (for us) ´white´ clothings reflect more sunlight (also IR?) then black.
The UV-Range of the sunlight is again different, here all i know is, that most materials absorb ultraviolet light quite well and i´d guess that the visible color is also not necessarily important for the UV-related properties.
So, sorry for my long story, but i discussed that much more already.
(pls let me know if you have great new findings!)

MT

 

[quark]

Actually I read in a Russian book (Tales about Metals) that Americans developed a jacket which can be used both in Summer and Winter as well. The jacket is coated with aluminium on oneside. If the al. side is kept out, it will reflect back the heat and can be used in summer. If al. side is kept inside, it reflects body heat and can be used in winter. Though I don't know the fact, this seems to be possible and so I vote for Martin.

Regards,

Believe it or not : Though he couldn't prove, Einstein never agreed with Heisenberg on his Uncertainty Principle.

 

[BlueBeam]

Well, metals have indeed special properties with radiation. And it seems that aluminum has a high reflectance for IR (heat radiation). Don´t ask me for UV. So, an umbrella made of aluminum (or at least with an aluminum layer) should work.
Furthermore i actually wrote the same as Martin, that the surface/material and not the color mainly determines the protection level.
So long,
MT

 

[martindup]

Dear kmpillai
On the use of different colours and its properties to pure radiation heat transfer it all comes down to the emmitance number. Black objects would absorb radiational heat very quickly. Black objects would also emmit heat better. Where white and silver objects would not absorb so much heat from radiation (and rather reflect it) the same lighter coloured object would be less effective for emmiting heat it already has. The reason for Asians using black is maybe more a cultural thing. I am no UN-diplomat but isn't white clothing supposed to be a colour of mourning for some Asian cultures? Could also be because white cloth tend to be more transparent than coloured because the colour die fills up the little holes in the material. Personally I'll wear white when it is a hot day.
For the more harmfull rays (UV's) metalic surfaces are used for protection. UV's might not heat something up a lot but it is damaging because of cancer and deteriation of chemicals. That is why some surfaces of spacecraft is made of gold foil. Yep, real gold, but not enough for retirement, unfortunatly.
martindup

 

[IRstuff]

Bedouins also wear black, so it's not a "cultural" thing.

Perhaps they simply know more about real-world heat transfer than you think:

http://search.csmonitor.com/durable/2000/12/05/p18s1.htm

TTFN

 

[TD2K]

Interesting article about the Bedouin's clothing.

I know in Saudi during the summer, I never saw any of the Saudi men who were wearing traditional clothing in anything but white. In winter when it's cooler, you see black, grey, blue, etc but they weren't worn in the summer.

The women's clothing on the other hand in public was always black.

 

[VSSriram]

TDK2 is absolutely right. Not only in Saudi, almost in all of the Middle East countries, where the Sun shows its power for most of the time, the traditional dress is white. Can't figure out why women wear black though. Possibly because, in the olden days, they were more at home than outdoors.

Feminist activitis - No offence intended. Don't shoot me !!!

 

[quark]

One important variable which was not mentioned in the link provided by IRStuff was dehydration rate. If the hot air movement increases, heat transmitted to the body also increases and so perspiration. If the humidity is low, we can't see body sweating but still loose tremendous amounts of body fluids.

My idea is that black might have been the cheapest color to dye a cloth.

Regards,


Believe it or not : Though he couldn't prove, Einstein never agreed with Heisenberg on his Uncertainty Principle.

 

[IRstuff]

Actually, white cloth is probably more expensive, since it needs to be bleached.

I suspect that most Saudi's visible to Westerners wear white as a sign of prosperity.

The Bedouins and Tauregs are not so enamored with Western ideals of prosperity and assuming that you could actually track a traditional one down, they would still be doing the black tents and black clothing.

TTFN

 

[25362]

I remember another thread dealing with the same subject: bedouins' black clothing. As I recall the proposed explanation was that with loose-fit black clothing the improved natural draft generated by the inner heated (by a few degrees) air provided more comfort to the wearer by its circulation beneath the black cloth than otherwise.