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#4526170 - 09/25/17 08:14 PM Radiative Cooling into space
Shannow Online   content


Registered: 12/12/02
Posts: 39838
Loc: 'Stralia
While investigating ice storage a few weeks ago, I found that they were making ice in Egypt in clay vessels at night, and apparently Iran's shah like sorbet that they made by placing dishes at the bottom of wells.

Apparently if you can exclude direct sunlight, plus get the emissivities right at different spectrums, you can radiate heat into the 3K sky in the middle of the day and cool stuff down.


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#4526175 - 09/25/17 08:22 PM Re: Radiative Cooling into space [Re: Shannow]
kschachn Offline


Registered: 12/26/05
Posts: 9379
Loc: Upper Midwest
This is an interesting topic and I remember one of my thermodynamics professors mentioning it in regards to the black body temperature of the night sky. I recall it depended on a lot of things but overall it could be around 0 degrees C.
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#4526229 - 09/25/17 09:22 PM Re: Radiative Cooling into space [Re: Shannow]
JohnnyJohnson Online   content


Registered: 10/22/09
Posts: 2731
Loc: Wet side WA
You mean we could generate Glo Bull Cooling?
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#4526236 - 09/25/17 09:29 PM Re: Radiative Cooling into space [Re: kschachn]
Kestas Offline



Registered: 06/04/02
Posts: 12324
Loc: The Motor City
Originally Posted By: kschachn
This is an interesting topic and I remember one of my thermodynamics professors mentioning it in regards to the black body temperature of the night sky. I recall it depended on a lot of things but overall it could be around 0 degrees C.

From my schooling I remember the sky in outer space is around 4 degrees Kelvin, around what Shannow posted.
If it was 0 degrees C, nowhere on Earth could it be below that.

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#4526243 - 09/25/17 09:38 PM Re: Radiative Cooling into space [Re: Shannow]
ZeeOSix Offline


Registered: 07/22/10
Posts: 17763
Loc: PNW
Yep, it's common to get ice on the top of cars on a clear night if there's moisture on them due to the direct view factor and heat loss to space, even though the ambient air temperature is above freezing.

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#4526250 - 09/25/17 09:58 PM Re: Radiative Cooling into space [Re: ZeeOSix]
Shannow Online   content


Registered: 12/12/02
Posts: 39838
Loc: 'Stralia
We can get that ice on roads here too, couple of local overpasses can lose a lot of heat that the morning fog freezes...

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#4526259 - 09/25/17 10:09 PM Re: Radiative Cooling into space [Re: ZeeOSix]
CR94 Offline


Registered: 03/20/16
Posts: 1120
Loc: Western S.C. since 1996
Originally Posted By: ZeeOSix
Yep, it's common to get ice on the top of cars on a clear night if there's moisture on them due to the direct view factor and heat loss to space, even though the ambient air temperature is above freezing.
There doesn't have to be moisture on them, other than moisture that condenses from the air (aka, dew) due to the same radiative cooling to space on a clear night. Basic Heat Transfer! For related reasons, a white roof can not only be less hot than a black roof on a sunny day, but also less hot than a shiny metal one.
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#4526261 - 09/25/17 10:12 PM Re: Radiative Cooling into space [Re: Shannow]
ZeeOSix Offline


Registered: 07/22/10
Posts: 17763
Loc: PNW
Originally Posted By: Shannow
We can get that ice on roads here too, couple of local overpasses can lose a lot of heat that the morning fog freezes...


Yep, another good example. Happens here all the time with the high humidity and clear nights even if the air temp is well above freezing. The highway department lays down that salt brine juice to keep the overpasses from getting black ice.

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#4526266 - 09/25/17 10:18 PM Re: Radiative Cooling into space [Re: CR94]
ZeeOSix Offline


Registered: 07/22/10
Posts: 17763
Loc: PNW
Originally Posted By: CR94
Originally Posted By: ZeeOSix
Yep, it's common to get ice on the top of cars on a clear night if there's moisture on them due to the direct view factor and heat loss to space, even though the ambient air temperature is above freezing.
There doesn't have to be moisture on them, other than moisture that condenses from the air (aka, dew) due to the same radiative cooling to space on a clear night. Basic Heat Transfer!


Yep, if the air humidity is high enough the cold sheet metal from losing heat to space on a clear night will condense moisture from the air and it may also freeze. Similar to an iced drink in a glass condensing moisture from humid air.

Originally Posted By: CR94
For related reasons, a white roof can not only be less hot than a black roof on a sunny day, but also less hot than a shiny metal one.


Now you're talking about solar absorption properties of surfaces along with their radiative and conductive losses. If the roof was chromed like a car bumper it would absorb less solar radiation than some white paints. Here's a quick search that shows a table of different surfaces ... chromium plate (I assume very shiny chrome surface) is only 0.20 absorption.

http://www.engineeringtoolbox.com/radiation-surface-absorptivity-d_1805.html

Here's another table showing solar absorption properties. Aluminum foil (which is pretty shiny) is alpha 0.15, and white paint ranges from 0.23 - 0.49 depending on what is is exactly.

http://www.solarmirror.com/fom/fom-serve/cache/43.html


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#4526379 - 09/26/17 05:31 AM Re: Radiative Cooling into space [Re: Shannow]
Claud Offline


Registered: 02/11/14
Posts: 593
Loc: Margate England
Apparantly more people die from hypothermia than heat stroke in high elevation deserts...

Claud.

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#4526386 - 09/26/17 05:47 AM Re: Radiative Cooling into space [Re: Shannow]
Shannow Online   content


Registered: 12/12/02
Posts: 39838
Loc: 'Stralia
Handy chart that shows the wavelength dependence discussed in the video



Having lived in a white painted fibro cement house for 20+ years, it's no surprise that at night, it released heat like a black body...red double brick in the new place makes cold mornings much more tolerable

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#4526395 - 09/26/17 06:05 AM Re: Radiative Cooling into space [Re: Shannow]
Ducked Online   content


Registered: 10/25/12
Posts: 4325
Loc: Taiwan
Coldest night I ever spent was in the Sierra Madre in Mexico, and I've camped out in the Scottish Highlands in winter quite a lot.

Very clear, no wind, lots of stars, bloody freezing.

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