IR thermometer pointed up into a cloudy sky?


by Spinnor
Tags: cloudy, pointed, thermometer
Spinnor
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#1
Mar28-13, 02:59 PM
P: 1,361
A physics stack question asks why when the air is hotter then your body thermodynamics allows your body to operate as a heat engine as a heat engine must have a sink for waste heat. See question here,

http://physics.stackexchange.com/que...-a-heat-engine

Part of the answer is that normally the sky (except for the sun) is a net heat sink not a net heat source, if we point our hands to a clear sky our palms would sense cold? A clear sky is even colder then an overcast sky? Other fun with an infrared sensor?

Edit, point your hands towards the sky and away from the sun for a cooling effect?

With infrared thermometer in hand I measured plastic tarp in the shade, the overcast sky, inside my mouth. The outside temp was 47F in the shade by a standard thermometer.

plastic outside in shade, 48F
overcast sky (cloud ceiling at 7500ft), 19F
inside of my mouth, 97.7F.

So part of what keeps us from overheating is the fact that the sky is relatively cool?

Thanks for any help!
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cjl
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#2
Mar28-13, 03:25 PM
P: 975
I doubt that's a significant portion of it. A much, much larger factor is the fact that evaporative cooling effectively allows you to use warm air (even if it's warmer than your skin) as a heat sink, so long as the humidity is relatively low. Specifically, look up the concept of the wet bulb temperature - that defines the lowest attainable temperature using evaporative cooling in a given environment.
Spinnor
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#3
Mar28-13, 04:54 PM
P: 1,361
Quote Quote by cjl View Post
I doubt that's a significant portion of it. A much, much larger factor is the fact that evaporative cooling effectively allows you to use warm air (even if it's warmer than your skin) as a heat sink, so long as the humidity is relatively low. Specifically, look up the concept of the wet bulb temperature - that defines the lowest attainable temperature using evaporative cooling in a given environment.
I was doubting along with you so did some Goggling,

https://www.google.com/webhp?hl=en#h...w=1093&bih=491

first listing,

http://books.google.com/books?id=n5V...adiate&f=false

seems to say otherwise on page 192,

"A naked human will radiate as much energy as 13 100W light-bulbs"?

That is off a bit as they use 98F as the surface temp of a naked human. I'm topless and have a surface temp of 84F belly, 92.2F arm, 93.4 back.

Using 88F body temp in their calculations with room walls at 73F I get a net power radiated 1150W - 940W = 210W for a naked person in a 73F room?

So probably still small compared to the heat that can be removed by evaporative cooling and conduction?


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