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Author Topic: Do plants radiate heat into the sky?  (Read 5338 times)

Offline Dave Powelson

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Do plants radiate heat into the sky?
« on: 23/11/2010 21:30:03 »
Dave Powelson  asked the Naked Scientists:
   
Naked Scientist,

You might want to bundle up for this question. 

Fruit growers and gardeners are always focused on the night time air temperature to decide if they need to protect their plants.  But aren't plants subject to radiative heat loss to the sky as well as conductive heat loss to the air?  I left a thermometer in the open next to a plant on a clear night and found it registered 10 degrees F colder than a covered thermometer.  

Dave Powelson
Logan, Utah, USA

What do you think?
« Last Edit: 23/11/2010 21:30:03 by _system »


 

Offline Airthumbs

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Do plants radiate heat into the sky?
« Reply #1 on: 10/01/2011 23:38:21 »
Does that mean they are always chilly?
 

Offline Pikaia

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Do plants radiate heat into the sky?
« Reply #2 on: 10/01/2011 23:48:16 »
Everything radiates heat. I protect slightly tender plants in my greenhouse by putting newspaper over them during freezing conditions, to reflect the heat back to them. It seems to work.
 

Offline JP

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Do plants radiate heat into the sky?
« Reply #3 on: 11/01/2011 04:51:17 »
For a plant, the loss of heat through radiation is going to be incredibly slow compared to the loss of heat through conduction.  That's why it's not really a big concern compared to the ambient air temperature.
 

Offline Geezer

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Do plants radiate heat into the sky?
« Reply #4 on: 11/01/2011 06:36:20 »
Everything radiates heat. I protect slightly tender plants in my greenhouse by putting newspaper over them during freezing conditions, to reflect the heat back to them. It seems to work.

Pikaia,

I think what you are doing there is mainly slowing the rate of heat loss from the soil so that a layer of warmer air is trapped under the newspaper. It works very well too.
 

Offline Pikaia

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Do plants radiate heat into the sky?
« Reply #5 on: 11/01/2011 08:31:48 »
I have done sums, and the heat radiated by a black body at zero celsius is a little over 300W per square metre. Plants are not black bodies so the actual figure will be rather less, but it is still rather substantial.
 

Offline JP

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Do plants radiate heat into the sky?
« Reply #6 on: 11/01/2011 11:45:17 »
Pikia, you have to take care with interpreting that number.  That is correct for the heat radiated away from the plant, but unless the plant is in deep space, it will receive radiation from its environment (which also radiates more or less as a black body).  If the plant is at 10 C and the ambient environment is at 0 C, I get roughly 50 Watts/m2 radiated.  It's still significant, but not nearly as big!  Of course, there are all sorts of factors that can change this number, but it's probably in the right ballpark.

It would be nice to be able to compare this to heat lost due to conduction or convection, but those calculations are beyond me.
 

Offline imatfaal

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Do plants radiate heat into the sky?
« Reply #7 on: 11/01/2011 17:05:25 »
We can set an upper limit on the amount of heat a viable plant radiates away. 

1. The sun's energy at the earth's surface is around 1.3-1.4 KW/m^2. 
2. a plant's energy comes entirely from the sunlight incident on their leaves (other than a few rare exceptions). 
3. Wikipedia states about 5% incident light is converted - so plant can count on about 70W/m^2 during sunlight hours.

-  The radiated energy must be less than this - the plant needs energy to build and respire as well.
 

Offline Airthumbs

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Do plants radiate heat into the sky?
« Reply #8 on: 11/01/2011 21:58:06 »
Try repeating the experiment with one, uncovered thermometer and another one, uncovered, next to a plant, and/or both covered. I would be interested to see the results?  ;D
 

Offline CliffordK

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Do plants radiate heat into the sky?
« Reply #9 on: 11/01/2011 22:25:34 »
3. Wikipedia states about 5% incident light is converted - so plant can count on about 70W/m^2 during sunlight hours.

There would be 3 types of heat & energy.
  • Reflected light.  (Green?)  It never gets absorbed, but is reflected back from the original light/energy source.
  • Energy absorbed as HEAT.  I.E. 5% is absorbed as energy.  The other 95% has to go somewhere...  I.E. either HEAT or reflected energy.  Especially in the summer.  While plants are often cooler than asphalt, for example, this additional energy has to go somewhere.  I believe that plants are supposed to have some internal temperature regulation.
  • Metabolism.  Energy absorbed by the chlorophyll, and used to power the plant, and probably also radiated as part of the metabolic process
 

Offline imatfaal

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Do plants radiate heat into the sky?
« Reply #10 on: 12/01/2011 15:00:53 »
3. Wikipedia states about 5% incident light is converted - so plant can count on about 70W/m^2 during sunlight hours.

There would be 3 types of heat & energy.
  • Reflected light.  (Green?)  It never gets absorbed, but is reflected back from the original light/energy source.
  • Energy absorbed as HEAT.  I.E. 5% is absorbed as energy.  The other 95% has to go somewhere...  I.E. either HEAT or reflected energy.  Especially in the summer.  While plants are often cooler than asphalt, for example, this additional energy has to go somewhere.  I believe that plants are supposed to have some internal temperature regulation.
  • Metabolism.  Energy absorbed by the chlorophyll, and used to power the plant, and probably also radiated as part of the metabolic process


Clifford - all of what you say is correct, but in trying to simplify - you can set an upper limit to the energy available to a plant to radiate by understanding its total energy income.

 A plant cannot use the vast majority of the spectrum of incident light - the UV and higher, the infrared and lower, and of course the green.  Chlorophyll only takes a portion of the light that is of the right band, and of that portion the energy per photon is most efficient towards the red end
 

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Do plants radiate heat into the sky?
« Reply #10 on: 12/01/2011 15:00:53 »

 

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