HEAT & RADIATION; Do you know the difference?
Why do explanations of Greenhouse Theory use the terms 'HEAT' and 'RADIATION' interchangeably... are they the same thing?
Greenhouse gases do not restrict the conduction of HEAT, like blankets do. Greenhouse gases do not restrict the convection of HEAT, like the glass walls of greenhouses do. So what exactly do they do?!
The Greenhouse Theory contains the central idea that greenhouse gases (GHG’s) absorb RADIATION, which is itself emitted from the sun-heated Earth.
(I wrote a ‘Greenhouse Theory 101’ in a previous newsletter should you wish to recap!)
When scratching just a little bit deeper, many explanations of the inner workings of the Greenhouse Effect use two words interchangeably when describing this process;
HEAT and RADIATION.
But what do these terms actually mean?
Are they the same thing, as is so often stated?
And why does it matter?
This newsletter attempts to explore these questions. And in doing so, I hope that you will gain a deeper insight into the scientific theory behind arguably the biggest cultural, political and economic issue of our time.
Radiation AKA Heat… Wait, What?
Have a look at the following quotes from some renowned institutions. Notice how the terms ‘HEAT’ and ‘RADIATION’ are used interchangeably (and in the NASA example, used within a false analogy to which I shall return later!) [emphasis added]:
‘‘The warmed surface then radiates heat, which is absorbed by greenhouse gases such as carbon dioxide.’’ Wikipedia
‘‘Greenhouse gas molecules absorb and emit this infrared [radiation], so heat up and emit radiant heat in all directions, warming other greenhouse gas molecules, and pass heat on to the surrounding air.’’ Wikipedia
‘‘A greenhouse gas is called that because it absorbs infrared radiation from the Sun in the form of heat.’’ British Geological Survey
‘‘These heat-trapping gases can be thought of as a blanket wrapped around Earth, keeping the planet toastier than it would be without them.’’ NASA
And here is yet another example in this nice little movie/ cartoon explaining in simple terms how the greenhouse theory works. At around the 0:45 second time-stamp this popular video states;
‘At the same time [the atmosphere] traps the infrared radiation, aka heat, radiating out from the Earth’s sun warmed surface [sic]’
There are of course countless more examples, but you get my point!
Some Definitions
What is ‘RADIATION’?
The Earth, the sea, a jam sandwich, even your cat; all substances emit RADIATION as long as it’s temperature is above ‘absolute zero’ (-273 Celsius).
But what exactly it is RADIATION?
First we have to understand that we are surrounded by invisible forces of electricity and magnetism. Known as the electromagnetic field; it’s everywhere, all the time.
RADIATION is a phenomenon of electromagnetism. From microwaves to radio waves, visible light to ionizing radiation; these are all manifestations of different electromagnetic field ‘perturbations’, or waves.
The following image (taken from the NASA website) is a representation of the electromagnetic spectrum which shows not only the various manifestations of electromagnetic field waves, but also of common day-to-day technologies that utilize such properties.
What is HEAT?
If you search for a definition online, you will find a more colloquial definition as something like;
‘the quality of being hot, or having a high temperature’.
A more science-based definition of HEAT is something like;
‘the transfer of kinetic energy from an object of higher energy to that of a lower energy’.
We can see that HEAT is a manifestation of kinetic energy, or movement, of molecules contained within a substance.
Moreover, HEAT is not simply the temperature of an object, but involves the transfer of kinetic energy from one substance to another. And there are three main pathways of HEAT transfer;
Conduction;
Convection; and last but not least
RADIATION*.
*Note: Radiation is unlike the other two mechanisms in that a physical medium isn’t required to transmit energy.
And here we find the overlap between HEAT and RADIATION!
When we see RADIATION - perhaps more accurately called THERMAL RADIATION in this context - used interchangeably with HEAT, technically speaking, it represents one of the three known pathways of HEAT transfer.
Put more simply, it would perhaps be more accurate to say that Thermal RADIATION is one sub-category of HEAT transfer.
Confused Yet?!
So HEAT is not RADIATION - the former being the relative movement of molecules within a substance, the latter being an electromagnetic field perturbation.
However, HEAT does cause RADIATION, and RADIATION does cause HEAT!
It may seem as though I am splitting hairs, but I do believe that it is important to understand that there is a difference between these phenomena, even if the subtleties are less obvious.
It is especially important if we are to really get our heads around understanding the inner workings of the Greenhouse Theory.
Heat Transfer & Greenhouse Theory
Do GHG’s insulate the atmosphere?
I don’t believe that the statement ‘Greenhouse Gases absorb HEAT’ is either technically correct, as described above, or indeed helpful, as I will describe below.
To say ‘absorb HEAT’ gives the misleading impression that there is some sort of loft insulation up in the sky! But of course insulation works by restricting conduction, not through the action of absorbing RADIATION.
And for that matter, neither does the Earth act like an actual greenhouse, which relies on preventing warmed air within the greenhouse mixing (i.e. convection) with external, colder air.
Converting Absorbed Radiation to Higher Temperatures
When we say that GHG’s absorb HEAT (which is technically incorrect), it doesn’t take much imagination to understand how that translates to rising temperatures. It is less obvious, however, how the absorption of RADIATION translates to rising temperatures.
What is the mechanism by which the absorption* of RADIATION by GHG’s translates into an increase in temperature, which is then measured in met stations around the world?
*defining what is meant by ‘absorption’ is a can of worms I will leave unopened for now!
There are different explanations to this question, even within the environmental and climate sciences themselves (see HERE for a summary). The answer certainly isn’t obvious, irrespective of whether some ‘experts’ try and tell you it is!
Those in the know will almost certainly give you an explanation of the Greenhouse Effect that mixes up the terms HEAT and RADIATION, and so we frustratingly end up back to where we started this newsletter from!
Summary
Here is what we’ve covered:
HEAT is not RADIATION, but HEAT does ‘radiate’.
RADIATION is not HEAT, but RADIATION can cause heating.
RADIATION is a manifestation of waves within the electromagnetic field; whereas HEAT is a manifestation of molecular movement within a substance.
All objects with a temperature above absolute zero emit infrared radiation, the amount being in proportion to its’ temperature.
RADIATION is one of three heat transfer mechanisms by which energy moves from a body of higher temperature to one of lower temperature (the others being conduction and convection).
GHG’s do not restrict the conduction of HEAT, like blankets do.
GHG’s do not restrict the convection of HEAT, like the glass walls of a greenhouses do.
GHG’s are said to absorb infrared RADIATION, which represents the third mechanism of HEAT transfer.
The precise mechanism by which the absorption of RADIATION is converted into rising temperatures (in an openly convective system) is, surprisingly, still an open question; see my earlier newsletter A Fork in the Road for more information.
And finally, if there would be one key takeaway from this Newsletter, it would be to think about the following statement:
‘‘Radiation and Heat; one is a manifestation of electromagnetic fields, whereas the other is a manifestation of molecular movement.’’
As ever, I hope you found this interesting!
Please don’t forget to share or subscribe (if you haven’t already)!
-Tristan