Seeing heat and the Greenhouse Effect
The connection between heat and light
We can "see" objects glowing when they get above about 1000 F.
Some snakes can sense much lower temperatures with their 'pit organs'.
Somehow, this is related to the Greenhouse Effect!
What is heat??
Atomically, it's related to the average speed of molecules. The higher the temperature, the faster molecules are moving. (Here's a more elaborate simulation).
Temperature is proportional to the average kinetic energy of the atoms. For a gas, all made up of identical atoms, with mass $m$... $$T=\frac32 k_B m\langle v^2\rangle =3k_B\cdot \frac 12m\langle v^2\rangle.$$
Losing heat
When a hot object cools down we say it loses energy. It's thermal energy is lower the colder it gets.
The most familiar way to lose thermal energy is by contact with something colder.
But there is a second way that hot objects can give off energy (and cool down): What form of energy is coming from the hot kiln that you can see?
Light = Electro-magnetic waves
Imagine you comb your hair, and your comb becomes statically charge with electricity...
Shaking charges on the comb back and forth will send out "electromagnetic waves". Sort of like a hand shaking a slinky sends out waves:
It takes energy to make the slinky shake, and the resulting wave is carrying energy away from the shaker.
Light is an electromagnetic wave, carrying energy.
Reversing the process,
- light -- an E-M wave -- arrives from the sun,
- it hits an object made of atoms (containing charges)
- This makes the atoms in the object shake faster, which means the object heats up.
A bunch of cats who don't want the sun's electro-magnetic waves to make their atoms shake faster...
Whenever you shake (accelerate) some charged particles you generate electro-magnetic waves.
Electro-magnetic waves are just another name for light. The faster the atoms are moving, the more frequently they bump into each other, and the average frequency of light increases when the temperature increases.
Frequency and wavelength
Frequency =
# of waves passing a point in a time interval - [1/s] or [(cycles)/sec] or
[Herz] =[Hz]
Electromagnetic spectrum
Higher temperature = faster shaking = higher frequency waves.
Blackbody radiation
Our sun has a surface temperature of ~5400 C~5700 K.
A Tungsten lightbulb filament has a melting temperature of ~3700 K.
The temperature of a glowing object is closely related to its apparent color.
We see objects glowing visibly, once they get above about 1000 F.
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Do we glow too?
We're much "cooler" than the sun, so if we "glow" it would be at a much lower frequency than visible light... Gee wouldn't it be nice if we had some way of sensing lower frequency light?
Special sensors can sense IR (infra-red) region of the spectrum, and then transpose it up into the visible range for us to see. This is the basis of an IR camera. Just like the visible scale, the camera can estimate the temperature from the wavelength of light given off:
Interesting: The incandescent bulb is much hotter than the fluorescent light in my ceiling... But much cooler than 1000 F? What happens when we turn off the light bulb?
You can also use this technology to look for heat leaks: Here are two sets of windows on a 6th St house on the same cold night:
 Older window |
 Replacement, highly insulated window |
|---|
Some snakes have "pit organs" that can sense IR radiation.
Transparency depends...
Whether an object:
- allows light to pass through ("transparent"), or
- does not allow light to pass through ("opaque" or "absorbs light")
...apparently depends on the frequency of the light.
We can "see" the sun very easily. Gases in the atmosphere like Nitrogen, $N_2$, oxygen, $O_2$, Carbon dioxide, $CO_2$, water vapor, $H_2O$ (though not condensed water vapor = clouds), methane, $CH_4$, helium, $He$, etc are transparent to visible light.
But some gases in the atmosphere are less transparent to infrared light. These are the "greenhouse gases":
- Carbon dioxide,
- water vapor,
- methane,
- $NO_x$
- CFC's (refrigerants from A/C systems)
[Basically, any molecules of 3 or more atoms are GHGs.]
The Greenhouse Effect
of Earth's atmosphere
- Visible light from the sun is transmitted through the atmosphere (including the GHGs):
- The visible light strikes Earth's surface where it is absorbed by land/roads/trees (friction): These objects heat up.
- Hot objects give off infrared (IR) radiation.
- But GHGs reflect and absorb IR radiation; They are not as transparent to IR light. [ IR=infrared: Light with a wavelength much longer than visible red light. ]
- So some of the sun's energy is trapped in the atmosphere.
Change
- "Earth's atmosphere"$\to$ "car window"
- "Earth's surface"$\to$ "dashboard and seats"
...and you have a description of the process that makes the inside of your parked car heat up on a sunny day:
So some energy from the sun remains trapped in the atmosphere, which heats up the atmosphere.
As a result Earth stays warmer than it would if there were no greenhouse gases. This is where gratitude kicks in:
Thanks be to God!
Scientists estimate (NASA) that the "natural greenhouse effect" is the reason why the average temperature on Earth is about 59F.
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If there were no greenhouse gases in Earth's atmosphere, Earth's average temperature (NASA-GISS) would be about 0F.
But, anthropogenic $CO_2$ is the causing *additional* warming.
Now, one of the biggest challenges of our generation...
Image credits
Insolation map: NREL
Phil G, D Searls, Flickr user Zen, Wikipedia user Kendrick7 and S.n.
